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

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

 public class Dog implements Cloneable,Comparable<Dog> {
 
     private String name;
     private String age;
 
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }
     public String getAge() {
         return age;
     }
     public void setAge(String age) {
         this.age = age;
     }
     public Dog(String name, String age) {
         super();
         this.name = name;
         this.age = age;
     }
     public Dog() {
         super();
     }
     @Override
     public String toString() {
         return "Dog [name=" + name + ", age=" + age + "]";
     }
 
     public Object clone() throws CloneNotSupportedException {
 
         return super.clone();
     }
     @Override
     public int compareTo(Dog o) {
 
         return this.getName().compareTo(o.getName());
     }
 }

测试类

 public class Test {
 
     public static void main(String[] args) {
         List<Dog> list = new ArrayList<Dog>();
         Dog d1 = new Dog("cac", "11");
         Dog d2 = new Dog("bac", "11");
         Dog d3 = new Dog("aac", "11");
         Dog d4 = new Dog("eac", "11");
         list.add(d1);
         list.add(d2);
         list.add(d3);
         list.add(d4);
 //        MyCompare mc = new MyCompare();
 //        Collections.sort(list,mc);
         Collections.sort(list);
         for (Dog dog : list) {
             System.out.println(dog);
         }
 
     }
 }

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

 public class Test {
 
     public static void main(String[] args) {
         List<Dog> list = new ArrayList<Dog>();
         Dog d1 = new Dog("cac", "11");
         Dog d2 = new Dog("bac", "11");
         Dog d3 = new Dog("aac", "11");
         Dog d4 = new Dog("eac", "11");
         list.add(d1);
         list.add(d2);
         list.add(d3);
         list.add(d4);
         MyCompare mc = new MyCompare();
         Collections.sort(list,mc);
         for (Dog dog : list) {
             System.out.println(dog);
         }
 
     }
 }
 
 class MyCompare implements Comparator<Dog>{
 
     @Override
     public int compare(Dog d1, Dog d2) {
         String s1 = d1.getName();
         String s2 = d2.getName();
         if(s1.equals(s2))
             return 0;
         else {
             return s1.compareTo(s2);
         }
     }
 
 }

结果：

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