TreeMap和TreeSet在排序时如何比较元素，Collections工具类中的sort()方法如何比较元素

Posted 2020-10-25 qf123

TreeSet和TreeMap排序时比较元素要求元素对象必须实现Comparable接口

Collections的sort方法比较元素有两种方法：

元素对象实现Comparable接口

实体类Dog

 1 public class Dog implements Cloneable,Comparable<Dog> {
 2 
 3     private String name;
 4     private String age;
 5     
 6     public String getName() {
 7         return name;
 8     }
 9     public void setName(String name) {
10         this.name = name;
11     }
12     public String getAge() {
13         return age;
14     }
15     public void setAge(String age) {
16         this.age = age;
17     }
18     public Dog(String name, String age) {
19         super();
20         this.name = name;
21         this.age = age;
22     }
23     public Dog() {
24         super();
25     }
26     @Override
27     public String toString() {
28         return "Dog [name=" + name + ", age=" + age + "]";
29     }
30     
31     public Object clone() throws CloneNotSupportedException {
32         
33         return super.clone();
34     }
35     @Override
36     public int compareTo(Dog o) {
37         
38         return this.getName().compareTo(o.getName());
39     }
40 }

测试类

 1 public class Test {
 2 
 3     public static void main(String[] args) {
 4         List<Dog> list = new ArrayList<Dog>();
 5         Dog d1 = new Dog("cac", "11");
 6         Dog d2 = new Dog("bac", "11");
 7         Dog d3 = new Dog("aac", "11");
 8         Dog d4 = new Dog("eac", "11");
 9         list.add(d1);
10         list.add(d2);
11         list.add(d3);
12         list.add(d4);
13 //        MyCompare mc = new MyCompare();
14 //        Collections.sort(list,mc);
15         Collections.sort(list);
16         for (Dog dog : list) {
17             System.out.println(dog);
18         }
19         
20     }
21 }

结果：

Dog [name=aac, age=11]
Dog [name=bac, age=11]
Dog [name=cac, age=11]
Dog [name=eac, age=11]

自定义比较器，Collections.sort(List list，Comparator<T> compare)，创建比较器类实现接口

 1 public class Test {
 2 
 3     public static void main(String[] args) {
 4         List<Dog> list = new ArrayList<Dog>();
 5         Dog d1 = new Dog("cac", "11");
 6         Dog d2 = new Dog("bac", "11");
 7         Dog d3 = new Dog("aac", "11");
 8         Dog d4 = new Dog("eac", "11");
 9         list.add(d1);
10         list.add(d2);
11         list.add(d3);
12         list.add(d4);
13         MyCompare mc = new MyCompare();
14         Collections.sort(list,mc);
15         for (Dog dog : list) {
16             System.out.println(dog);
17         }
18         
19     }
20 }
21 
22 class MyCompare implements Comparator<Dog>{
23 
24     @Override
25     public int compare(Dog d1, Dog d2) {
26         String s1 = d1.getName();
27         String s2 = d2.getName();
28         if(s1.equals(s2))
29             return 0;
30         else {
31             return s1.compareTo(s2);
32         }
33     }
34 
35 }

结果：

Dog [name=aac, age=11]
Dog [name=bac, age=11]
Dog [name=cac, age=11]
Dog [name=eac, age=11]