Added ImmutablePair factory methods left() and right().

src/changes/changes.xml

@@ -79,6 +79,7 @@ The <action> type attribute can be add,update,fix,remove.
     <action issue="LANG-1556" type="add" dev="ggregory" due-to="XenoAmess">Use Java 8 lambdas and Map operations.</action>
     <action issue="LANG-1565" type="add" dev="ggregory" due-to="XenoAmess">Change removeLastFieldSeparator to use endsWith #550.</action>
     <action issue="LANG-1557" type="add" dev="ggregory" due-to="XenoAmess, Gary Gregory">Change a Pattern to a static final field, for not letting it compile each time the function invoked. #542.</action>
+    <action                   type="add" dev="ggregory">Added ImmutablePair factory methods left() and right().</action>
   </release>
 
   <release version="3.10" date="2020-03-22" description="New features and bug fixes. Requires Java 8, supports Java 9, 10, 11.">

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

@@ -69,6 +69,21 @@ public final class ImmutablePair<L, R> extends Pair<L, R> {
         return (ImmutablePair<L, R>[]) EMPTY_ARRAY;
     }
 
+    /**
+     * <p>Creates an immutable pair of two objects inferring the generic types.</p>
+     *
+     * <p>This factory allows the pair to be created using inference to
+     * obtain the generic types.</p>
+     *
+     * @param <L> the left element type
+     * @param left  the left element, may be null
+     * @return a pair formed from the two parameters, not null
+     * @since 3.11
+     */
+    public static <L, R> Pair<L, R> left(final L left) {
+        return ImmutablePair.of(left, null);
+    }
+
     /**
      * Returns an immutable pair of nulls.
      *
@@ -122,6 +137,21 @@ public final class ImmutablePair<L, R> extends Pair<L, R> {
         return new ImmutablePair<>(left, right);
     }
 
+    /**
+     * <p>Creates an immutable pair of two objects inferring the generic types.</p>
+     *
+     * <p>This factory allows the pair to be created using inference to
+     * obtain the generic types.</p>
+     *
+     * @param <R> the right element type
+     * @param right  the right element, may be null
+     * @return a pair formed from the two parameters, not null
+     * @since 3.11
+     */
+    public static <L, R> Pair<L, R> right(final R right) {
+        return ImmutablePair.of(null, right);
+    }
+
     /** Left object */
     public final L left;
 

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

@@ -21,6 +21,7 @@ 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.assertTrue;
 
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
@@ -209,4 +210,40 @@ public class ImmutablePairTest {
             assertEquals(item.getLeft() + "" + item.getRight(), entry.getValue());
         }
     }
+
+    @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 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());
+    }
 }