Add method listFilter()

CHANGELOG.md

@@ -5,7 +5,7 @@
 - Require Java 11
 - `StringsPlume`: add `rpad` that pads with an arbitrary character
 - `CollectionsPlume`:
-   * add method `duplicates()`
+   * add methods `duplicates()` and `listFilter()`
    * add an overload for `mapCapacity()`
 
 ## 1.8.1 (2023-06-02)

src/main/java/org/plumelib/util/CollectionsPlume.java

@@ -25,6 +25,7 @@
 import java.util.TreeSet;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Function;
+import java.util.function.Predicate;
 import org.checkerframework.checker.index.qual.NonNegative;
 import org.checkerframework.checker.index.qual.Positive;
 import org.checkerframework.checker.lock.qual.GuardSatisfied;
@@ -552,6 +553,31 @@ List<TO> transform(
     return result;
   }
 
+  /**
+   * Returns a new list containing only the elements for which the filter returns true. To modify
+   * the collection in place, use {@code Collection#removeIf}.
+   *
+   * <p>Using streams gives an equivalent list but is less efficient and more verbose:
+   *
+   * <pre>{@code
+   * coll.stream().filter(filter).collect(Collectors.toList());
+   * }</pre>
+   *
+   * @param <T> the type of elements
+   * @param coll a collection
+   * @param filter a predicate
+   * @return a new list with the elements for which the filter returns true
+   */
+  public static <T> List<T> listFilter(Collection<T> coll, Predicate<? super T> filter) {
+    List<T> result = new ArrayList<>();
+    for (T elt : coll) {
+      if (filter.test(elt)) {
+        result.add(elt);
+      }
+    }
+    return result;
+  }
+
   ///////////////////////////////////////////////////////////////////////////
   /// SortedSet
   ///

src/test/java/org/plumelib/util/CollectionsPlumeTest.java

@@ -420,6 +420,15 @@ public void testTransform() {
     assertEquals(out, CollectionsPlume.transform(in, Object::toString));
   }
 
+  @Test
+  public void testListFilter() {
+    List<Integer> in = Arrays.asList(new Integer[] {1, 2, 3, 4, 5});
+    List<Integer> odd = Arrays.asList(new Integer[] {1, 3, 5});
+    List<Integer> even = Arrays.asList(new Integer[] {2, 4});
+    assertEquals(odd, CollectionsPlume.listFilter(in, i -> i % 2 == 1));
+    assertEquals(even, CollectionsPlume.listFilter(in, i -> i % 2 == 0));
+  }
+
   @Test
   public void testSortedSetEquals() {
     TreeSet<Integer> s2 = new TreeSet<>(Arrays.asList(1, 2));