list集合能不能重复（List集合序列排序的两种方法）

//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<Integer> nums = new ArrayList<Integer>(); 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<User>{ 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<User> users = new ArrayList<User>(); 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<T>)方法中List中的T必须实现Comparable<T>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。

//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<T> list, Comparator<? super T> 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> students = new ArrayList<Students>(); 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<Students>() { @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<T>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。