diff --git a/java/src/com/android/inputmethod/keyboard/Key.java b/java/src/com/android/inputmethod/keyboard/Key.java
index 06e552e3d..d1db3d4c5 100644
--- a/java/src/com/android/inputmethod/keyboard/Key.java
+++ b/java/src/com/android/inputmethod/keyboard/Key.java
@@ -43,6 +43,9 @@ import com.android.inputmethod.latin.common.StringUtils;
 import java.util.Arrays;
 import java.util.Locale;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 /**
  * Class for describing the position and characteristics of a single key in the keyboard.
  */
@@ -113,9 +116,11 @@ public class Key implements Comparable<Key> {
     /** Y coordinate of the top-left corner of the key in the keyboard layout, excluding the gap. */
     private final int mY;
     /** Hit bounding box of the key */
+    @Nonnull
     private final Rect mHitBox = new Rect();
 
     /** More keys. It is guaranteed that this is null or an array of one or more elements */
+    @Nullable
     private final MoreKeySpec[] mMoreKeys;
     /** More keys column number and flags */
     private final int mMoreKeysColumnAndFlags;
@@ -158,8 +163,9 @@ public class Key implements Comparable<Key> {
     private static final int ACTION_FLAGS_ALT_CODE_WHILE_TYPING = 0x04;
     private static final int ACTION_FLAGS_ENABLE_LONG_PRESS = 0x08;
 
+    @Nullable
     private final KeyVisualAttributes mKeyVisualAttributes;
-
+    @Nullable
     private final OptionalAttributes mOptionalAttributes;
 
     private static final class OptionalAttributes {
@@ -181,6 +187,7 @@ public class Key implements Comparable<Key> {
             mVisualInsetsRight = visualInsetsRight;
         }
 
+        @Nullable
         public static OptionalAttributes newInstance(final String outputText, final int altCode,
                 final int disabledIconId, final int visualInsetsLeft, final int visualInsetsRight) {
             if (outputText == null && altCode == CODE_UNSPECIFIED
@@ -204,10 +211,10 @@ public class Key implements Comparable<Key> {
      * Constructor for a key on <code>MoreKeyKeyboard</code>, on <code>MoreSuggestions</code>,
      * and in a <GridRows/>.
      */
-    public Key(final String label, final int iconId, final int code, final String outputText,
-            final String hintLabel, final int labelFlags, final int backgroundType, final int x,
-            final int y, final int width, final int height, final int horizontalGap,
-            final int verticalGap) {
+    public Key(@Nullable final String label, final int iconId, final int code,
+            @Nullable final String outputText, @Nullable final String hintLabel,
+            final int labelFlags, final int backgroundType, final int x, final int y,
+            final int width, final int height, final int horizontalGap, final int verticalGap) {
         mWidth = width - horizontalGap;
         mHeight = height - verticalGap;
         mHorizontalGap = horizontalGap;
@@ -245,8 +252,9 @@ public class Key implements Comparable<Key> {
      * @param row the row that this key belongs to. row's x-coordinate will be the right edge of
      *        this key.
      */
-    public Key(final String keySpec, final TypedArray keyAttr, final KeyStyle style,
-            final KeyboardParams params, final KeyboardRow row) {
+    public Key(@Nullable final String keySpec, @Nonnull final TypedArray keyAttr,
+            @Nonnull final KeyStyle style, @Nonnull final KeyboardParams params,
+            @Nonnull final KeyboardRow row) {
         mHorizontalGap = isSpacer() ? 0 : params.mHorizontalGap;
         mVerticalGap = params.mVerticalGap;
 
@@ -403,11 +411,11 @@ public class Key implements Comparable<Key> {
      *
      * @param key the original key.
      */
-    protected Key(final Key key) {
+    protected Key(@Nonnull final Key key) {
         this(key, key.mMoreKeys);
     }
 
-    private Key(final Key key, final MoreKeySpec[] moreKeys) {
+    private Key(@Nonnull final Key key, @Nullable final MoreKeySpec[] moreKeys) {
         // Final attributes.
         mCode = key.mCode;
         mLabel = key.mLabel;
@@ -433,8 +441,9 @@ public class Key implements Comparable<Key> {
         mEnabled = key.mEnabled;
     }
 
-    public static Key removeRedundantMoreKeys(final Key key,
-            final MoreKeySpec.LettersOnBaseLayout lettersOnBaseLayout) {
+    @Nonnull
+    public static Key removeRedundantMoreKeys(@Nonnull final Key key,
+            @Nonnull final MoreKeySpec.LettersOnBaseLayout lettersOnBaseLayout) {
         final MoreKeySpec[] moreKeys = key.getMoreKeys();
         final MoreKeySpec[] filteredMoreKeys = MoreKeySpec.removeRedundantMoreKeys(
                 moreKeys, lettersOnBaseLayout);
@@ -554,14 +563,17 @@ public class Key implements Comparable<Key> {
         return mCode;
     }
 
+    @Nullable
     public String getLabel() {
         return mLabel;
     }
 
+    @Nullable
     public String getHintLabel() {
         return mHintLabel;
     }
 
+    @Nullable
     public MoreKeySpec[] getMoreKeys() {
         return mMoreKeys;
     }
@@ -620,6 +632,7 @@ public class Key implements Comparable<Key> {
         return mKeyVisualAttributes;
     }
 
+    @Nonnull
     public final Typeface selectTypeface(final KeyDrawParams params) {
         switch (mLabelFlags & LABEL_FLAGS_FONT_MASK) {
         case LABEL_FLAGS_FONT_NORMAL:
@@ -696,6 +709,7 @@ public class Key implements Comparable<Key> {
         return params.mLetterSize;
     }
 
+    @Nonnull
     public Typeface selectPreviewTypeface(final KeyDrawParams params) {
         if (previewHasLetterSize()) {
             return selectTypeface(params);
@@ -780,6 +794,7 @@ public class Key implements Comparable<Key> {
         return (mMoreKeysColumnAndFlags & MORE_KEYS_FLAGS_NO_PANEL_AUTO_MORE_KEY) != 0;
     }
 
+    @Nullable
     public final String getOutputText() {
         final OptionalAttributes attrs = mOptionalAttributes;
         return (attrs != null) ? attrs.mOutputText : null;
@@ -794,6 +809,7 @@ public class Key implements Comparable<Key> {
         return mIconId;
     }
 
+    @Nullable
     public Drawable getIcon(final KeyboardIconsSet iconSet, final int alpha) {
         final OptionalAttributes attrs = mOptionalAttributes;
         final int disabledIconId = (attrs != null) ? attrs.mDisabledIconId : ICON_UNDEFINED;
@@ -805,6 +821,7 @@ public class Key implements Comparable<Key> {
         return icon;
     }
 
+    @Nullable
     public Drawable getPreviewIcon(final KeyboardIconsSet iconSet) {
         return iconSet.getIconDrawable(getIconId());
     }
@@ -897,6 +914,7 @@ public class Key implements Comparable<Key> {
         mEnabled = enabled;
     }
 
+    @Nonnull
     public Rect getHitBox() {
         return mHitBox;
     }
@@ -968,8 +986,10 @@ public class Key implements Comparable<Key> {
      * @return the background drawable of the key.
      * @see android.graphics.drawable.StateListDrawable#setState(int[])
      */
-    public final Drawable selectBackgroundDrawable(final Drawable keyBackground,
-            final Drawable functionalKeyBackground, final Drawable spacebarBackground) {
+    @Nonnull
+    public final Drawable selectBackgroundDrawable(@Nonnull final Drawable keyBackground,
+            @Nonnull final Drawable functionalKeyBackground,
+            @Nonnull final Drawable spacebarBackground) {
         final Drawable background;
         if (mBackgroundType == BACKGROUND_TYPE_FUNCTIONAL) {
             background = functionalKeyBackground;
diff --git a/java/src/com/android/inputmethod/keyboard/Keyboard.java b/java/src/com/android/inputmethod/keyboard/Keyboard.java
index 619b801f4..2055a59bb 100644
--- a/java/src/com/android/inputmethod/keyboard/Keyboard.java
+++ b/java/src/com/android/inputmethod/keyboard/Keyboard.java
@@ -28,6 +28,9 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 /**
  * Loads an XML description of a keyboard and stores the attributes of the keys. A keyboard
  * consists of rows of keys.
@@ -47,6 +50,7 @@ import java.util.List;
  * </pre>
  */
 public class Keyboard {
+    @Nonnull
     public final KeyboardId mId;
     public final int mThemeId;
 
@@ -78,17 +82,22 @@ public class Keyboard {
     public final int mMaxMoreKeysKeyboardColumn;
 
     /** List of keys in this keyboard */
+    @Nonnull
     private final List<Key> mSortedKeys;
+    @Nonnull
     public final List<Key> mShiftKeys;
+    @Nonnull
     public final List<Key> mAltCodeKeysWhileTyping;
+    @Nonnull
     public final KeyboardIconsSet mIconsSet;
 
     private final SparseArray<Key> mKeyCache = new SparseArray<>();
 
+    @Nonnull
     private final ProximityInfo mProximityInfo;
     private final boolean mProximityCharsCorrectionEnabled;
 
-    public Keyboard(final KeyboardParams params) {
+    public Keyboard(@Nonnull final KeyboardParams params) {
         mId = params.mId;
         mThemeId = params.mThemeId;
         mOccupiedHeight = params.mOccupiedHeight;
@@ -114,7 +123,7 @@ public class Keyboard {
         mProximityCharsCorrectionEnabled = params.mProximityCharsCorrectionEnabled;
     }
 
-    protected Keyboard(final Keyboard keyboard) {
+    protected Keyboard(@Nonnull final Keyboard keyboard) {
         mId = keyboard.mId;
         mThemeId = keyboard.mThemeId;
         mOccupiedHeight = keyboard.mOccupiedHeight;
@@ -150,6 +159,7 @@ public class Keyboard {
         return canAssumeNativeHasProximityCharsInfoOfAllKeys || Character.isLetter(code);
     }
 
+    @Nonnull
     public ProximityInfo getProximityInfo() {
         return mProximityInfo;
     }
@@ -160,10 +170,12 @@ public class Keyboard {
      * The list may contain {@link Key.Spacer} object as well.
      * @return the sorted unmodifiable list of {@link Key}s of this keyboard.
      */
+    @Nonnull
     public List<Key> getSortedKeys() {
         return mSortedKeys;
     }
 
+    @Nullable
     public Key getKey(final int code) {
         if (code == Constants.CODE_UNSPECIFIED) {
             return null;
@@ -185,7 +197,7 @@ public class Keyboard {
         }
     }
 
-    public boolean hasKey(final Key aKey) {
+    public boolean hasKey(@Nonnull final Key aKey) {
         if (mKeyCache.indexOfValue(aKey) >= 0) {
             return true;
         }
@@ -211,6 +223,7 @@ public class Keyboard {
      * @return the list of the nearest keys to the given point. If the given
      * point is out of range, then an array of size zero is returned.
      */
+    @Nonnull
     public List<Key> getNearestKeys(final int x, final int y) {
         // Avoid dead pixels at edges of the keyboard
         final int adjustedX = Math.max(0, Math.min(x, mOccupiedWidth - 1));
@@ -218,7 +231,8 @@ public class Keyboard {
         return mProximityInfo.getNearestKeys(adjustedX, adjustedY);
     }
 
-    public int[] getCoordinates(final int[] codePoints) {
+    @Nonnull
+    public int[] getCoordinates(@Nonnull final int[] codePoints) {
         final int length = codePoints.length;
         final int[] coordinates = CoordinateUtils.newCoordinateArray(length);
         for (int i = 0; i < length; ++i) {
diff --git a/java/src/com/android/inputmethod/keyboard/KeyboardLayoutSet.java b/java/src/com/android/inputmethod/keyboard/KeyboardLayoutSet.java
index b1051385d..5d9b8a712 100644
--- a/java/src/com/android/inputmethod/keyboard/KeyboardLayoutSet.java
+++ b/java/src/com/android/inputmethod/keyboard/KeyboardLayoutSet.java
@@ -72,6 +72,7 @@ public final class KeyboardLayoutSet {
     private static final String KEYBOARD_LAYOUT_SET_RESOURCE_PREFIX = "keyboard_layout_set_";
 
     private final Context mContext;
+    @Nonnull
     private final Params mParams;
 
     // How many layouts we forcibly keep in cache. This only includes ALPHABET (default) and
@@ -84,6 +85,7 @@ public final class KeyboardLayoutSet {
     private static final Keyboard[] sForcibleKeyboardCache = new Keyboard[FORCIBLE_CACHE_SIZE];
     private static final HashMap<KeyboardId, SoftReference<Keyboard>> sKeyboardCache =
             new HashMap<>();
+    @Nonnull
     private static final KeysCache sKeysCache = new KeysCache();
     private final static HashMap<InputMethodSubtype, Integer> sScriptIdsForSubtypes =
             new HashMap<>();
@@ -145,7 +147,8 @@ public final class KeyboardLayoutSet {
         sKeysCache.clear();
     }
 
-    public static int getScriptId(final Resources resources, final InputMethodSubtype subtype) {
+    public static int getScriptId(final Resources resources,
+            @Nonnull final InputMethodSubtype subtype) {
         final Integer value = sScriptIdsForSubtypes.get(subtype);
         if (null == value) {
             final int scriptId = Builder.readScriptId(resources, subtype);
@@ -155,11 +158,12 @@ public final class KeyboardLayoutSet {
         return value;
     }
 
-    KeyboardLayoutSet(final Context context, final Params params) {
+    KeyboardLayoutSet(final Context context, @Nonnull final Params params) {
         mContext = context;
         mParams = params;
     }
 
+    @Nonnull
     public Keyboard getKeyboard(final int baseKeyboardLayoutSetElementId) {
         final int keyboardLayoutSetElementId;
         switch (mParams.mMode) {
@@ -203,6 +207,7 @@ public final class KeyboardLayoutSet {
         }
     }
 
+    @Nonnull
     private Keyboard getKeyboard(final ElementParams elementParams, final KeyboardId id) {
         final SoftReference<Keyboard> ref = sKeyboardCache.get(id);
         final Keyboard cachedKeyboard = (ref == null) ? null : ref.get();
diff --git a/java/src/com/android/inputmethod/keyboard/KeyboardView.java b/java/src/com/android/inputmethod/keyboard/KeyboardView.java
index 01980ea6e..27e538cb7 100644
--- a/java/src/com/android/inputmethod/keyboard/KeyboardView.java
+++ b/java/src/com/android/inputmethod/keyboard/KeyboardView.java
@@ -103,7 +103,8 @@ public class KeyboardView extends View {
     // TODO: Consider having a dummy keyboard object to make this @Nonnull
     @Nullable
     private Keyboard mKeyboard;
-    protected final KeyDrawParams mKeyDrawParams = new KeyDrawParams();
+    @Nonnull
+    private final KeyDrawParams mKeyDrawParams = new KeyDrawParams();
 
     // Drawing
     /** True if all keys should be drawn */
@@ -120,6 +121,7 @@ public class KeyboardView extends View {
     @Nonnull
     private final Paint mPaint = new Paint();
     private final Paint.FontMetrics mFontMetrics = new Paint.FontMetrics();
+
     public KeyboardView(final Context context, final AttributeSet attrs) {
         this(context, attrs, R.attr.keyboardViewStyle);
     }
@@ -210,6 +212,11 @@ public class KeyboardView extends View {
         return mVerticalCorrection;
     }
 
+    @Nonnull
+    protected KeyDrawParams getKeyDrawParams() {
+        return mKeyDrawParams;
+    }
+
     protected void updateKeyDrawParams(final int keyHeight) {
         mKeyDrawParams.updateParams(keyHeight, mKeyVisualAttributes);
     }
diff --git a/java/src/com/android/inputmethod/keyboard/MainKeyboardView.java b/java/src/com/android/inputmethod/keyboard/MainKeyboardView.java
index fc6d43919..eeac4755d 100644
--- a/java/src/com/android/inputmethod/keyboard/MainKeyboardView.java
+++ b/java/src/com/android/inputmethod/keyboard/MainKeyboardView.java
@@ -484,7 +484,7 @@ public final class MainKeyboardView extends KeyboardView implements DrawingProxy
 
         locatePreviewPlacerView();
         getLocationInWindow(mOriginCoords);
-        mKeyPreviewChoreographer.placeAndShowKeyPreview(key, keyboard.mIconsSet, mKeyDrawParams,
+        mKeyPreviewChoreographer.placeAndShowKeyPreview(key, keyboard.mIconsSet, getKeyDrawParams(),
                 getWidth(), mOriginCoords, mDrawingPreviewPlacerView, isHardwareAccelerated());
     }
 
diff --git a/java/src/com/android/inputmethod/keyboard/MoreKeysKeyboard.java b/java/src/com/android/inputmethod/keyboard/MoreKeysKeyboard.java
index f0de86ff9..a021e5e2d 100644
--- a/java/src/com/android/inputmethod/keyboard/MoreKeysKeyboard.java
+++ b/java/src/com/android/inputmethod/keyboard/MoreKeysKeyboard.java
@@ -27,6 +27,8 @@ import com.android.inputmethod.latin.R;
 import com.android.inputmethod.latin.common.StringUtils;
 import com.android.inputmethod.latin.utils.TypefaceUtils;
 
+import javax.annotation.Nonnull;
+
 public final class MoreKeysKeyboard extends Keyboard {
     private final int mDefaultKeyCoordX;
 
@@ -328,6 +330,7 @@ public final class MoreKeysKeyboard extends Keyboard {
         }
 
         @Override
+        @Nonnull
         public MoreKeysKeyboard build() {
             final MoreKeysKeyboardParams params = mParams;
             final int moreKeyFlags = mParentKey.getMoreKeyLabelFlags();
diff --git a/java/src/com/android/inputmethod/keyboard/ProximityInfo.java b/java/src/com/android/inputmethod/keyboard/ProximityInfo.java
index ab2323b06..228b964ea 100644
--- a/java/src/com/android/inputmethod/keyboard/ProximityInfo.java
+++ b/java/src/com/android/inputmethod/keyboard/ProximityInfo.java
@@ -28,6 +28,8 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
+import javax.annotation.Nonnull;
+
 public class ProximityInfo {
     private static final String TAG = ProximityInfo.class.getSimpleName();
     private static final boolean DEBUG = false;
@@ -36,6 +38,7 @@ public class ProximityInfo {
     public static final int MAX_PROXIMITY_CHARS_SIZE = 16;
     /** Number of key widths from current touch point to search for nearest keys. */
     private static final float SEARCH_DISTANCE = 1.2f;
+    @Nonnull
     private static final List<Key> EMPTY_KEY_LIST = Collections.emptyList();
     private static final float DEFAULT_TOUCH_POSITION_CORRECTION_RADIUS = 0.15f;
 
@@ -49,13 +52,16 @@ public class ProximityInfo {
     private final int mKeyboardHeight;
     private final int mMostCommonKeyWidth;
     private final int mMostCommonKeyHeight;
+    @Nonnull
     private final List<Key> mSortedKeys;
+    @Nonnull
     private final List<Key>[] mGridNeighbors;
 
     @SuppressWarnings("unchecked")
     ProximityInfo(final int gridWidth, final int gridHeight, final int minWidth, final int height,
-            final int mostCommonKeyWidth, final int mostCommonKeyHeight, final List<Key> sortedKeys,
-            final TouchPositionCorrection touchPositionCorrection) {
+            final int mostCommonKeyWidth, final int mostCommonKeyHeight,
+            @Nonnull final List<Key> sortedKeys,
+            @Nonnull final TouchPositionCorrection touchPositionCorrection) {
         mGridWidth = gridWidth;
         mGridHeight = gridHeight;
         mGridSize = mGridWidth * mGridHeight;
@@ -104,7 +110,8 @@ public class ProximityInfo {
         return count;
     }
 
-    private long createNativeProximityInfo(final TouchPositionCorrection touchPositionCorrection) {
+    private long createNativeProximityInfo(
+            @Nonnull final TouchPositionCorrection touchPositionCorrection) {
         final List<Key>[] gridNeighborKeys = mGridNeighbors;
         final int[] proximityCharsArray = new int[mGridSize * MAX_PROXIMITY_CHARS_SIZE];
         Arrays.fill(proximityCharsArray, Constants.NOT_A_CODE);
@@ -163,7 +170,7 @@ public class ProximityInfo {
             infoIndex++;
         }
 
-        if (touchPositionCorrection != null && touchPositionCorrection.isValid()) {
+        if (touchPositionCorrection.isValid()) {
             if (DEBUG) {
                 Log.d(TAG, "touchPositionCorrection: ON");
             }
@@ -385,10 +392,8 @@ y |---+---+---+---+-v-+-|-+---+---+---+---+---|          | thresholdBase and get
         }
     }
 
+    @Nonnull
     public List<Key> getNearestKeys(final int x, final int y) {
-        if (mGridNeighbors == null) {
-            return EMPTY_KEY_LIST;
-        }
         if (x >= 0 && x < mKeyboardMinWidth && y >= 0 && y < mKeyboardHeight) {
             int index = (y / mCellHeight) * mGridWidth + (x / mCellWidth);
             if (index < mGridSize) {
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeyDrawParams.java b/java/src/com/android/inputmethod/keyboard/internal/KeyDrawParams.java
index df50efdc1..3ef9ea1dc 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeyDrawParams.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeyDrawParams.java
@@ -20,8 +20,12 @@ import android.graphics.Typeface;
 
 import com.android.inputmethod.latin.utils.ResourceUtils;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 public final class KeyDrawParams {
-    public Typeface mTypeface;
+    @Nonnull
+    public Typeface mTypeface = Typeface.DEFAULT;
 
     public int mLetterSize;
     public int mLabelSize;
@@ -49,7 +53,7 @@ public final class KeyDrawParams {
 
     public KeyDrawParams() {}
 
-    private KeyDrawParams(final KeyDrawParams copyFrom) {
+    private KeyDrawParams(@Nonnull final KeyDrawParams copyFrom) {
         mTypeface = copyFrom.mTypeface;
 
         mLetterSize = copyFrom.mLetterSize;
@@ -77,7 +81,7 @@ public final class KeyDrawParams {
         mAnimAlpha = copyFrom.mAnimAlpha;
     }
 
-    public void updateParams(final int keyHeight, final KeyVisualAttributes attr) {
+    public void updateParams(final int keyHeight, @Nullable final KeyVisualAttributes attr) {
         if (attr == null) {
             return;
         }
@@ -117,8 +121,9 @@ public final class KeyDrawParams {
                 attr.mHintLabelOffCenterRatio, mHintLabelOffCenterRatio);
     }
 
+    @Nonnull
     public KeyDrawParams mayCloneAndUpdateParams(final int keyHeight,
-            final KeyVisualAttributes attr) {
+            @Nullable final KeyVisualAttributes attr) {
         if (attr == null) {
             return this;
         }
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeySpecParser.java b/java/src/com/android/inputmethod/keyboard/internal/KeySpecParser.java
index 63aab968b..3eb62e7a6 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeySpecParser.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeySpecParser.java
@@ -22,6 +22,9 @@ import static com.android.inputmethod.latin.common.Constants.CODE_UNSPECIFIED;
 import com.android.inputmethod.latin.common.Constants;
 import com.android.inputmethod.latin.common.StringUtils;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 /**
  * The string parser of the key specification.
  *
@@ -53,11 +56,11 @@ public final class KeySpecParser {
         // Intentional empty constructor for utility class.
     }
 
-    private static boolean hasIcon(final String keySpec) {
+    private static boolean hasIcon(@Nonnull final String keySpec) {
         return keySpec.startsWith(KeyboardIconsSet.PREFIX_ICON);
     }
 
-    private static boolean hasCode(final String keySpec, final int labelEnd) {
+    private static boolean hasCode(@Nonnull final String keySpec, final int labelEnd) {
         if (labelEnd <= 0 || labelEnd + 1 >= keySpec.length()) {
             return false;
         }
@@ -72,7 +75,8 @@ public final class KeySpecParser {
         return false;
     }
 
-    private static String parseEscape(final String text) {
+    @Nonnull
+    private static String parseEscape(@Nonnull final String text) {
         if (text.indexOf(BACKSLASH) < 0) {
             return text;
         }
@@ -91,7 +95,7 @@ public final class KeySpecParser {
         return sb.toString();
     }
 
-    private static int indexOfLabelEnd(final String keySpec) {
+    private static int indexOfLabelEnd(@Nonnull final String keySpec) {
         final int length = keySpec.length();
         if (keySpec.indexOf(BACKSLASH) < 0) {
             final int labelEnd = keySpec.indexOf(VERTICAL_BAR);
@@ -116,22 +120,25 @@ public final class KeySpecParser {
         return -1;
     }
 
-    private static String getBeforeLabelEnd(final String keySpec, final int labelEnd) {
+    @Nonnull
+    private static String getBeforeLabelEnd(@Nonnull final String keySpec, final int labelEnd) {
         return (labelEnd < 0) ? keySpec : keySpec.substring(0, labelEnd);
     }
 
-    private static String getAfterLabelEnd(final String keySpec, final int labelEnd) {
+    @Nonnull
+    private static String getAfterLabelEnd(@Nonnull final String keySpec, final int labelEnd) {
         return keySpec.substring(labelEnd + /* VERTICAL_BAR */1);
     }
 
-    private static void checkDoubleLabelEnd(final String keySpec, final int labelEnd) {
+    private static void checkDoubleLabelEnd(@Nonnull final String keySpec, final int labelEnd) {
         if (indexOfLabelEnd(getAfterLabelEnd(keySpec, labelEnd)) < 0) {
             return;
         }
         throw new KeySpecParserError("Multiple " + VERTICAL_BAR + ": " + keySpec);
     }
 
-    public static String getLabel(final String keySpec) {
+    @Nullable
+    public static String getLabel(@Nullable final String keySpec) {
         if (keySpec == null) {
             // TODO: Throw {@link KeySpecParserError} once Key.keyLabel attribute becomes mandatory.
             return null;
@@ -147,7 +154,8 @@ public final class KeySpecParser {
         return label;
     }
 
-    private static String getOutputTextInternal(final String keySpec, final int labelEnd) {
+    @Nullable
+    private static String getOutputTextInternal(@Nonnull final String keySpec, final int labelEnd) {
         if (labelEnd <= 0) {
             return null;
         }
@@ -155,7 +163,8 @@ public final class KeySpecParser {
         return parseEscape(getAfterLabelEnd(keySpec, labelEnd));
     }
 
-    public static String getOutputText(final String keySpec) {
+    @Nullable
+    public static String getOutputText(@Nullable final String keySpec) {
         if (keySpec == null) {
             // TODO: Throw {@link KeySpecParserError} once Key.keyLabel attribute becomes mandatory.
             return null;
@@ -184,7 +193,7 @@ public final class KeySpecParser {
         return (StringUtils.codePointCount(label) == 1) ? null : label;
     }
 
-    public static int getCode(final String keySpec) {
+    public static int getCode(@Nullable final String keySpec) {
         if (keySpec == null) {
             // TODO: Throw {@link KeySpecParserError} once Key.keyLabel attribute becomes mandatory.
             return CODE_UNSPECIFIED;
@@ -211,7 +220,7 @@ public final class KeySpecParser {
         return (StringUtils.codePointCount(label) == 1) ? label.codePointAt(0) : CODE_OUTPUT_TEXT;
     }
 
-    public static int parseCode(final String text, final int defaultCode) {
+    public static int parseCode(@Nullable final String text, final int defaultCode) {
         if (text == null) {
             return defaultCode;
         }
@@ -226,7 +235,7 @@ public final class KeySpecParser {
         return defaultCode;
     }
 
-    public static int getIconId(final String keySpec) {
+    public static int getIconId(@Nullable final String keySpec) {
         if (keySpec == null) {
             // TODO: Throw {@link KeySpecParserError} once Key.keyLabel attribute becomes mandatory.
             return KeyboardIconsSet.ICON_UNDEFINED;
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeyStyle.java b/java/src/com/android/inputmethod/keyboard/internal/KeyStyle.java
index 7941ddd41..28aa22c16 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeyStyle.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeyStyle.java
@@ -18,18 +18,22 @@ package com.android.inputmethod.keyboard.internal;
 
 import android.content.res.TypedArray;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 public abstract class KeyStyle {
     private final KeyboardTextsSet mTextsSet;
 
-    public abstract String[] getStringArray(TypedArray a, int index);
-    public abstract String getString(TypedArray a, int index);
+    public abstract @Nullable String[] getStringArray(TypedArray a, int index);
+    public abstract @Nullable String getString(TypedArray a, int index);
     public abstract int getInt(TypedArray a, int index, int defaultValue);
     public abstract int getFlags(TypedArray a, int index);
 
-    protected KeyStyle(final KeyboardTextsSet textsSet) {
+    protected KeyStyle(@Nonnull final KeyboardTextsSet textsSet) {
         mTextsSet = textsSet;
     }
 
+    @Nullable
     protected String parseString(final TypedArray a, final int index) {
         if (a.hasValue(index)) {
             return mTextsSet.resolveTextReference(a.getString(index));
@@ -37,6 +41,7 @@ public abstract class KeyStyle {
         return null;
     }
 
+    @Nullable
     protected String[] parseStringArray(final TypedArray a, final int index) {
         if (a.hasValue(index)) {
             final String text = mTextsSet.resolveTextReference(a.getString(index));
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeyStylesSet.java b/java/src/com/android/inputmethod/keyboard/internal/KeyStylesSet.java
index 5cbb34119..61f98c8ff 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeyStylesSet.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeyStylesSet.java
@@ -29,33 +29,42 @@ import org.xmlpull.v1.XmlPullParserException;
 import java.util.Arrays;
 import java.util.HashMap;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 public final class KeyStylesSet {
     private static final String TAG = KeyStylesSet.class.getSimpleName();
     private static final boolean DEBUG = false;
 
+    @Nonnull
     private final HashMap<String, KeyStyle> mStyles = new HashMap<>();
 
+    @Nonnull
     private final KeyboardTextsSet mTextsSet;
+    @Nonnull
     private final KeyStyle mEmptyKeyStyle;
+    @Nonnull
     private static final String EMPTY_STYLE_NAME = "<empty>";
 
-    public KeyStylesSet(final KeyboardTextsSet textsSet) {
+    public KeyStylesSet(@Nonnull final KeyboardTextsSet textsSet) {
         mTextsSet = textsSet;
         mEmptyKeyStyle = new EmptyKeyStyle(textsSet);
         mStyles.put(EMPTY_STYLE_NAME, mEmptyKeyStyle);
     }
 
     private static final class EmptyKeyStyle extends KeyStyle {
-        EmptyKeyStyle(final KeyboardTextsSet textsSet) {
+        EmptyKeyStyle(@Nonnull final KeyboardTextsSet textsSet) {
             super(textsSet);
         }
 
         @Override
+        @Nullable
         public String[] getStringArray(final TypedArray a, final int index) {
             return parseStringArray(a, index);
         }
 
         @Override
+        @Nullable
         public String getString(final TypedArray a, final int index) {
             return parseString(a, index);
         }
@@ -76,14 +85,16 @@ public final class KeyStylesSet {
         private final String mParentStyleName;
         private final SparseArray<Object> mStyleAttributes = new SparseArray<>();
 
-        public DeclaredKeyStyle(final String parentStyleName, final KeyboardTextsSet textsSet,
-                final HashMap<String, KeyStyle> styles) {
+        public DeclaredKeyStyle(@Nonnull final String parentStyleName,
+                @Nonnull final KeyboardTextsSet textsSet,
+                @Nonnull final HashMap<String, KeyStyle> styles) {
             super(textsSet);
             mParentStyleName = parentStyleName;
             mStyles = styles;
         }
 
         @Override
+        @Nullable
         public String[] getStringArray(final TypedArray a, final int index) {
             if (a.hasValue(index)) {
                 return parseStringArray(a, index);
@@ -98,6 +109,7 @@ public final class KeyStylesSet {
         }
 
         @Override
+        @Nullable
         public String getString(final TypedArray a, final int index) {
             if (a.hasValue(index)) {
                 return parseString(a, index);
@@ -176,37 +188,43 @@ public final class KeyStylesSet {
     public void parseKeyStyleAttributes(final TypedArray keyStyleAttr, final TypedArray keyAttrs,
             final XmlPullParser parser) throws XmlPullParserException {
         final String styleName = keyStyleAttr.getString(R.styleable.Keyboard_KeyStyle_styleName);
+        if (styleName == null) {
+            throw new XmlParseUtils.ParseException(
+                    KeyboardBuilder.TAG_KEY_STYLE + " has no styleName attribute", parser);
+        }
         if (DEBUG) {
             Log.d(TAG, String.format("<%s styleName=%s />",
                     KeyboardBuilder.TAG_KEY_STYLE, styleName));
             if (mStyles.containsKey(styleName)) {
-                Log.d(TAG, "key-style " + styleName + " is overridden at "
+                Log.d(TAG, KeyboardBuilder.TAG_KEY_STYLE + " " + styleName + " is overridden at "
                         + parser.getPositionDescription());
             }
         }
 
-        String parentStyleName = EMPTY_STYLE_NAME;
-        if (keyStyleAttr.hasValue(R.styleable.Keyboard_KeyStyle_parentStyle)) {
-            parentStyleName = keyStyleAttr.getString(R.styleable.Keyboard_KeyStyle_parentStyle);
-            if (!mStyles.containsKey(parentStyleName)) {
-                throw new XmlParseUtils.ParseException(
-                        "Unknown parentStyle " + parentStyleName, parser);
-            }
+        final String parentStyleInAttr = keyStyleAttr.getString(
+                R.styleable.Keyboard_KeyStyle_parentStyle);
+        if (parentStyleInAttr != null && !mStyles.containsKey(parentStyleInAttr)) {
+            throw new XmlParseUtils.ParseException(
+                    "Unknown parentStyle " + parentStyleInAttr, parser);
         }
+        final String parentStyleName = (parentStyleInAttr == null) ? EMPTY_STYLE_NAME
+                : parentStyleInAttr;
         final DeclaredKeyStyle style = new DeclaredKeyStyle(parentStyleName, mTextsSet, mStyles);
         style.readKeyAttributes(keyAttrs);
         mStyles.put(styleName, style);
     }
 
+    @Nonnull
     public KeyStyle getKeyStyle(final TypedArray keyAttr, final XmlPullParser parser)
             throws XmlParseUtils.ParseException {
-        if (!keyAttr.hasValue(R.styleable.Keyboard_Key_keyStyle)) {
+        final String styleName = keyAttr.getString(R.styleable.Keyboard_Key_keyStyle);
+        if (styleName == null) {
             return mEmptyKeyStyle;
         }
-        final String styleName = keyAttr.getString(R.styleable.Keyboard_Key_keyStyle);
-        if (!mStyles.containsKey(styleName)) {
+        final KeyStyle style = mStyles.get(styleName);
+        if (style == null) {
             throw new XmlParseUtils.ParseException("Unknown key style: " + styleName, parser);
         }
-        return mStyles.get(styleName);
+        return style;
     }
 }
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeyVisualAttributes.java b/java/src/com/android/inputmethod/keyboard/internal/KeyVisualAttributes.java
index c60d587db..6f000d294 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeyVisualAttributes.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeyVisualAttributes.java
@@ -23,7 +23,11 @@ import android.util.SparseIntArray;
 import com.android.inputmethod.latin.R;
 import com.android.inputmethod.latin.utils.ResourceUtils;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 public final class KeyVisualAttributes {
+    @Nullable
     public final Typeface mTypeface;
 
     public final float mLetterRatio;
@@ -81,7 +85,8 @@ public final class KeyVisualAttributes {
         }
     }
 
-    public static KeyVisualAttributes newInstance(final TypedArray keyAttr) {
+    @Nullable
+    public static KeyVisualAttributes newInstance(@Nonnull final TypedArray keyAttr) {
         final int indexCount = keyAttr.getIndexCount();
         for (int i = 0; i < indexCount; i++) {
             final int attrId = keyAttr.getIndex(i);
@@ -93,7 +98,7 @@ public final class KeyVisualAttributes {
         return null;
     }
 
-    private KeyVisualAttributes(final TypedArray keyAttr) {
+    private KeyVisualAttributes(@Nonnull final TypedArray keyAttr) {
         if (keyAttr.hasValue(R.styleable.Keyboard_Key_keyTypeface)) {
             mTypeface = Typeface.defaultFromStyle(
                     keyAttr.getInt(R.styleable.Keyboard_Key_keyTypeface, Typeface.NORMAL));
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeyboardBuilder.java b/java/src/com/android/inputmethod/keyboard/internal/KeyboardBuilder.java
index 51f89c122..5743ef967 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeyboardBuilder.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeyboardBuilder.java
@@ -139,6 +139,7 @@ public class KeyboardBuilder<KP extends KeyboardParams> {
     private static final int DEFAULT_KEYBOARD_COLUMNS = 10;
     private static final int DEFAULT_KEYBOARD_ROWS = 4;
 
+    @Nonnull
     protected final KP mParams;
     protected final Context mContext;
     protected final Resources mResources;
@@ -149,7 +150,7 @@ public class KeyboardBuilder<KP extends KeyboardParams> {
     private boolean mTopEdge;
     private Key mRightEdgeKey = null;
 
-    public KeyboardBuilder(final Context context, final KP params) {
+    public KeyboardBuilder(final Context context, @Nonnull final KP params) {
         mContext = context;
         final Resources res = context.getResources();
         mResources = res;
@@ -194,6 +195,7 @@ public class KeyboardBuilder<KP extends KeyboardParams> {
         mParams.mProximityCharsCorrectionEnabled = enabled;
     }
 
+    @Nonnull
     public Keyboard build() {
         return new Keyboard(mParams);
     }
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeyboardIconsSet.java b/java/src/com/android/inputmethod/keyboard/internal/KeyboardIconsSet.java
index e1f302c1e..15a5bd456 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeyboardIconsSet.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeyboardIconsSet.java
@@ -26,6 +26,9 @@ import com.android.inputmethod.latin.R;
 
 import java.util.HashMap;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 public final class KeyboardIconsSet {
     private static final String TAG = KeyboardIconsSet.class.getSimpleName();
 
@@ -127,6 +130,7 @@ public final class KeyboardIconsSet {
         return iconId >= 0 && iconId < ICON_NAMES.length;
     }
 
+    @Nonnull
     public static String getIconName(final int iconId) {
         return isValidIconId(iconId) ? ICON_NAMES[iconId] : "unknown<" + iconId + ">";
     }
@@ -147,6 +151,7 @@ public final class KeyboardIconsSet {
         throw new RuntimeException("unknown icon name: " + name);
     }
 
+    @Nullable
     public Drawable getIconDrawable(final int iconId) {
         if (isValidIconId(iconId)) {
             return mIcons[iconId];
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeyboardParams.java b/java/src/com/android/inputmethod/keyboard/internal/KeyboardParams.java
index fb5e97757..432687635 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeyboardParams.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeyboardParams.java
@@ -49,6 +49,7 @@ public class KeyboardParams {
     public int mLeftPadding;
     public int mRightPadding;
 
+    @Nullable
     public KeyVisualAttributes mKeyVisualAttributes;
 
     public int mDefaultRowHeight;
@@ -63,14 +64,22 @@ public class KeyboardParams {
     public int GRID_HEIGHT;
 
     // Keys are sorted from top-left to bottom-right order.
+    @Nonnull
     public final SortedSet<Key> mSortedKeys = new TreeSet<>(ROW_COLUMN_COMPARATOR);
+    @Nonnull
     public final ArrayList<Key> mShiftKeys = new ArrayList<>();
+    @Nonnull
     public final ArrayList<Key> mAltCodeKeysWhileTyping = new ArrayList<>();
+    @Nonnull
     public final KeyboardIconsSet mIconsSet = new KeyboardIconsSet();
+    @Nonnull
     public final KeyboardTextsSet mTextsSet = new KeyboardTextsSet();
+    @Nonnull
     public final KeyStylesSet mKeyStyles = new KeyStylesSet(mTextsSet);
 
-    @Nullable public KeysCache mKeysCache;
+    // TODO: Make this @Nonnull
+    @Nullable
+    public KeysCache mKeysCache;
     public boolean mAllowRedundantMoreKeys;
 
     public int mMostCommonKeyHeight = 0;
@@ -78,6 +87,7 @@ public class KeyboardParams {
 
     public boolean mProximityCharsCorrectionEnabled;
 
+    @Nonnull
     public final TouchPositionCorrection mTouchPositionCorrection =
             new TouchPositionCorrection();
 
@@ -100,7 +110,9 @@ public class KeyboardParams {
     }
 
     public void onAddKey(@Nonnull final Key newKey) {
-        final Key key = (mKeysCache != null) ? mKeysCache.get(newKey) : newKey;
+        // To avoid possible null pointer access.
+        final KeysCache keysCache = mKeysCache;
+        final Key key = (keysCache != null) ? keysCache.get(newKey) : newKey;
         final boolean isSpacer = key.isSpacer();
         if (isSpacer && key.getWidth() == 0) {
             // Ignore zero width {@link Spacer}.
@@ -128,12 +140,14 @@ public class KeyboardParams {
         for (final Key key : mSortedKeys) {
             lettersOnBaseLayout.addLetter(key);
         }
+        // To avoid possible null pointer access.
+        final KeysCache keysCache = mKeysCache;
         final ArrayList<Key> allKeys = new ArrayList<>(mSortedKeys);
         mSortedKeys.clear();
         for (final Key key : allKeys) {
             final Key filteredKey = Key.removeRedundantMoreKeys(key, lettersOnBaseLayout);
-            if (mKeysCache != null) {
-                mKeysCache.replace(key, filteredKey);
+            if (keysCache != null) {
+                keysCache.replace(key, filteredKey);
             }
             mSortedKeys.add(filteredKey);
         }
diff --git a/java/src/com/android/inputmethod/keyboard/internal/KeysCache.java b/java/src/com/android/inputmethod/keyboard/internal/KeysCache.java
index e8678637b..6ad450c29 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/KeysCache.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/KeysCache.java
@@ -20,6 +20,7 @@ import com.android.inputmethod.keyboard.Key;
 
 import java.util.HashMap;
 
+// TODO: Rename more appropriate name.
 public final class KeysCache {
     private final HashMap<Key, Key> mMap = new HashMap<>();
 
@@ -27,6 +28,7 @@ public final class KeysCache {
         mMap.clear();
     }
 
+    // TODO: Rename more descriptive name.
     public Key get(final Key key) {
         final Key existingKey = mMap.get(key);
         if (existingKey != null) {
@@ -37,6 +39,7 @@ public final class KeysCache {
         return key;
     }
 
+    // TODO: Rename more descriptive name.
     public Key replace(final Key oldKey, final Key newKey) {
         if (oldKey.equals(newKey)) {
             return oldKey;
diff --git a/java/src/com/android/inputmethod/keyboard/internal/MoreKeySpec.java b/java/src/com/android/inputmethod/keyboard/internal/MoreKeySpec.java
index 87c96cc0d..0bd42fc13 100644
--- a/java/src/com/android/inputmethod/keyboard/internal/MoreKeySpec.java
+++ b/java/src/com/android/inputmethod/keyboard/internal/MoreKeySpec.java
@@ -24,14 +24,13 @@ import com.android.inputmethod.keyboard.Key;
 import com.android.inputmethod.latin.common.CollectionUtils;
 import com.android.inputmethod.latin.common.Constants;
 import com.android.inputmethod.latin.common.StringUtils;
-import com.android.inputmethod.latin.define.DebugFlags;
 
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.HashSet;
 import java.util.Locale;
 
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 
 /**
  * The more key specification object. The more keys are an array of {@link MoreKeySpec}.
@@ -47,12 +46,15 @@ import javax.annotation.Nonnull;
 // TODO: Should extend the key specification object.
 public final class MoreKeySpec {
     public final int mCode;
+    @Nullable
     public final String mLabel;
+    @Nullable
     public final String mOutputText;
     public final int mIconId;
 
-    public MoreKeySpec(final String moreKeySpec, boolean needsToUpperCase, final Locale locale) {
-        if (TextUtils.isEmpty(moreKeySpec)) {
+    public MoreKeySpec(@Nonnull final String moreKeySpec, boolean needsToUpperCase,
+            @Nonnull final Locale locale) {
+        if (moreKeySpec.isEmpty()) {
             throw new KeySpecParser.KeySpecParserError("Empty more key spec");
         }
         final String label = KeySpecParser.getLabel(moreKeySpec);
@@ -76,7 +78,7 @@ public final class MoreKeySpec {
 
     @Nonnull
     public Key buildKey(final int x, final int y, final int labelFlags,
-            final KeyboardParams params) {
+            @Nonnull final KeyboardParams params) {
         return new Key(mLabel, mIconId, mCode, mOutputText, null /* hintLabel */, labelFlags,
                 Key.BACKGROUND_TYPE_NORMAL, x, y, params.mDefaultKeyWidth, params.mDefaultRowHeight,
                 params.mHorizontalGap, params.mVerticalGap);
@@ -87,14 +89,18 @@ public final class MoreKeySpec {
         int hashCode = 1;
         hashCode = 31 + mCode;
         hashCode = hashCode * 31 + mIconId;
-        hashCode = hashCode * 31 + (mLabel == null ? 0 : mLabel.hashCode());
-        hashCode = hashCode * 31 + (mOutputText == null ? 0 : mOutputText.hashCode());
+        final String label = mLabel;
+        hashCode = hashCode * 31 + (label == null ? 0 : label.hashCode());
+        final String outputText = mOutputText;
+        hashCode = hashCode * 31 + (outputText == null ? 0 : outputText.hashCode());
         return hashCode;
     }
 
     @Override
     public boolean equals(final Object o) {
-        if (this == o) return true;
+        if (this == o) {
+            return true;
+        }
         if (o instanceof MoreKeySpec) {
             final MoreKeySpec other = (MoreKeySpec)o;
             return mCode == other.mCode
@@ -121,7 +127,7 @@ public final class MoreKeySpec {
         private final SparseIntArray mCodes = new SparseIntArray();
         private final HashSet<String> mTexts = new HashSet<>();
 
-        public void addLetter(final Key key) {
+        public void addLetter(@Nonnull final Key key) {
             final int code = key.getCode();
             if (CharacterCompat.isAlphabetic(code)) {
                 mCodes.put(code, 0);
@@ -130,7 +136,7 @@ public final class MoreKeySpec {
             }
         }
 
-        public boolean contains(final MoreKeySpec moreKey) {
+        public boolean contains(@Nonnull final MoreKeySpec moreKey) {
             final int code = moreKey.mCode;
             if (CharacterCompat.isAlphabetic(code) && mCodes.indexOfKey(code) >= 0) {
                 return true;
@@ -141,8 +147,9 @@ public final class MoreKeySpec {
         }
     }
 
-    public static MoreKeySpec[] removeRedundantMoreKeys(final MoreKeySpec[] moreKeys,
-            final LettersOnBaseLayout lettersOnBaseLayout) {
+    @Nullable
+    public static MoreKeySpec[] removeRedundantMoreKeys(@Nullable final MoreKeySpec[] moreKeys,
+            @Nonnull final LettersOnBaseLayout lettersOnBaseLayout) {
         if (moreKeys == null) {
             return null;
         }
@@ -162,7 +169,6 @@ public final class MoreKeySpec {
         return filteredMoreKeys.toArray(new MoreKeySpec[size]);
     }
 
-    private static final boolean DEBUG = DebugFlags.DEBUG_ENABLED;
     // Constants for parsing.
     private static final char COMMA = Constants.CODE_COMMA;
     private static final char BACKSLASH = Constants.CODE_BACKSLASH;
@@ -180,7 +186,8 @@ public final class MoreKeySpec {
      * @return an array of key specification text. Null if the specified <code>text</code> is empty
      * or has no key specifications.
      */
-    public static String[] splitKeySpecs(final String text) {
+    @Nullable
+    public static String[] splitKeySpecs(@Nullable final String text) {
         if (TextUtils.isEmpty(text)) {
             return null;
         }
@@ -222,9 +229,11 @@ public final class MoreKeySpec {
         return list.toArray(new String[list.size()]);
     }
 
+    @Nonnull
     private static final String[] EMPTY_STRING_ARRAY = new String[0];
 
-    private static String[] filterOutEmptyString(final String[] array) {
+    @Nonnull
+    private static String[] filterOutEmptyString(@Nullable final String[] array) {
         if (array == null) {
             return EMPTY_STRING_ARRAY;
         }
@@ -245,8 +254,8 @@ public final class MoreKeySpec {
         return out.toArray(new String[out.size()]);
     }
 
-    public static String[] insertAdditionalMoreKeys(final String[] moreKeySpecs,
-            final String[] additionalMoreKeySpecs) {
+    public static String[] insertAdditionalMoreKeys(@Nullable final String[] moreKeySpecs,
+            @Nullable final String[] additionalMoreKeySpecs) {
         final String[] moreKeys = filterOutEmptyString(moreKeySpecs);
         final String[] additionalMoreKeys = filterOutEmptyString(additionalMoreKeySpecs);
         final int moreKeysCount = moreKeys.length;
@@ -280,11 +289,6 @@ public final class MoreKeySpec {
         if (additionalCount > 0 && additionalIndex == 0) {
             // No '%' marker is found in more keys.
             // Insert all additional more keys to the head of more keys.
-            if (DEBUG && out != null) {
-                throw new RuntimeException("Internal logic error:"
-                        + " moreKeys=" + Arrays.toString(moreKeys)
-                        + " additionalMoreKeys=" + Arrays.toString(additionalMoreKeys));
-            }
             out = CollectionUtils.arrayAsList(additionalMoreKeys, additionalIndex, additionalCount);
             for (int i = 0; i < moreKeysCount; i++) {
                 out.add(moreKeys[i]);
@@ -292,11 +296,6 @@ public final class MoreKeySpec {
         } else if (additionalIndex < additionalCount) {
             // The number of '%' markers are less than additional more keys.
             // Append remained additional more keys to the tail of more keys.
-            if (DEBUG && out != null) {
-                throw new RuntimeException("Internal logic error:"
-                        + " moreKeys=" + Arrays.toString(moreKeys)
-                        + " additionalMoreKeys=" + Arrays.toString(additionalMoreKeys));
-            }
             out = CollectionUtils.arrayAsList(moreKeys, 0, moreKeysCount);
             for (int i = additionalIndex; i < additionalCount; i++) {
                 out.add(additionalMoreKeys[additionalIndex]);
@@ -311,7 +310,7 @@ public final class MoreKeySpec {
         }
     }
 
-    public static int getIntValue(final String[] moreKeys, final String key,
+    public static int getIntValue(@Nullable final String[] moreKeys, final String key,
             final int defaultValue) {
         if (moreKeys == null) {
             return defaultValue;
@@ -338,7 +337,7 @@ public final class MoreKeySpec {
         return value;
     }
 
-    public static boolean getBooleanValue(final String[] moreKeys, final String key) {
+    public static boolean getBooleanValue(@Nullable final String[] moreKeys, final String key) {
         if (moreKeys == null) {
             return false;
         }