【java】TreeSet、Comparable、Comparator、内部类、匿名类

package com.tn.treeSet;

public class Student {
    private String name;
    private int age;
    public Student(){}
    public Student(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + "]";
    }    
}

package com.tn.treeSet;

import java.util.TreeSet;

public class TreeSetDemo {
    public static void main(String[] args){
        TreeSet<Student> students=new TreeSet<>();//后一个尖括号内可以省略类型
        Student student1=new Student("武松",30);
        Student student2=new Student("林冲",31);
        Student student3=new Student("鲁智深",29);
        students.add(student1);
        students.add(student2);
        students.add(student3);
        System.out.println(students);
    }
}

 运行结果：

原因:TreeSet中加入的对象需要能进行比较，即实现Comparable接口

---

 改造一：在Student类中实现Comparable接口

package com.tn.treeSet;

public class Student implements Comparable<Student> {
    private String name;
    private int age;
    public Student(){}
    public Student(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + "]";
    }
    @Override
    public int compareTo(Student o) {
        if(!this.equals(o)){
//            return this.name.compareTo(o.name);
            return o.name.compareTo(this.name);//和上面语句打印顺序颠倒。
        }
        return 0;
    }    
}

package com.tn.treeSet;

import java.util.TreeSet;

public class TreeSetDemo {
    public static void main(String[] args){
        TreeSet<Student> students=new TreeSet<>();//后一个尖括号内可以省略类型
        Student student1=new Student("武松",30);
        Student student2=new Student("林冲",31);
        Student student3=new Student("鲁智深",29);
        students.add(student1);
        students.add(student2);
        students.add(student3);
        System.out.println(students);
    };//方法体结结尾大括号后有;不会报错
};//类体最后一个大括号后有;不会报错

---

改造二：用内部类实现Comparator接口

package com.tn.treeSet;

public class Student{
    private String name;
    private int age;
    public Student(){}
    public Student(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + "]";
    }
}

package com.tn.treeSet;

import java.util.Comparator;
import java.util.TreeSet;

public class TreeSetDemo {
    public static void main(String[] args){
        Student student1=new Student("武松",30);
        Student student2=new Student("林冲",31);
        Student student3=new Student("鲁智深",29);
        //TreeSet构造时用Comparator作为构造函数参数
        TreeSet<Student> students=new TreeSet<Student>(new ComparatorDemo());
        students.add(student1);
        students.add(student2);
        students.add(student3);
        System.out.println(students);
    }
    static class ComparatorDemo implements Comparator<Student>{
//        内部类要写在类体里，但不能写进类中方法体内。
        @Override
        public int compare(Student o1, Student o2) {
            return o1.getName().compareTo(o2.getName());
        }        
    }
}

---

 改造三：用匿名类实现Comparator接口

package com.tn.treeSet;

public class Student{
    private String name;
    private int age;
    public Student(){}
    public Student(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student [name=" + name + "]";
    }
}

package com.tn.treeSet;

import java.util.Comparator;
import java.util.TreeSet;

public class TreeSetDemo {
    public static void main(String[] args) {
        Student student1 = new Student("武松", 30);
        Student student2 = new Student("林冲", 31);
        Student student3 = new Student("鲁智深", 29);
        
        TreeSet<Student> students = new TreeSet<Student>(
                new Comparator<Student>() {
                    @Override
                    public int compare(Student o1, Student o2) {
                        return o1.getName().compareTo(o2.getName());
                    }
                });
        students.add(student1);
        students.add(student2);
        students.add(student3);
        System.out.println(students);
    }
}

 总结：

TreeSet容器中的对象要能排序，两种实现排序方法：

1.TreeSet使用无参构造函数，容器中的对象实现Comparable接口，见改造一；

2.TreeSet构造时使用Comparator作为构造函数参数；

　　比较方法如果返回0，则对象不能重复加入。

TreeSet底层是TreeMap