Map接口之TreeMap

*TreeMap*

**在使用TreeSet或者TreeMap来保存自定义对象时，必须让自定义对象的类实现Comparable接口,并重写其compareTo()方法，否则会报cannot be cast to java.lang.Comparable异常。*

小案例

package com.treemappractice;

/**
 * 学生实体类
 */
public class Student implements Comparable<Student>{
    private String name;
    private int stuNo;

    public Student() {
    }

    public Student(String name, int stuNo) {
        this.name = name;
        this.stuNo = stuNo;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getStuNo() {
        return stuNo;
    }

    public void setStuNo(int stuNo) {
        this.stuNo = stuNo;
    }
    //去除重复 重写hashcode方法和equals方法
        //重写hashcode方法进行内存地址的判断
    @Override
    public int hashCode() {
        int n1 = this.name.hashCode();
        int n2 = this.stuNo;
        return n1 + n2;
    }

    //必须重写equals方法才能判断添加的对象是否重复
    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (obj instanceof Student) {
            Student person = (Student) obj;
            if (this.name.equals(person.getName()) && this.stuNo == person.getStuNo()) {
                return true;
            }
        }
        return false;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", stuNo=" + stuNo +
                '}';
    }

    public int compareTo(Student o) {
        int n2=this.stuNo-o.getStuNo();
        return n2;
    }
}

package com.treemappractice;

import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

/**
 * 联系TreeMap集合的使用
 * 存储结构：红黑树
 */
public class TreeMapDemo01 {
    public static void main(String[] args) {
        //1.添加元素 定制比较器
//        TreeMap<Student, String> treeMap = new TreeMap<Student, String>(new Comparator<Student>() {
//            public int compare(Student o1, Student o2) {
//                return 0;
//            }
//        });
        TreeMap<Student, String> treeMap = new TreeMap<Student, String>();
        Student stu1 = new Student("孙悟空", 100);
        Student stu2 = new Student("猪八戒", 101);
        Student stu3 = new Student("沙和尚", 102);
        treeMap.put(stu1,"金箍棒");
        treeMap.put(stu2,"菜耙子");
        treeMap.put(stu3,"降魔杖");
        System.out.println("元素的个数："+treeMap.size());
        System.out.println("集合内容："+treeMap);
        //2.删除
//        treeMap.remove(stu1);
//        System.out.println("删除之后元素的个数："+treeMap.size());
//        System.out.println("删除之后集合内容："+treeMap);
        //3.遍历
        System.out.println("==========keySet==========");
        for (Student key : treeMap.keySet()) {
            System.out.println(key+"--"+treeMap.get(key));
        }
        System.out.println("==========entrySet==========");
        for (Map.Entry<Student, String> stringEntry : treeMap.entrySet()) {
            System.out.println(stringEntry.getKey()+"--"+stringEntry.getValue());
        }
    }
}

运行结果：
元素的个数：3
集合内容：{Student{name='孙悟空', stuNo=100}=金箍棒, Student{name='猪八戒', stuNo=101}=菜耙子, Student{name='沙和尚', stuNo=102}=降魔杖}
==========keySet==========
Student{name='孙悟空', stuNo=100}--金箍棒
Student{name='猪八戒', stuNo=101}--菜耙子
Student{name='沙和尚', stuNo=102}--降魔杖
==========entrySet==========
Student{name='孙悟空', stuNo=100}--金箍棒
Student{name='猪八戒', stuNo=101}--菜耙子
Student{name='沙和尚', stuNo=102}--降魔杖