refactor: Add a CollectionUtils with a safe toString

src/main/java/org/opentripplanner/framework/collection/CollectionUtils.java

@@ -0,0 +1,35 @@
+package org.opentripplanner.framework.collection;
+
+import java.util.Collection;
+import java.util.Set;
+import java.util.SortedSet;
+import java.util.stream.Collectors;
+import javax.annotation.Nullable;
+
+public class CollectionUtils {
+
+  /**
+   * A null-safe version of toString() for a collections.
+   * <p>
+   * If the collection is {@code null} the given {@code nullText} is returned.
+   * <p>
+   * All elements are also converted to a sting using the {@code toString()} method or if
+   * {@code null} to the given {@code nullText}.
+   * <p>
+   * If the collection is a set, but not a SortedSet then the elements are sorted. This is done
+   * to return the elements in a deterministic manner, witch is important if this is used in
+   * for example a unit-test.
+   * <p>
+   * The final result string examples: {@code "[]", "[a]", "[a, b, c]"}
+   */
+  public static <T> String toString(@Nullable Collection<T> c, String nullText) {
+    if (c == null) {
+      return nullText;
+    }
+    var stream = c.stream().map(it -> it == null ? nullText : it.toString());
+    if (c instanceof Set && !(c instanceof SortedSet<T>)) {
+      stream = stream.sorted();
+    }
+    return stream.collect(Collectors.joining(", ", "[", "]"));
+  }
+}

src/test/java/org/opentripplanner/framework/collection/CollectionUtilsTest.java

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+package org.opentripplanner.framework.collection;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import com.google.type.Month;
+import java.time.Duration;
+import java.util.ArrayList;
+import java.util.EnumSet;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import org.junit.jupiter.api.Test;
+
+class CollectionUtilsTest {
+
+  public static final String NULL_STRING = "<null>";
+
+  @Test
+  void testToString() {
+    assertEquals("<null>", CollectionUtils.toString(null, NULL_STRING));
+    assertEquals("[]", CollectionUtils.toString(Set.of(), NULL_STRING));
+    assertEquals("[]", CollectionUtils.toString(List.of(), NULL_STRING));
+    assertEquals("[JUNE]", CollectionUtils.toString(EnumSet.of(Month.JUNE), NULL_STRING));
+    assertEquals(
+      "[APRIL, JANUARY, JUNE]",
+      CollectionUtils.toString(EnumSet.of(Month.JANUARY, Month.JUNE, Month.APRIL), "#")
+    );
+
+    // Given a list of objects
+    List<Object> list = new ArrayList<>();
+    list.add(Duration.ofHours(3));
+    list.add(null);
+    list.add(Month.JUNE);
+    list.add(Month.APRIL);
+
+    // Then: keep list order, do not sort
+    assertEquals("[PT3H, <null>, JUNE, APRIL]", CollectionUtils.toString(list, NULL_STRING));
+
+    // And: Set should be sorted alphabetically
+    Set<Object> set = new HashSet<>(list);
+    assertEquals("[<null>, APRIL, JUNE, PT3H]", CollectionUtils.toString(set, NULL_STRING));
+  }
+}