TreeMap和TreeSet在排序時如何比較元素，Collections工具類中的sort()方法如何比較元素

TreeSet和TreeMap排序時比較元素要求元素對象必須實現Comparable接口

Collections的sort方法比較元素有兩種方法：

元素對象實現Comparable接口

實體類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());
    }
}

測試類

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);
        }
        
    }
}

結果：

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

自定義比較器，Collections.sort(List list，Comparator<T> compare)，創建比較器類實現接口

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);
        }
    }

}

結果：

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