Add documentation for CollectionUtils#arrayAsList

(This is a cherrypick of http://ag/576739)

Also, modify the IllegalArgumentException to capture the
values passed for start, end, and array.length to make
debugging easier in the unlikely event that it is thrown.
Add corresponding tests for the exceptions.

Tested:
tapas LatinImeGoogle LatinImeGoogleTests userdebug
make -j30 && \
adb install -r \
out/target/product/generic/data/app/LatinImeGoogleTests/LatinImeGoogleTests.apk && \
adb install -r \
out/target/product/generic/system/app/LatinImeGoogle/LatinImeGoogle.apk && \
adb shell am instrument -w -e class \
com.android.inputmethod.latin.utils.CollectionUtilsTests \
com.google.android.inputmethod.latin.tests/android.support.test.runner.AndroidJUnitRunner

Change-Id: I0c579ca4f4f2a9b2575aa6f8fa75e3ee4a21cf8a

1 files changed, 12 insertions, 1 deletions

diff --git a/java/src/com/android/inputmethod/latin/utils/CollectionUtils.java b/java/src/com/android/inputmethod/latin/utils/CollectionUtils.java
index f9839eb91..01f5e1079 100644
--- a/java/src/com/android/inputmethod/latin/utils/CollectionUtils.java
+++ b/java/src/com/android/inputmethod/latin/utils/CollectionUtils.java
@@ -22,16 +22,27 @@ import java.util.Collection;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
+/**
+ * Utility methods for working with collections.
+ */
 public final class CollectionUtils {
     private CollectionUtils() {
         // This utility class is not publicly instantiable.
     }
 
+    /**
+     * Converts a sub-range of the given array to an ArrayList of the appropriate type.
+     * @param array Array to be converted.
+     * @param start First index inclusive to be converted.
+     * @param end Last index exclusive to be converted.
+     * @throws IllegalArgumentException if start or end are out of range or start > end.
+     */
     @Nonnull
     public static <E> ArrayList<E> arrayAsList(@Nonnull final E[] array, final int start,
             final int end) {
         if (start < 0 || start > end || end > array.length) {
-            throw new IllegalArgumentException();
+            throw new IllegalArgumentException("Invalid start: " + start + " end: " + end
+                    + " with array.length: " + array.length);
         }
 
         final ArrayList<E> list = new ArrayList<>(end - start);