java中Comparable和Comparator兩種比較器的區別

　Comparable和Comparator接口都是為了對類進行比較，眾所周知，諸如Integer，double等基本數據類型，java可以對他們進行比較，而對於類的比較，需要人工定義比較用到的字段比較邏輯。可以把Comparable理解為內部比較器，而Comparator是外部比較器，基本的寫法如下：

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;
        }
    }
}

　　實現了Comparable接口的類需要實現compareTo()方法，傳入一個外部參數進行比對，實現了Comparator接口的方法需要實現compare()方法，對外部傳入的兩個類進行比較，從而讓外部方法在比較時調用。

　　兩者的區別是實現Comparator接口代碼更加靈活，可以定義某個類的多個比較器，從而在排序時根據實際場景自由調用，而Comparable接口實現后便不能改動。兩種接口的調用方式如下：

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);

    }
}

　　上述代碼存在的問題，不能在比較器中進行double類型的減法操作，因為對於值比較大的double，減法操作容易導致值的溢出，java7對每一種包裝類型的比較新增了compare()方法，改造后的代碼如下：

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的源碼如下

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)
    }