util: Add Lazy

kohlschutter-util/src/main/java/com/kohlschutter/util/Lazy.java

@@ -0,0 +1,70 @@
+package com.kohlschutter.util;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.function.Supplier;
+
+/**
+ * Wrapper for a lazy-initialized object.
+ * 
+ * @param <V> The object type.
+ * @author Christian Kohlschütter
+ */
+public final class Lazy<V> {
+  private final CompletableFuture<V> future = new CompletableFuture<V>();
+  private final AtomicBoolean supplied = new AtomicBoolean();
+  private final Supplier<V> supplier;
+
+  /**
+   * Creates a lazy-load wrapper, using the given supplier to supply the object upon the first call
+   * to {@link #get()}.
+   * 
+   * @param <V> The object type.
+   * @param supplier The object supplier.
+   * @return The wrapper instance.
+   */
+  public static final <V> Lazy<V> of(Supplier<V> supplier) {
+    return new Lazy<>(supplier);
+  }
+
+  private Lazy(Supplier<V> supplier) {
+    this.supplier = supplier;
+  }
+
+  /**
+   * Returns the object. If this is the first call, the object is retrieved from the configured
+   * supplier, and transitions this instance to a completed satate.
+   * 
+   * @return The object.
+   */
+  public V get() {
+    if (!future.isDone() && supplied.compareAndSet(false, true)) {
+      future.complete(supplier.get());
+    }
+    try {
+      return future.get();
+    } catch (InterruptedException | ExecutionException e) {
+      throw new RuntimeException(e);
+    }
+  }
+
+  /**
+   * If not already completed, sets the value returned by {@link #get()} and related methods to the
+   * given value, side-stepping the value that would be retrieved from the supplier.
+   *
+   * @param value the result value
+   * @return {@code true} if this invocation caused this instance to transition to a completed
+   *         state, else {@code false}
+   */
+  public boolean complete(V value) {
+    supplied.set(true); // NOTE: We do not compareAndSet here to allow #set from within supplier.get
+    return future.complete(value);
+  }
+
+  @SuppressWarnings("null")
+  @Override
+  public String toString() {
+    return super.toString() + "[supplied=" + supplied + "; value=" + future.getNow(null) + "]";
+  }
+}