List集合序列排序的两种方法

首先讲一下Comparable接口和Comparator接口,以及他们之间的差异。有助于Collections.sort()方法的使用。请参考

1.Comparable自然规则排序
//在自定义类Student里面实现Comparable接口,并重写抽象方法compareTo(Student o);
//Collections.sort(集合);

先看一个简单的例子:

    public static void main(String[] args) {  
        List nums = new ArrayList();  
            nums.add(3);  
            nums.add(5);  
            nums.add(1);  
            nums.add(0);  
            System.out.println(nums);  
            Collections.sort(nums);  
            System.out.println(nums);  
    }  

输出结果:
[3, 5, 1, 0]
[0, 1, 3, 5]

稍微复杂的List里面放一个复杂的对象

package core.java.collection.collections;  
  
public class User implements Comparable{  
      
    private int score;  
      
    private int age;  
      
    public User(int score, int age){  
        super();  
        this.score = score;  
        this.age = age;  
    }  
  
    public int getScore() {  
        return score;  
    }  
  
    public void setScore(int score) {  
        this.score = score;  
    }  
  
    public int getAge() {  
        return age;  
    }  
  
    public void setAge(int age) {  
        this.age = age;  
    }  
  
    @Override  
    public int compareTo(User o) {  
        int i = this.getAge() - o.getAge();//先按照年龄排序  
        if(i == 0){  
            return this.score - o.getScore();//如果年龄相等了再用分数进行排序  
        }  
        return i;  
    }  
      
}  
  
public static void main(String[] args) {  
        List users = new ArrayList();  
        users.add(new User(78, 26));  
        users.add(new User(67, 23));  
        users.add(new User(34, 56));  
        users.add(new User(55, 23));  
        Collections.sort(users);  
        for(User user : users){  
            System.out.println(user.getScore() + "," + user.getAge());  
        }  
} 

输出结果:
55,23
67,23
78,26
34,56

我们会发现sort(List)方法中List中的T必须实现Comparable接口,然后实现compareTo()方法,该方法的返回值0代表相等,正数表示大于,负数表示小于;为什么在简单例子中没有看到实现Comparable接口呢?是因为Integer类其实自己已经实现了Comparable接口,Java已经给我们做好了。

2.Comparator专门规则排序(l临时排序)
//新建一个实现了Comparator接口的类,并重写抽象方法compare(Student o1, Student o2);
//Collections.sort(集合,实现了Comparator接口的类的实例化对象);

Collections提供的第二种排序方法sort(List list, Comparator c)
先看例子:

package core.java.collection.collections;  
  
public class Students {  
      
    private int age;  
    private int score;  
      
    public Students(int age, int score){  
        super();  
        this.age = age;  
        this.score = score;  
    }  
      
    public int getAge() {  
        return age;  
    }  
    public void setAge(int age) {  
        this.age = age;  
    }  
    public int getScore() {  
        return score;  
    }  
    public void setScore(int score) {  
        this.score = score;  
    }  
}  
public static void main(String[] args) {  
        List students = new ArrayList();  
        students.add(new Students(23, 100));  
        students.add(new Students(27, 98));  
        students.add(new Students(29, 99));  
        students.add(new Students(29, 98));  
        students.add(new Students(22, 89));  
        Collections.sort(students, new Comparator() {  
  
            @Override  
            public int compare(Students o1, Students o2) {  
                int i = o1.getScore() - o2.getScore();  
                if(i == 0){  
                    return o1.getAge() - o2.getAge();  
                }  
                return i;  
            }  
        });  
        for(Students stu : students){  
            System.out.println("score:" + stu.getScore() + ":age" + stu.getAge());  
        }  
}

输出结果:
score:89:age22
score:98:age27
score:98:age29
score:99:age29
score:100:age23

从上面的例子我们可以看出Students类没有实现Comparable接口,只是在sort()方法中多传入一个参数,只不过该参数是一个接口我们需要实现其compare方法。
 

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