Java Comparable vs Comparator

Collections.sort() 和 Arrays.sort()方法能夠對Object數組進行排序，可是這種方法排序要求對象所在的類必須實現Comparable接口，此接口用於指定排序規則

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class HDTV implements Comparable<HDTV> {

    private int size;
    private String brand;

    public HDTV(int size, String brand) {
        this.size = size;
        this.brand = brand;
    }

public int getSize() {
    return size;
}

public void setSize(int size) {
    this.size = size;
}

public String getBrand() {
    return brand;
}

public void setBrand(String brand) {
    this.brand = brand;
}

@Override
public int compareTo(HDTV tv) {
    if (this.getSize() > tv.getSize())
        return 1;
    else if (this.getSize() < tv.getSize())
        return -1;
    else
        return 0;
}

public static void main(String[] args) {
    HDTV tv1 = new HDTV(55, "Samsung");
    HDTV tv2 = new HDTV(60, "Sony");
    HDTV tv3 = new HDTV(35, "Apple");

    List<HDTV> list = new ArrayList<HDTV>();
    list.add(tv1);list.add(tv2);list.add(tv3);

    Collections.sort(list);

    for (HDTV tv : list) {
        System.out.println(tv.getSize()+"---"+tv.getBrand());
    }


}

若是一個類已經開發完成，可是在此類創建的初期並無實現Comparable接口，此時確定是沒法進行對象排序操做的，因此爲了解決這個問題，java定義了另外一個比較器的操做接口--Comparator

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class HDTV {
    private int size;
    private String brand;

    public HDTV(int size, String brand) {
        this.size = size;
        this.brand = brand;
    }

    public int getSize() {
        return size;
    }

    public void setSize(int size) {
        this.size = size;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }
}

class SizeComparator implements Comparator<HDTV> {
    @Override
    public int compare(HDTV tv1, HDTV tv2) {
        int tv1Size = tv1.getSize();
        int tv2Size = tv2.getSize();

        if (tv1Size > tv2Size) {
            return 1;
        } else if (tv1Size < tv2Size) {
            return -1;
        } else {
            return 0;
        }
    }
}

public class Main {
    public static void main(String[] args) {
        HDTV tv1 = new HDTV(55, "Samsung");
        HDTV tv2 = new HDTV(60, "Sony");
        HDTV tv3 = new HDTV(42, "Panasonic");

    ArrayList<HDTV> al = new ArrayList<HDTV>();
    al.add(tv1);
    al.add(tv2);
    al.add(tv3);

    Collections.sort(al, new SizeComparator());
    for (HDTV a : al) {
        System.out.println(a.getBrand());
    }
}

}

總結：
Comparable 是一個對象自己就已經支持自比較所須要實現的接口（如 String、Integer 本身就能夠完成比較大小操做，已經實現了Comparable接口）

用 Comparator 是策略模式（strategy design pattern），就是不改變對象自身，而用一個策略對象（strategy object）來改變它的行爲

參考：Comparable vs Comparator in Java

其餘資料：
Comparable與Comparator的區別

Java Sorting: Comparator vs Comparable Tutorial