The source code implementation of guava ComparisonChain

The implementation of the guava ComparisonChain source code is very subtle, using the use of inner classes to implement the singleton design pattern and the use of the state design pattern. The code is very elegant.

Appreciate the source code:
public abstract class ComparisonChain {
  //Constructor private singleton mode
  private ComparisonChain() {}

  /**
   * Begins a new chained comparison statement. See example in the class
   * documentation.
   */
  public static ComparisonChain start() {
    return ACTIVE;
  }
  //Use the inner class to implement the singleton design pattern
  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));
        }
        //Delegate to the current
        stateComparisonChain classify(int result) {
          return (result < 0) ? LESS : (result > 0) ? GREATER : ACTIVE;
        }

        @Override
        public int result() {
          return 0;
        }
      };
  //Status The embodiment of the design pattern realizes two values ​​save greater and less than two states  
  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();
}