java中Comparable和Comparator两种比较器的区别

　Comparable和Comparator接口都是为了对类进行比较，众所周知，诸如Integer，double等基本数据类型，java可以对他们进行比较，而对于类的比较，需要人工定义比较用到的字段比较逻辑。可以把Comparable理解为内部比较器，而Comparator是外部比较器，基本的写法如下：

class Apple implements Comparable<Apple>{
    int id;
    double price;

    public Apple(int id, double price) {
        this.id = id;
        this.price = price;
    }
    public int compareTo(Apple o) {
        //return Double.compare(this.getPrice(),o.getPrice());
        if (Math.abs(this.price-o.price)<0.001)
            return 0;
        else
            return (o.price-this.price)>0?1:-1;
    }
    @Override
    public String toString() {
        return "Apple{" +
                "id=" + id +
                ", price=" + price +
                '}';
    }
}

class AESComparator implements Comparator<Apple>{

    public int compare(Apple o1, Apple o2) {
        if (Math.abs(o1.price-o2.price)<0.001)
            return 0;
        else{
            return (o1.price-o2.price)>0?1:-1;
        }
    }
}

　　实现了Comparable接口的类需要实现compareTo()方法，传入一个外部参数进行比对，实现了Comparator接口的方法需要实现compare()方法，对外部传入的两个类进行比较，从而让外部方法在比较时调用。

　　两者的区别是实现Comparator接口代码更加灵活，可以定义某个类的多个比较器，从而在排序时根据实际场景自由调用，而Comparable接口实现后便不能改动。两种接口的调用方式如下：

class AESComparator implements Comparator<Apple>{

    public int compare(Apple o1, Apple o2) {
        if (Math.abs(o1.price-o2.price)<0.001)
            return 0;
        else{
            return (o1.price-o2.price)>0?1:-1;
        }
    }
}
class  DESComparator implements Comparator<Apple>{

    public int compare(Apple o1, Apple o2) {
        if (Math.abs(o1.price-o2.price)<0.001)
            return 0;
        else {
            return (o1.price-o2.price)>0?-1:1;
        }
    }
}

public static void main(String[] args) {
        Apple apple1 = new Apple(1,4.8);
        Apple apple2 = new Apple(2,5.9);
        Apple apple3 = new Apple(3,8.5);
        List<Apple> list = new ArrayList<Apple>();
        list.add(apple1);
        list.add(apple3);
        list.add(apple2);
        System.out.println("Comparable==========");
        System.out.printf("this list of apples: %s\n",list);
        Collections.sort(list);
        System.out.printf("this list of apples: %s\n",list);
        System.out.println("Comparator==========");
        System.out.printf("this list of apples: %s\n",list);
        Collections.sort(list,new DESComparator());
        System.out.printf("this list of apples: %s\n",list);
        Collections.sort(list,new AESComparator());
        System.out.printf("this list of apples: %s\n",list);

    }
}

　　上述代码存在的问题，不能在比较器中进行double类型的减法操作，因为对于值比较大的double，减法操作容易导致值的溢出，java7对每一种包装类型的比较新增了compare()方法，改造后的代码如下：

class Apple implements Comparable<Apple>{
    int id;
    double price;

    public Apple(int id, double price) {
        this.id = id;
        this.price = price;
    }
    public int compareTo(Apple o) {
        return Double.compare(this.price,o.price);
    }
    @Override
    public String toString() {
        return "Apple{" +
                "id=" + id +
                ", price=" + price +
                '}';
    }
}
class AESComparator implements Comparator<Apple>{

    public int compare(Apple o1, Apple o2) {
        return Double.compare(o1.price,o2.price);
    }
}
class  DESComparator implements Comparator<Apple>{

    public int compare(Apple o1, Apple o2) {
        return Double.compare(o2.price,o1.price);
    }
}

　　查看Double.compare的源码如下

public static int compare(double d1, double d2) {
        if (d1 < d2)
            return -1;           // Neither val is NaN, thisVal is smaller
        if (d1 > d2)
            return 1;            // Neither val is NaN, thisVal is larger

        // Cannot use doubleToRawLongBits because of possibility of NaNs.
        long thisBits    = Double.doubleToLongBits(d1);
        long anotherBits = Double.doubleToLongBits(d2);
        return (thisBits == anotherBits ?  0 : // Values are equal
                (thisBits < anotherBits ? -1 : // (-0.0, 0.0) or (!NaN, NaN)
                        1));                          // (0.0, -0.0) or (NaN, !NaN)
    }