common/base/Objects.java

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * 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 static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtCompatible;

import java.util.Arrays;

import javax.annotation.Nullable;

/**
 * Helper functions that can operate on any {@code Object}.
 *
 * <p>See the Guava User Guide on <a
 * href="http://code.google.com/p/guava-libraries/wiki/CommonObjectUtilitiesExplained">writing
 * {@code Object} methods with {@code Objects}</a>.
 *
 * @author Laurence Gonsalves
 * @since 2.0 (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(@Nullable Object... objects) {
    return Arrays.hashCode(objects);
  }

  /**
   * Creates an instance of {@link ToStringHelper}.
   *
   * <p>This is helpful for implementing {@link Object#toString()}.
   * Specification by example: <pre>   {@code
   *   // Returns "ClassName{}"
   *   Objects.toStringHelper(this)
   *       .toString();
   *
   *   // Returns "ClassName{x=1}"
   *   Objects.toStringHelper(this)
   *       .add("x", 1)
   *       .toString();
   *
   *   // Returns "MyObject{x=1}"
   *   Objects.toStringHelper("MyObject")
   *       .add("x", 1)
   *       .toString();
   *
   *   // Returns "ClassName{x=1, y=foo}"
   *   Objects.toStringHelper(this)
   *       .add("x", 1)
   *       .add("y", "foo")
   *       .toString();
   *   }}
   *
   *   // Returns "ClassName{x=1}"
   *   Objects.toStringHelper(this)
   *       .omitNullValues()
   *       .add("x", 1)
   *       .add("y", null)
   *       .toString();
   *   }}</pre>
   *
   * <p>Note that in GWT, class names are often obfuscated.
   *
   * @param self the object to generate the string for (typically {@code this}),
   *        used only for its class name
   * @since 2.0
   */
  public static ToStringHelper toStringHelper(Object self) {
    return new ToStringHelper(simpleName(self.getClass()));
  }

  /**
   * Creates an instance of {@link ToStringHelper} in the same manner as
   * {@link Objects#toStringHelper(Object)}, but using the name of {@code clazz}
   * instead of using an instance's {@link Object#getClass()}.
   *
   * <p>Note that in GWT, class names are often obfuscated.
   *
   * @param clazz the {@link Class} of the instance
   * @since 7.0 (source-compatible since 2.0)
   */
  public static ToStringHelper toStringHelper(Class<?> clazz) {
    return new ToStringHelper(simpleName(clazz));
  }

  /**
   * Creates an instance of {@link ToStringHelper} in the same manner as
   * {@link Objects#toStringHelper(Object)}, but using {@code className} instead
   * of using an instance's {@link Object#getClass()}.
   *
   * @param className the name of the instance type
   * @since 7.0 (source-compatible since 2.0)
   */
  public static ToStringHelper toStringHelper(String className) {
    return new ToStringHelper(className);
  }

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

    // the nth anonymous class has a class name ending in "Outer$n"
    // and local inner classes have names ending in "Outer.$1Inner"
    name = name.replaceAll("\\$[0-9]+", "\\$");

    // 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);
  }

  /**
   * Returns the first of two given parameters that is not {@code null}, if
   * either is, or otherwise throws a {@link NullPointerException}.
   *
   * <p><b>Note:</b> if {@code first} is represented as an {@code Optional<T>},
   * this can be accomplished with {@code first.or(second)}. That approach also
   * allows for lazy evaluation of the fallback instance, using
   * {@code first.or(Supplier)}.
   *
   * @return {@code first} if {@code first} is not {@code null}, or
   *     {@code second} if {@code first} is {@code null} and {@code second} is
   *     not {@code null}
   * @throws NullPointerException if both {@code first} and {@code second} were
   *     {@code null}
   * @since 3.0
   */
  public static <T> T firstNonNull(@Nullable T first, @Nullable T second) {
    return first != null ? first : checkNotNull(second);
  }

  /**
   * Support class for {@link Objects#toStringHelper}.
   *
   * @author Jason Lee
   * @since 2.0
   */
  public static final class ToStringHelper {
    private final String className;
    private ValueHolder holderHead = new ValueHolder();
    private ValueHolder holderTail = holderHead;
    private boolean omitNullValues = false;

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

    /**
     * Configures the {@link ToStringHelper} so {@link #toString()} will ignore
     * properties with null value. The order of calling this method, relative
     * to the {@code add()}/{@code addValue()} methods, is not significant.
     *
     * @since 12.0
     */
    public ToStringHelper omitNullValues() {
      omitNullValues = true;
      return this;
    }

    /**
     * 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, unless {@link #omitNullValues()} is called, in which case this
     * name/value pair will not be added.
     */
    public ToStringHelper add(String name, @Nullable Object value) {
      return addHolder(name, value);
    }

    /**
     * Adds a name/value pair to the formatted output in {@code name=value}
     * format.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper add(String name, boolean value) {
      return addHolder(name, String.valueOf(value));
    }

    /**
     * Adds a name/value pair to the formatted output in {@code name=value}
     * format.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper add(String name, char value) {
      return addHolder(name, String.valueOf(value));
    }

    /**
     * Adds a name/value pair to the formatted output in {@code name=value}
     * format.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper add(String name, double value) {
      return addHolder(name, String.valueOf(value));
    }

    /**
     * Adds a name/value pair to the formatted output in {@code name=value}
     * format.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper add(String name, float value) {
      return addHolder(name, String.valueOf(value));
    }

    /**
     * Adds a name/value pair to the formatted output in {@code name=value}
     * format.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper add(String name, int value) {
      return addHolder(name, String.valueOf(value));
    }

    /**
     * Adds a name/value pair to the formatted output in {@code name=value}
     * format.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper add(String name, long value) {
      return addHolder(name, String.valueOf(value));
    }

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

    /**
     * Adds an unnamed value to the formatted output.
     *
     * <p>It is strongly encouraged to use {@link #add(String, boolean)} instead
     * and give value a readable name.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper addValue(boolean value) {
      return addHolder(String.valueOf(value));
    }

    /**
     * Adds an unnamed value to the formatted output.
     *
     * <p>It is strongly encouraged to use {@link #add(String, char)} instead
     * and give value a readable name.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper addValue(char value) {
      return addHolder(String.valueOf(value));
    }

    /**
     * Adds an unnamed value to the formatted output.
     *
     * <p>It is strongly encouraged to use {@link #add(String, double)} instead
     * and give value a readable name.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper addValue(double value) {
      return addHolder(String.valueOf(value));
    }

    /**
     * Adds an unnamed value to the formatted output.
     *
     * <p>It is strongly encouraged to use {@link #add(String, float)} instead
     * and give value a readable name.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper addValue(float value) {
      return addHolder(String.valueOf(value));
    }

    /**
     * Adds an unnamed value to the formatted output.
     *
     * <p>It is strongly encouraged to use {@link #add(String, int)} instead
     * and give value a readable name.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper addValue(int value) {
      return addHolder(String.valueOf(value));
    }

    /**
     * Adds an unnamed value to the formatted output.
     *
     * <p>It is strongly encouraged to use {@link #add(String, long)} instead
     * and give value a readable name.
     *
     * @since 11.0 (source-compatible since 2.0)
     */
    public ToStringHelper addValue(long value) {
      return addHolder(String.valueOf(value));
    }

    /**
     * Returns a string in the format specified by {@link
     * Objects#toStringHelper(Object)}.
     *
     * <p>After calling this method, you can keep adding more properties to later
     * call toString() again and get a more complete representation of the
     * same object; but properties cannot be removed, so this only allows
     * limited reuse of the helper instance. The helper allows duplication of
     * properties (multiple name/value pairs with the same name can be added).
     */
    @Override
    public String toString() {
      // create a copy to keep it consistent in case value changes
      boolean omitNullValuesSnapshot = omitNullValues;
      String nextSeparator = "";
      StringBuilder builder = new StringBuilder(32).append(className).append('{');
      for (ValueHolder valueHolder = holderHead.next; valueHolder != null; valueHolder = valueHolder.next) {
        if (!omitNullValuesSnapshot || valueHolder.value != null) {
          builder.append(nextSeparator);
          nextSeparator = ", ";

          if (valueHolder.name != null) {
            builder.append(valueHolder.name).append('=');
          }
          builder.append(valueHolder.value);
        }
      }
      return builder.append('}').toString();
    }

    private ValueHolder addHolder() {
      ValueHolder valueHolder = new ValueHolder();
      holderTail = holderTail.next = valueHolder;
      return valueHolder;
    }

    private ToStringHelper addHolder(@Nullable Object value) {
      ValueHolder valueHolder = addHolder();
      valueHolder.value = value;
      return this;
    }

    private ToStringHelper addHolder(String name, @Nullable Object value) {
      ValueHolder valueHolder = addHolder();
      valueHolder.value = value;
      valueHolder.name = checkNotNull(name);
      return this;
    }

    private static final class ValueHolder {
      String name;
      Object value;
      ValueHolder next;
    }
  }
}