(feat) MemoizingSupplier.isInitialized

Author: alexey-pelykh

guava-tests/test/com/google/common/base/SuppliersTest.java

@@ -81,12 +81,12 @@ static class SerializableThrowingSupplier extends ThrowingSupplier implements Se
   static void checkMemoize(CountingSupplier countingSupplier, Supplier<Integer> memoizedSupplier) {
     // the underlying supplier hasn't executed yet
     assertEquals(0, countingSupplier.calls);
-
+    assertFalse(memoizedSupplier.isInitialized());
     assertEquals(10, (int) memoizedSupplier.get());
 
     // now it has
     assertEquals(1, countingSupplier.calls);
-
+    assertTrue(memoizedSupplier.isInitialized());
     assertEquals(10, (int) memoizedSupplier.get());
 
     // it still should only have executed once due to memoization
@@ -165,7 +165,7 @@ public void testMemoizeSerializable() throws Exception {
     Object unused2 = memoizedSupplier.get();
 
     CountingSupplier countingCopy =
-        (CountingSupplier) ((Suppliers.MemoizingSupplier<Integer>) copy).delegate;
+        (CountingSupplier) ((Suppliers.SerializableMemoizingSupplier<Integer>) copy).delegate;
     checkMemoize(countingCopy, copy);
   }
 

guava/src/com/google/common/base/Suppliers.java

@@ -110,18 +110,34 @@ public String toString() {
    * <p>If {@code delegate} is an instance created by an earlier call to {@code memoize}, it is
    * returned directly.
    */
-  public static <T extends @Nullable Object> Supplier<T> memoize(Supplier<T> delegate) {
+  public static <T extends @Nullable Object> MemoizingSupplier<T> memoize(Supplier<T> delegate) {
     if (delegate instanceof NonSerializableMemoizingSupplier
-        || delegate instanceof MemoizingSupplier) {
-      return delegate;
+        || delegate instanceof SerializableMemoizingSupplier) {
+      return (MemoizingSupplier<T>) delegate;
     }
     return delegate instanceof Serializable
-        ? new MemoizingSupplier<T>(delegate)
+        ? new SerializableMemoizingSupplier<T>(delegate)
         : new NonSerializableMemoizingSupplier<T>(delegate);
   }
 
+  /**
+   * A supplier that memoizes the result of the first call to {@link #get()} and returns the same
+   * result on subsequent calls to {@link #get()}.
+   *
+   * @author Alexey Pelykh
+   */
+  @ElementTypesAreNonnullByDefault
+  public interface MemoizingSupplier<T extends @Nullable Object> extends Supplier<T> {
+    /**
+     * Returns {@code true} if the supplier has been initialized, i.e. if the first call to
+     * {@link #get()} has been made or if the supplier has been explicitly initialized.
+     */
+    boolean isInitialized();
+  }
+
   @VisibleForTesting
-  static class MemoizingSupplier<T extends @Nullable Object> implements Supplier<T>, Serializable {
+  static class SerializableMemoizingSupplier<T extends @Nullable Object>
+    implements MemoizingSupplier<T>, Serializable {
     private transient Object lock = new Object();
 
     final Supplier<T> delegate;
@@ -130,7 +146,7 @@ static class MemoizingSupplier<T extends @Nullable Object> implements Supplier<T
     // on volatile read of "initialized".
     @CheckForNull transient T value;
 
-    MemoizingSupplier(Supplier<T> delegate) {
+    SerializableMemoizingSupplier(Supplier<T> delegate) {
       this.delegate = checkNotNull(delegate);
     }
 
@@ -154,6 +170,11 @@ public T get() {
       return uncheckedCastNullableTToT(value);
     }
 
+    @Override
+    public boolean isInitialized() {
+      return initialized;
+    }
+
     @Override
     public String toString() {
       return "Suppliers.memoize("
@@ -172,7 +193,8 @@ private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundE
   }
 
   @VisibleForTesting
-  static class NonSerializableMemoizingSupplier<T extends @Nullable Object> implements Supplier<T> {
+  static class NonSerializableMemoizingSupplier<T extends @Nullable Object>
+    implements MemoizingSupplier<T> {
     private final Object lock = new Object();
 
     @SuppressWarnings("UnnecessaryLambda") // Must be a fixed singleton object
@@ -208,6 +230,11 @@ public T get() {
       return uncheckedCastNullableTToT(value);
     }
 
+    @Override
+    public boolean isInitialized() {
+      return delegate == SUCCESSFULLY_COMPUTED;
+    }
+
     @Override
     public String toString() {
       Supplier<T> delegate = this.delegate;