common/base/Objects.java

/*
 * Copyright (C) 2007 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.common.base;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import javax.annotation.Nullable;

/**
 * Helper functions that can operate on any {@code Object}.
 *
 * @author Laurence Gonsalves
 * @since 2010.01.04 <b>stable</b> (imported from Google Collections Library)
 */
@GwtCompatible
public final class Objects {
  private Objects() {}

  /**
   * Determines whether two possibly-null objects are equal. Returns:
   *
   * <ul>
   * <li>{@code true} if {@code a} and {@code b} are both null.
   * <li>{@code true} if {@code a} and {@code b} are both non-null and they are
   *     equal according to {@link Object#equals(Object)}.
   * <li>{@code false} in all other situations.
   * </ul>
   *
   * <p>This assumes that any non-null objects passed to this function conform
   * to the {@code equals()} contract.
   */
  public static boolean equal(@Nullable Object a, @Nullable Object b) {
    return a == b || (a != null && a.equals(b));
  }

  /**
   * Generates a hash code for multiple values. The hash code is generated by
   * calling {@link Arrays#hashCode(Object[])}.
   *
   * <p>This is useful for implementing {@link Object#hashCode()}. For example,
   * in an object that has three properties, {@code x}, {@code y}, and
   * {@code z}, one could write:
   * <pre>
   * public int hashCode() {
   *   return Objects.hashCode(getX(), getY(), getZ());
   * }</pre>
   *
   * <b>Warning</b>: When a single object is supplied, the returned hash code
   * does not equal the hash code of that object.
   */
  public static int hashCode(Object... objects) {
    return Arrays.hashCode(objects);
  }

  /**
   * Creates an instance of {@link ToStringHelper}.
   *
   * <p>This is helpful for implementing {@link Object#toString()}. For
   * example, in an object that contains two member variables, {@code x},
   * and {@code y}, one could write:<pre>   <tt>
   *   public class ClassName {
   *     public String toString() {
   *       return Objects.toStringHelper(this)
   *           .add("x", x)
   *           .add("y", y)
   *           .toString();
   *     }
   *   }</tt>
   * </pre>
   *
   * Assuming the values of {@code x} and {@code y} are 1 and 2,
   * this code snippet returns the string <tt>"ClassName{x=1, y=2}"</tt>.
   *
   * @since 2010.01.04 <b>tentative</b>
   */
  public static ToStringHelper toStringHelper(Object object) {
    return new ToStringHelper(object);
  }

  /**
   * Support class for {@link Objects#toStringHelper}.
   *
   * @author Jason Lee
   * @since 2010.01.04 <b>tentative</b>
   */
  public static class ToStringHelper {
    private final List<String> fieldString = new ArrayList<String>();
    private final Object instance;

    /**
     * Use {@link Objects#toStringHelper(Object)} to create an instance.
     */
    private ToStringHelper(Object instance) {
      this.instance = Preconditions.checkNotNull(instance);
    }

    /**
     * Adds a name/value pair to the formatted output in {@code name=value}
     * format. If {@code value} is {@code null}, the string {@code "null"}
     * is used.
     */
    public ToStringHelper add(String name, @Nullable Object value) {
      return addValue(Preconditions.checkNotNull(name) + "=" + value);
    }

    /**
     * Adds a value to the formatted output in {@code value} format.<p/>
     *
     * It is strongly encouraged to use {@link #add(String, Object)} instead and
     * give value a readable name.
     */
    public ToStringHelper addValue(@Nullable Object value) {
      fieldString.add(String.valueOf(value));
      return this;
    }

    private static final Joiner JOINER = Joiner.on(", ");

    /**
     * Returns the formatted string.
     */
    @Override public String toString() {
      StringBuilder builder = new StringBuilder(100)
          .append(simpleName(instance.getClass()))
          .append('{');
      return JOINER.appendTo(builder, fieldString)
          .append('}')
          .toString();
    }

    /**
     * {@link Class#getSimpleName()} is not GWT compatible yet, so we
     * provide our own implementation.
     */
    @VisibleForTesting
    static String simpleName(Class<?> clazz) {
      String name = clazz.getName();

      // we want the name of the inner class all by its lonesome
      int start = name.lastIndexOf('$');

      // if this isn't an inner class, just find the start of the
      // top level class name.
      if (start == -1) {
        start = name.lastIndexOf('.');
      }
      return name.substring(start + 1);
    }
  }
}