pylucene 3.5.0-3

[pylucene.git] / lucene-java-3.5.0 / lucene / src / java / org / apache / lucene / util / SetOnce.java

diff --git a/lucene-java-3.5.0/lucene/src/java/org/apache/lucene/util/SetOnce.java b/lucene-java-3.5.0/lucene/src/java/org/apache/lucene/util/SetOnce.java
new file mode 100644 (file)
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+++ b/lucene-java-3.5.0/lucene/src/java/org/apache/lucene/util/SetOnce.java
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+package org.apache.lucene.util;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * A convenient class which offers a semi-immutable object wrapper
+ * implementation which allows one to set the value of an object exactly once,
+ * and retrieve it many times. If {@link #set(Object)} is called more than once,
+ * {@link AlreadySetException} is thrown and the operation
+ * will fail.
+ *
+ * @lucene.experimental
+ */
+public final class SetOnce<T> {
+
+  /** Thrown when {@link SetOnce#set(Object)} is called more than once. */
+  public static final class AlreadySetException extends RuntimeException {
+    public AlreadySetException() {
+      super("The object cannot be set twice!");
+    }
+  }
+  
+  private volatile T obj = null;
+  private final AtomicBoolean set;
+  
+  /**
+   * A default constructor which does not set the internal object, and allows
+   * setting it by calling {@link #set(Object)}.
+   */
+  public SetOnce() {
+    set = new AtomicBoolean(false);
+  }
+
+  /**
+   * Creates a new instance with the internal object set to the given object.
+   * Note that any calls to {@link #set(Object)} afterwards will result in
+   * {@link AlreadySetException}
+   *
+   * @throws AlreadySetException if called more than once
+   * @see #set(Object)
+   */
+  public SetOnce(T obj) {
+    this.obj = obj;
+    set = new AtomicBoolean(true);
+  }
+  
+  /** Sets the given object. If the object has already been set, an exception is thrown. */
+  public final void set(T obj) {
+    if (set.compareAndSet(false, true)) {
+      this.obj = obj;
+    } else {
+      throw new AlreadySetException();
+    }
+  }
+  
+  /** Returns the object set by {@link #set(Object)}. */
+  public final T get() {
+    return obj;
+  }
+}