guava ComparisonChain的源码实现

guava ComparisonChain源码的实现十分精妙，用到了利用内部类实现单例设计模式以及利用状态设计模式。代码十分优雅。

上源码欣赏：
public abstract class ComparisonChain {
  //构造函数私有 单例模式
  private ComparisonChain() {}

  /**
   * Begins a new chained comparison statement. See example in the class
   * documentation.
   */
  public static ComparisonChain start() {
    return ACTIVE;
  }
  //利用内部类的方式来实现单例设计模式
  private static final ComparisonChain ACTIVE =
      new ComparisonChain() {
        @SuppressWarnings("unchecked")
        @Override
        public ComparisonChain compare(Comparable left, Comparable right) {
          return classify(left.compareTo(right));
        }

        @Override
        public <T> ComparisonChain compare(
            @Nullable T left, @Nullable T right, Comparator<T> comparator) {
          return classify(comparator.compare(left, right));
        }

        @Override
        public ComparisonChain compare(int left, int right) {
          return classify(Ints.compare(left, right));
        }

        @Override
        public ComparisonChain compare(long left, long right) {
          return classify(Longs.compare(left, right));
        }

        @Override
        public ComparisonChain compare(float left, float right) {
          return classify(Float.compare(left, right));
        }

        @Override
        public ComparisonChain compare(double left, double right) {
          return classify(Double.compare(left, right));
        }

        @Override
        public ComparisonChain compareTrueFirst(boolean left, boolean right) {
          return classify(Booleans.compare(right, left)); // reversed
        }

        @Override
        public ComparisonChain compareFalseFirst(boolean left, boolean right) {
          return classify(Booleans.compare(left, right));
        }
        //委托到当前状态
        ComparisonChain classify(int result) {
          return (result < 0) ? LESS : (result > 0) ? GREATER : ACTIVE;
        }

        @Override
        public int result() {
          return 0;
        }
      };
  //状态设计模式的体现 实现两个值保存大于和小于两种状态  
  private static final ComparisonChain LESS = new InactiveComparisonChain(-1);

  private static final ComparisonChain GREATER = new InactiveComparisonChain(1);

  private static final class InactiveComparisonChain extends ComparisonChain {
    final int result;

    InactiveComparisonChain(int result) {
      this.result = result;
    }
    //返回当前状态
    @Override
    public ComparisonChain compare(@Nullable Comparable left, @Nullable Comparable right) {
      return this;
    }

    @Override
    public <T> ComparisonChain compare(
        @Nullable T left, @Nullable T right, @Nullable Comparator<T> comparator) {
      return this;
    }

    @Override
    public ComparisonChain compare(int left, int right) {
      return this;
    }

    @Override
    public ComparisonChain compare(long left, long right) {
      return this;
    }

    @Override
    public ComparisonChain compare(float left, float right) {
      return this;
    }

    @Override
    public ComparisonChain compare(double left, double right) {
      return this;
    }

    @Override
    public ComparisonChain compareTrueFirst(boolean left, boolean right) {
      return this;
    }

    @Override
    public ComparisonChain compareFalseFirst(boolean left, boolean right) {
      return this;
    }

    @Override
    public int result() {
      return result;
    }
  }

  /**
   * Compares two comparable objects as specified by {@link
   * Comparable#compareTo}, <i>if</i> the result of this comparison chain
   * has not already been determined.
   */
  public abstract ComparisonChain compare(Comparable<?> left, Comparable<?> right);

  /**
   * Compares two objects using a comparator, <i>if</i> the result of this
   * comparison chain has not already been determined.
   */
  public abstract <T> ComparisonChain compare(
      @Nullable T left, @Nullable T right, Comparator<T> comparator);

  /**
   * Compares two {@code int} values as specified by {@link Ints#compare},
   * <i>if</i> the result of this comparison chain has not already been
   * determined.
   */
  public abstract ComparisonChain compare(int left, int right);

  /**
   * Compares two {@code long} values as specified by {@link Longs#compare},
   * <i>if</i> the result of this comparison chain has not already been
   * determined.
   */
  public abstract ComparisonChain compare(long left, long right);

  /**
   * Compares two {@code float} values as specified by {@link
   * Float#compare}, <i>if</i> the result of this comparison chain has not
   * already been determined.
   */
  public abstract ComparisonChain compare(float left, float right);

  /**
   * Compares two {@code double} values as specified by {@link
   * Double#compare}, <i>if</i> the result of this comparison chain has not
   * already been determined.
   */
  public abstract ComparisonChain compare(double left, double right);

  /**
   * Discouraged synonym for {@link #compareFalseFirst}.
   *
   * @deprecated Use {@link #compareFalseFirst}; or, if the parameters passed
   *     are being either negated or reversed, undo the negation or reversal and
   *     use {@link #compareTrueFirst}.
   * @since 19.0
   */
  @Deprecated
  public final ComparisonChain compare(Boolean left, Boolean right) {
    return compareFalseFirst(left, right);
  }

  /**
   * Compares two {@code boolean} values, considering {@code true} to be less
   * than {@code false}, <i>if</i> the result of this comparison chain has not
   * already been determined.
   *
   * @since 12.0
   */
  public abstract ComparisonChain compareTrueFirst(boolean left, boolean right);

  /**
   * Compares two {@code boolean} values, considering {@code false} to be less
   * than {@code true}, <i>if</i> the result of this comparison chain has not
   * already been determined.
   *
   * @since 12.0 (present as {@code compare} since 2.0)
   */
  public abstract ComparisonChain compareFalseFirst(boolean left, boolean right);

  /**
   * Ends this comparison chain and returns its result: a value having the
   * same sign as the first nonzero comparison result in the chain, or zero if
   * every result was zero.
   */
  public abstract int result();
}