Comparator是一個專用的比較器，當(dāng)這個對象不支持自比較或者自比較函數(shù)不能滿足要求時，可寫一個比較器來完成兩個對象之間大小的比較。Comparator體現(xiàn)了一種策略模式(strategy design pattern)，就是不改變對象自身，而用一個策略對象(strategy object)來改變它的行為。

比如我有一組數(shù)字需要對其進行正序排序,java8之前我們通常的做法是:

 Integer [] values = new Integer[]{1,0,3,5,7,8,5,4,3,7,9,56,3,23};
Arrays.sort(values, new Comparator<Integer>() {
            @Override
            public int compare(Integer value1, Integer value2) {
                return value1- value2;
            }
        });

而在java8中,由于新增lambda表達(dá)式,我們可以很簡潔的使用如下的方式:

Integer [] values = new Integer[]{1,0,3,5,7,8,5,4,3,7,9,56,3,23};
 Arrays.sort(values, (x,y) -> x-y);

可以看到代碼量更加簡潔更加清晰,jdk甚至為了排序做了一些定制的方法,比如上面的代碼可以寫成如下的方式,更加讓人容易理解:

Integer [] values = new Integer[]{1,0,3,5,7,8,5,4,3,7,9,56,3,23};
Arrays.sort(values, Comparator.comparingInt(x -> x));

由于本文不會專門講解java8中關(guān)于lambda表達(dá)式的內(nèi)容,只講解主題相關(guān)的Comparator的使用,如果想了解更多關(guān)于lambda表達(dá)式的內(nèi)容,還請參考其他文章.

Comparator使用

Comparator是一個函數(shù)式接口,只有一個 int compare(T o1, T o2);方法供外部使用,但是java8在其基礎(chǔ)上定制了許多default方法,這些default方法,開箱即用,下面我們就看看有哪些default方法以及如何使用.

reversed()

default Comparator<T> reversed() {
        return Collections.reverseOrder(this);
    }

顧名思義,可以知道這個方法是用來生成一個逆序器,比如我們開始需要得到一個正序的排序序列,然后又想得到一個反轉(zhuǎn)的排序序列,就可以使用該方法,比如:

Integer [] values = new Integer[]{1,0,3,5,7,8,5,4,3,7,9,56,3,23};
Comparator<Integer> comparator = Comparator.comparingInt(x -> x);
Arrays.sort(values, comparator);
System.out.println(JSON.toJSONString(values));
Arrays.sort(values, comparator.reversed());
System.out.println(JSON.toJSONString(values));

結(jié)果如下:

[0,1,3,3,3,4,5,5,7,7,8,9,23,56]
[56,23,9,8,7,7,5,5,4,3,3,3,1,0]

可以看到,先得到正序排序的序列,然后調(diào)用comparator.reversed(),我們將內(nèi)容進行反轉(zhuǎn)

thenComparing(Comparator<? super T> other)

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);
        };
    }

顧名思義,該方法是在原有的比較器上再加入一個比較器,比如我們有一些用戶,在對其進行排序的時候,我們首先對他的名字長度進行排序,然后再對他的年齡進行排序,如下:

person類

class Person{
        private int age;
        private String name;
        public int getAge() {
            return age;
        }
        public void setAge(int age) {
            this.age = age;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public Person(int age, String name) {
            this.age = age;
            this.name = name;
        }
        @Override
        public String toString() {
            return "Person{" +
                    "age=" + age +
                    ", name='" + name + '\'' +
                    '}';
        }
    }

測試代碼:

Person [] persons = new Person[]{
                new Person(22,"zhangsan"),
                new Person(23,"lisi"),
                new Person(23,"wangwu"),
                new Person(22,"wangwu"),
                new Person(22,"lisi"),
                new Person(23,"zhangsan")
        };
        System.out.println(JSON.toJSONString(persons));
        Comparator<Person> comparator = Comparator.comparingInt(x -> x.name.length());
        Arrays.sort(persons,comparator);
        System.out.println(JSON.toJSONString(persons));
        comparator = comparator.thenComparing(Comparator.comparingInt(x->x.age));
        Arrays.sort(persons,comparator);
        System.out.println(JSON.toJSONString(persons));

結(jié)果如下:

[{"age":22,"name":"zhangsan"},{"age":23,"name":"lisi"},{"age":23,"name":"wangwu"},{"age":22,"name":"wangwu"},{"age":22,"name":"lisi"},{"age":23,"name":"zhangsan"}]
[{"age":23,"name":"lisi"},{"age":22,"name":"lisi"},{"age":23,"name":"wangwu"},{"age":22,"name":"wangwu"},{"age":22,"name":"zhangsan"},{"age":23,"name":"zhangsan"}]
[{"age":22,"name":"lisi"},{"age":23,"name":"lisi"},{"age":22,"name":"wangwu"},{"age":23,"name":"wangwu"},{"age":22,"name":"zhangsan"},{"age":23,"name":"zhangsan"}]

上述代碼甚至可以簡化成如下:

Comparator<Person> comparator = Comparator.comparing(Person::getName).thenComparing(Person::getAge);

default <U> Comparator<T> thenComparing(
Function<? super T, ? extends U> keyExtractor,
Comparator<? super U> keyComparator)

default <U> Comparator<T> thenComparing(
            Function<? super T, ? extends U> keyExtractor,
            Comparator<? super U> keyComparator)
    {
        return thenComparing(comparing(keyExtractor, keyComparator));
    }

待補充