java中Comparator和 Comparable的介绍及区别实例

本文，先介绍Comparable 和Comparator两个接口，以及它们的差异；接着，通过示例，对它们的使用方法进行说明。

Comparable 简介

Comparable 是排序接口。

若一个类实现了Comparable接口，就意味着“该类支持排序”。 即然实现Comparable接口的类支持排序，假设现在存在“实现Comparable接口的类的对象的List列表(或数组)”，则该List列表(或数组)可以通过 Collections.sort（或 Arrays.sort）进行排序。

此外，“实现Comparable接口的类的对象”可以用作“有序映射(如TreeMap)”中的键或“有序集合(TreeSet)”中的元素，而不需要指定比较器。

1、Comparable的代码如下：

public interface Comparable<T> {public int compareTo(T o);
}

2、Comparator的代码如下

public interface Comparator<T> {int compare(T o1, T o2);boolean equals(Object obj);    // jdk1.8 后的方法default Comparator<T> reversed() {return Collections.reverseOrder(this);
    }default Comparator<T> thenComparing(Comparator<? super T> other) {
        Objects.requireNonNull(other);return (Comparator<T> & Serializable) (c1, c2) -> {int res = compare(c1, c2);return (res != 0) ? res : other.compare(c1, c2);
        };
    }default <U> Comparator<T> thenComparing(
            Function<? super T, ? extends U> keyExtractor,
            Comparator<? super U> keyComparator)
    {return thenComparing(comparing(keyExtractor, keyComparator));
    }default <U extends Comparable<? super U>> Comparator<T> thenComparing(
            Function<? super T, ? extends U> keyExtractor)
    {return thenComparing(comparing(keyExtractor));
    }default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor) {return thenComparing(comparingInt(keyExtractor));
    }default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor) {return thenComparing(comparingLong(keyExtractor));
    }default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) {return thenComparing(comparingDouble(keyExtractor));
    }public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {return Collections.reverseOrder();
    }

    @SuppressWarnings("unchecked")public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
    }public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {return new Comparators.NullComparator<>(true, comparator);
    }public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {return new Comparators.NullComparator<>(false, comparator);
    }public static <T, U> Comparator<T> comparing(
            Function<? super T, ? extends U> keyExtractor,
            Comparator<? super U> keyComparator)
    {
        Objects.requireNonNull(keyExtractor);
        Objects.requireNonNull(keyComparator);return (Comparator<T> & Serializable)
            (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
                                              keyExtractor.apply(c2));
    }public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
            Function<? super T, ? extends U> keyExtractor)
    {
        Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)
            (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
    }public static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor) {
        Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)
            (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
    }public static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor) {
        Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)
            (c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
    }public static<T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor) {
        Objects.requireNonNull(keyExtractor);return (Comparator<T> & Serializable)
            (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
    }
}

Comparable和Comparator的主要区别在于：

　　(1).Comparator 和 Comparable都是Java中的内部比较器接口，都是用来实现对一个自定义的类进行排序

　　(2). 不同的是实现Comparable接口是定义在类的内部，比较代码需要嵌入类的内部结构中

　　(3). Comparator 实现在类的外部，单独实现第一个比较器，不需要对原来的类进行结构上的变化，属于无侵入式的。

例如实现Comparable接口：

    class Score implements Comparable<Score>{  
      public int score;  
      public int time;  
        public int getScore() {  return score;  
    }  
      public void setScore(int score) {  this.score = score;  
    }  
      public int getTime() {  return time;  
    }  
      public void setTime(int time) {  this.time = time;  
    }  
      
        @Override  public int compareTo(Score o) {  if(this.time>o.time) return 1;  else if(this.time==o.time) return 0;  else return -1;  
        }  
      public Score(int score, int time) {  super();  this.score = score;  this.time = time;  
        }  
    }

实现Comparator接口的例子：

    class ScoreComparator implements Comparator<Score>{  
      
        @Override  public int compare(Score o1, Score o2) {  if(o1.time>o2.time) return 1;  else if(o1.time==o2.time) return 0;  else return -1;  
        }     
    }

然后调用ScoreComparator ：

Arrays.sort(score, new ScoreComparator());

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