[LANG-1458] Add EMPTY_ARRAY constants to classes in

org.apache.commons.lang3.tuple

src/main/java/org/apache/commons/lang3/tuple/ImmutablePair.java

@@ -53,7 +53,7 @@ public final class ImmutablePair<L, R> extends Pair<L, R> {
      * @since 3.10.
      */
     @SuppressWarnings("unchecked")
-    public static final <L, R> ImmutablePair<L, R>[] emptyArray() {
+    public static <L, R> ImmutablePair<L, R>[] emptyArray() {
         return (ImmutablePair<L, R>[]) EMPTY_ARRAY;
     }
 

src/main/java/org/apache/commons/lang3/tuple/ImmutableTriple.java

@@ -55,7 +55,7 @@ public final class ImmutableTriple<L, M, R> extends Triple<L, M, R> {
      * @since 3.10.
      */
     @SuppressWarnings("unchecked")
-    public static final <L, M, R> ImmutableTriple<L, M, R>[] emptyArray() {
+    public static <L, M, R> ImmutableTriple<L, M, R>[] emptyArray() {
         return (ImmutableTriple<L, M, R>[]) EMPTY_ARRAY;
     }
 

src/main/java/org/apache/commons/lang3/tuple/MutablePair.java

@@ -48,7 +48,7 @@ public class MutablePair<L, R> extends Pair<L, R> {
      * @since 3.10.
      */
     @SuppressWarnings("unchecked")
-    public static final <L, R> MutablePair<L, R>[] emptyArray() {
+    public static <L, R> MutablePair<L, R>[] emptyArray() {
         return (MutablePair<L, R>[]) EMPTY_ARRAY;
     }
 

src/main/java/org/apache/commons/lang3/tuple/MutableTriple.java

@@ -50,7 +50,7 @@ public class MutableTriple<L, M, R> extends Triple<L, M, R> {
      * @since 3.10.
      */
     @SuppressWarnings("unchecked")
-    public static final <L, M, R> MutableTriple<L, M, R>[] emptyArray() {
+    public static <L, M, R> MutableTriple<L, M, R>[] emptyArray() {
         return (MutableTriple<L, M, R>[]) EMPTY_ARRAY;
     }
 

src/main/java/org/apache/commons/lang3/tuple/Pair.java

@@ -43,6 +43,51 @@ public abstract class Pair<L, R> implements Map.Entry<L, R>, Comparable<Pair<L,
     /** Serialization version */
     private static final long serialVersionUID = 4954918890077093841L;
 
+    private static final class PairAdapter<L, R> extends Pair<L, R> {
+
+        private static final long serialVersionUID = 1L;
+
+        @Override
+        public L getLeft() {
+            return null;
+        }
+
+        @Override
+        public R getRight() {
+            return null;
+        }
+
+        @Override
+        public R setValue(R value) {
+            return null;
+        }
+
+    }
+
+    /**
+     * An empty array.
+     * <p>
+     * Consider using {@link #emptyArray()} to avoid generics warnings.
+     * </p>
+     *
+     * @since 3.10.
+     */
+    public static final Pair<?, ?>[] EMPTY_ARRAY = new PairAdapter[0];
+
+    /**
+     * Returns the empty array singleton that can be assigned without compiler warning.
+     *
+     * @param <L> the left element type
+     * @param <R> the right element type
+     * @return the empty array singleton that can be assigned without compiler warning.
+     *
+     * @since 3.10.
+     */
+    @SuppressWarnings("unchecked")
+    public static <L, R> Pair<L, R>[] emptyArray() {
+        return (Pair<L, R>[]) EMPTY_ARRAY;
+    }
+
     /**
      * <p>Obtains an immutable pair of two objects inferring the generic types.</p>
      *

src/main/java/org/apache/commons/lang3/tuple/Triple.java

@@ -42,6 +42,52 @@ public abstract class Triple<L, M, R> implements Comparable<Triple<L, M, R>>, Se
     /** Serialization version */
     private static final long serialVersionUID = 1L;
 
+    private static final class TripleAdapter<L, M, R> extends Triple<L, M, R> {
+
+        private static final long serialVersionUID = 1L;
+
+        @Override
+        public L getLeft() {
+            return null;
+        }
+
+        @Override
+        public M getMiddle() {
+            return null;
+        }
+
+        @Override
+        public R getRight() {
+            return null;
+        }
+
+    }
+
+    /**
+     * An empty array.
+     * <p>
+     * Consider using {@link #emptyArray()} to avoid generics warnings.
+     * </p>
+     *
+     * @since 3.10.
+     */
+    public static final Triple<?, ?, ?>[] EMPTY_ARRAY = new TripleAdapter[0];
+
+    /**
+     * Returns the empty array singleton that can be assigned without compiler warning.
+     *
+     * @param <L> the left element type
+     * @param <M> the middle element type
+     * @param <R> the right element type
+     * @return the empty array singleton that can be assigned without compiler warning.
+     *
+     * @since 3.10.
+     */
+    @SuppressWarnings("unchecked")
+    public static <L, M, R> Triple<L, M, R>[] emptyArray() {
+        return (Triple<L, M, R>[]) EMPTY_ARRAY;
+    }
+
     /**
      * <p>Obtains an immutable triple of three objects inferring the generic types.</p>
      *

src/test/java/org/apache/commons/lang3/tuple/PairTest.java

@@ -33,6 +33,19 @@ import org.junit.jupiter.api.Test;
  */
 public class PairTest {
 
+    @Test
+    public void testEmptyArrayLength() {
+        @SuppressWarnings("unchecked")
+        final Pair<Integer, String>[] empty = (Pair<Integer, String>[]) Pair.EMPTY_ARRAY;
+        assertEquals(0, empty.length);
+    }
+
+    @Test
+    public void testEmptyArrayGenerics() {
+        final Pair<Integer, String>[] empty = Pair.emptyArray();
+        assertEquals(0, empty.length);
+    }
+
     @Test
     public void testComparable1() {
         final Pair<String, String> pair1 = Pair.of("A", "D");

src/test/java/org/apache/commons/lang3/tuple/TripleTest.java

@@ -30,6 +30,19 @@ import org.junit.jupiter.api.Test;
  */
 public class TripleTest {
 
+    @Test
+    public void testEmptyArrayLength() {
+        @SuppressWarnings("unchecked")
+        final Triple<Integer, String, Boolean>[] empty = (Triple<Integer, String, Boolean>[]) Triple.EMPTY_ARRAY;
+        assertEquals(0, empty.length);
+    }
+
+    @Test
+    public void testEmptyArrayGenerics() {
+        final Triple<Integer, String, Boolean>[] empty = Triple.emptyArray();
+        assertEquals(0, empty.length);
+    }
+
     @Test
     public void testComparable1() {
         final Triple<String, String, String> triple1 = Triple.of("A", "D", "A");