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diff --git a/src/test/java/org/apache/commons/lang3/tuple/ImmutablePairTest.java b/src/test/java/org/apache/commons/lang3/tuple/ImmutablePairTest.java
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+++ b/src/test/java/org/apache/commons/lang3/tuple/ImmutablePairTest.java
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.lang3.tuple;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map.Entry;
+import java.util.TreeMap;
+
+import org.apache.commons.lang3.AbstractLangTest;
+import org.apache.commons.lang3.SerializationUtils;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Test the Pair class.
+ */
+public class ImmutablePairTest extends AbstractLangTest {
+
+    @Test
+    public void testBasic() {
+        ImmutablePair<Integer, String> oldPair = new ImmutablePair<>(0, "foo");
+        ImmutablePair<Integer, String> nowPair;
+        for (int i=0; i<4; i++) {
+            nowPair = ImmutablePair.of(oldPair);
+            assertEquals(0, nowPair.left.intValue());
+            assertEquals(0, nowPair.getLeft().intValue());
+            assertEquals("foo", nowPair.right);
+            assertEquals("foo", nowPair.getRight());
+            assertEquals(oldPair, nowPair);
+            oldPair = nowPair;
+        }
+
+        ImmutablePair<Object, String> oldPair2 = new ImmutablePair<>(null, "bar");
+        ImmutablePair<Object, String> nowPair2;
+        for (int i=0; i<4; i++) {
+            nowPair2 = ImmutablePair.of(oldPair2);
+            assertNull(nowPair2.left);
+            assertNull(nowPair2.getLeft());
+            assertEquals("bar", nowPair2.right);
+            assertEquals("bar", nowPair2.getRight());
+            oldPair2 = nowPair2;
+        }
+    }
+
+    @Test
+    public void testComparableLeftOnly() {
+        final Pair<String, String> pair1 = ImmutablePair.left("A");
+        final Pair<String, String> pair2 = ImmutablePair.left("B");
+        assertEquals("A", pair1.getLeft());
+        assertEquals("B", pair2.getLeft());
+        assertEquals(0, pair1.compareTo(pair1));
+        assertTrue(pair1.compareTo(pair2) < 0);
+        assertEquals(0, pair2.compareTo(pair2));
+        assertTrue(pair2.compareTo(pair1) > 0);
+    }
+
+    @Test
+    public void testComparableRightOnly() {
+        final Pair<String, String> pair1 = ImmutablePair.right("A");
+        final Pair<String, String> pair2 = ImmutablePair.right("B");
+        assertEquals("A", pair1.getRight());
+        assertEquals("B", pair2.getRight());
+        assertEquals(0, pair1.compareTo(pair1));
+        assertTrue(pair1.compareTo(pair2) < 0);
+        assertEquals(0, pair2.compareTo(pair2));
+        assertTrue(pair2.compareTo(pair1) > 0);
+    }
+
+    @Test
+    public void testEmptyArrayGenerics() {
+        final ImmutablePair<Integer, String>[] empty = ImmutablePair.emptyArray();
+        assertEquals(0, empty.length);
+    }
+
+    @Test
+    public void testEmptyArrayLength() {
+        @SuppressWarnings("unchecked")
+        final ImmutablePair<Integer, String>[] empty = (ImmutablePair<Integer, String>[]) ImmutablePair.EMPTY_ARRAY;
+        assertEquals(0, empty.length);
+    }
+
+    @Test
+    public void testEquals() {
+        assertEquals(ImmutablePair.of(null, "foo"), ImmutablePair.of(null, "foo"));
+        assertNotEquals(ImmutablePair.of("foo", 0), ImmutablePair.of("foo", null));
+        assertNotEquals(ImmutablePair.of("foo", "bar"), ImmutablePair.of("xyz", "bar"));
+
+        final ImmutablePair<String, String> p = ImmutablePair.of("foo", "bar");
+        assertEquals(p, p);
+        assertNotEquals(p, new Object());
+    }
+
+    @Test
+    public void testHashCode() {
+        assertEquals(ImmutablePair.of(null, "foo").hashCode(), ImmutablePair.of(null, "foo").hashCode());
+    }
+
+    @Test
+    public void testNullPairEquals() {
+        assertEquals(ImmutablePair.nullPair(), ImmutablePair.nullPair());
+    }
+
+    @Test
+    public void testNullPairKey() {
+        assertNull(ImmutablePair.nullPair().getKey());
+    }
+
+    @Test
+    public void testNullPairLeft() {
+        assertNull(ImmutablePair.nullPair().getLeft());
+    }
+
+    @Test
+    public void testNullPairRight() {
+        assertNull(ImmutablePair.nullPair().getRight());
+    }
+
+    @Test
+    public void testNullPairSame() {
+        assertSame(ImmutablePair.nullPair(), ImmutablePair.nullPair());
+    }
+
+    @Test
+    public void testNullPairTyped() {
+        // No compiler warnings
+        // How do we assert that?
+        final ImmutablePair<String, String> pair = ImmutablePair.nullPair();
+        assertNotNull(pair);
+    }
+
+    @Test
+    public void testNullPairValue() {
+        assertNull(ImmutablePair.nullPair().getValue());
+    }
+
+    @Test
+    public void testOfNonNull() {
+        assertThrows(NullPointerException.class, () -> ImmutablePair.ofNonNull(null, null));
+        assertThrows(NullPointerException.class, () -> ImmutablePair.ofNonNull(null, "x"));
+        assertThrows(NullPointerException.class, () -> ImmutablePair.ofNonNull("x", null));
+        final ImmutablePair<String, String> pair = ImmutablePair.ofNonNull("x", "y");
+        assertEquals("x", pair.left);
+        assertEquals("y", pair.right);
+    }
+
+    @Test
+    public void testPairOfMapEntry() {
+        final HashMap<Integer, String> map = new HashMap<>();
+        map.put(0, "foo");
+        final Entry<Integer, String> entry = map.entrySet().iterator().next();
+        final Pair<Integer, String> pair = ImmutablePair.of(entry);
+        assertEquals(entry.getKey(), pair.getLeft());
+        assertEquals(entry.getValue(), pair.getRight());
+    }
+
+    @Test
+    public void testPairOfObjects() {
+        final ImmutablePair<Integer, String> pair = ImmutablePair.of(0, "foo");
+        assertEquals(0, pair.left.intValue());
+        assertEquals(0, pair.getLeft().intValue());
+        assertEquals("foo", pair.right);
+        assertEquals("foo", pair.getRight());
+        final ImmutablePair<Object, String> pair2 = ImmutablePair.of(null, "bar");
+        assertNull(pair2.left);
+        assertNull(pair2.getLeft());
+        assertEquals("bar", pair2.right);
+        assertEquals("bar", pair2.getRight());
+        final ImmutablePair<?, ?> pair3 = ImmutablePair.of(null, null);
+        assertNull(pair3.left);
+        assertNull(pair3.right);
+    }
+
+    @Test
+    public void testSerialization() throws Exception {
+        final ImmutablePair<Integer, String> origPair = ImmutablePair.of(0, "foo");
+        final ImmutablePair<Integer, String> deserializedPair = SerializationUtils.roundtrip(origPair);
+        assertEquals(origPair, deserializedPair);
+        assertEquals(origPair.hashCode(), deserializedPair.hashCode());
+    }
+
+    @Test
+    public void testToString() {
+        assertEquals("(null,null)", ImmutablePair.of(null, null).toString());
+        assertEquals("(null,two)", ImmutablePair.of(null, "two").toString());
+        assertEquals("(one,null)", ImmutablePair.of("one", null).toString());
+        assertEquals("(one,two)", ImmutablePair.of("one", "two").toString());
+    }
+
+    @Test
+    public void testToStringLeft() {
+        final Pair<String, String> pair = ImmutablePair.left("Key");
+        assertEquals("(Key,null)", pair.toString());
+    }
+
+    @Test
+    public void testToStringRight() {
+        final Pair<String, String> pair = ImmutablePair.right("Value");
+        assertEquals("(null,Value)", pair.toString());
+    }
+
+    @Test
+    public void testUseAsKeyOfHashMap() {
+        final HashMap<ImmutablePair<Object, Object>, String> map = new HashMap<>();
+        final Object o1 = new Object();
+        final Object o2 = new Object();
+        final ImmutablePair<Object, Object> key1 = ImmutablePair.of(o1, o2);
+        final String value1 = "a1";
+        map.put(key1, value1);
+        assertEquals(value1, map.get(key1));
+        assertEquals(value1, map.get(ImmutablePair.of(o1, o2)));
+    }
+
+    @Test
+    public void testUseAsKeyOfTreeMap() {
+        final TreeMap<ImmutablePair<Integer, Integer>, String> map = new TreeMap<>();
+        map.put(ImmutablePair.of(1, 2), "12");
+        map.put(ImmutablePair.of(1, 1), "11");
+        map.put(ImmutablePair.of(0, 1), "01");
+        final ArrayList<ImmutablePair<Integer, Integer>> expected = new ArrayList<>();
+        expected.add(ImmutablePair.of(0, 1));
+        expected.add(ImmutablePair.of(1, 1));
+        expected.add(ImmutablePair.of(1, 2));
+        final Iterator<Entry<ImmutablePair<Integer, Integer>, String>> it = map.entrySet().iterator();
+        for (final ImmutablePair<Integer, Integer> item : expected) {
+            final Entry<ImmutablePair<Integer, Integer>, String> entry = it.next();
+            assertEquals(item, entry.getKey());
+            assertEquals(item.getLeft() + "" + item.getRight(), entry.getValue());
+        }
+    }
+
+    @Test
+    public void testUnsupportedOperation() {
+        final ImmutablePair<Integer, String> pair = new ImmutablePair<>(0, "foo");
+        assertThrows(UnsupportedOperationException.class, () -> pair.setValue("any"));
+
+    }
+}