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 ComparisonChain compare(
@Nullable T left, @Nullable T right, Comparator 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 ComparisonChain compare(
@Nullable T left, @Nullable T right, @Nullable Comparator 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}, if 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, if the result of this
* comparison chain has not already been determined.
*/
public abstract ComparisonChain compare(
@Nullable T left, @Nullable T right, Comparator comparator);

/**
* Compares two {@code int} values as specified by {@link Ints#compare},
* if 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},
* if 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}, if 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}, if 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}, if 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}, if 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();
}