gson-comments/gson/src/main/java/com/google/gson/JsonNull.java

/*
 * Copyright (C) 2008 Google Inc.
 *
 * Licensed 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.
 */

package com.google.gson;

import java.io.IOException;

/**
 * A class representing a Json {@code null} value.
 *
 * @author Inderjeet Singh
 * @author Joel Leitch
 * @since 1.2
 */
public final class JsonNull extends JsonElement {
  private static final JsonNull INSTANCE = new JsonNull();

  /**
   * Creates a new JsonNull object.
   */
  public JsonNull() {
    // Do nothing
  }
  
  @Override
  protected void toString(Appendable sb, Escaper escaper) throws IOException {
    sb.append("null");
  }
  
  /**
   * All instances of JsonNull have the same hash code since they are indistinguishable
   */
  @Override
  public int hashCode() {
    return JsonNull.class.hashCode();
  }
  
  /**
   * All instances of JsonNull are the same
   */
  @Override
  public boolean equals(Object other) {
    return this == other || other instanceof JsonNull;
  }
  
  /**
   * Creation method used to return an instance of a {@link JsonNull}.  To reduce the memory
   * footprint, a single object has been created for this class; therefore the same instance is
   * being returned for each invocation of this method. This method is kept private since we 
   * prefer the users to use {@link JsonNull#JsonNull()} which is similar to how other JsonElements
   * are created. Note that all instances of JsonNull return true for {@link #equals(Object)} 
   * when compared to each other.
   *
   * @return a instance of a {@link JsonNull}
   */
  static JsonNull createJsonNull() {
    return INSTANCE;
  }
}