List集合排序

直接使用Collections中的方法（主要是单属性）

如对String、Integer类型等非引用类型排序。List<String>集合

升序 Collections.sort(list)

public static void main(String[] args) {

    List<String> list = new ArrayList<>();
    list.add("abc");
    list.add("ecd");
    list.add("aaa");
    list.add("bef");
    list.add("cmc");
    list.add("cab");
    Collections.sort(list); //升序排序
    for (String s : list) {
        System.out.println(s);
    }
}

降序 Collections.reverse(list)

public static void main(String[] args) {

    List<String> list = new ArrayList<>();
    list.add("abc");
    list.add("ecd");
    list.add("aaa");
    list.add("bef");
    list.add("cmc");
    list.add("cab");
    Collections.sort(list); //升序排序
    for (String s : list) {
        System.out.println(s);
    }
    Collections.reverse(list); //降序排序
    for (String s : list) {
        System.out.println(s);
    }
}

结果

自定义排序规则（主要用于对象的排序）

如根据用户的年龄排序，对List<User>集合进行排序

如果需要降序，值需要改变规则即可，但是使用的还是Collections.sort()方法

方式一：实现Comparator接口

测试的list中Collections.sort()方法

Collections.sort(userList, new Comparator() {})

public static void main(String[] args) {
    List<User> userList = new ArrayList<>();
    userList.add(new User(19,"hello"));
    userList.add(new User(40,"hello2"));
    userList.add(new User(20,"hello3"));
    userList.add(new User(11,"hello4"));
    userList.add(new User(19,"hello5"));
    userList.add(new User(30,"hello6"));
    Collections.sort(userList, new Comparator<User>() {
        @Override
        public int compare(User o1, User o2) {
            if (o1.getAge() > o2.getAge()) {
                return 1;
            }else if (o1.getAge() < o2.getAge()){
                return -1;
            }else {
                return 0;
            }
        }
    });
    for (User user : userList) {
        System.out.println(user);
    }

}

User对象只是JavaBean

class User {
    int age;
    String name;

    public User(int age, String name) {
        this.age = age;
        this.name = name;
    }

    @Override
    public String toString() {
        return "User{" +
                "age=" + age +
                ", name='" + 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;
    }

}

结果：

升序结果

方式二：实现Comparable接口

测试数据正常排序

public static void main(String[] args) {
    List<User> userList = new ArrayList<>();
    userList.add(new User(19,"hello"));
    userList.add(new User(40,"hello2"));
    userList.add(new User(20,"hello3"));
    userList.add(new User(11,"hello4"));
    userList.add(new User(19,"hello5"));
    userList.add(new User(30,"hello6"));
    Collections.sort(userList);
    for (User user : userList) {
        System.out.println(user);
    }

}

User对象实现Comparable接口：

class User implements Comparable<User>{
    int age;
    String name;

    public User(int age, String name) {
        this.age = age;
        this.name = name;
    }

    @Override
    public String toString() {
        return "User{" +
                "age=" + age +
                ", name='" + 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;
    }

    @Override
    public int compareTo(User o) {
        if (o.age > age) {
            return -1;
        } else if (o.age < age) {
            return 1;
        } else {
            return 0;
        }
    }
}