集合框架之Map接口

• Map接口

特点：存储一对数据，无序、无下标，键不可重复，值可重复

package com.oop.demo11;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

/**
 * Map接口使用
 * 特点：存储键值对；键不能重复；值可以重复；无序
 */
public class Demo12 {
    public static void main(String[] args) {
        //创建Map集合
        Map<String,String> map=new HashMap<>();
        //1.添加元素
        map.put("cn","中国");
        map.put("uk","英国");
        map.put("usa","美国");
        System.out.println("元素个数："+map.size());
        System.out.println(map.toString());
        System.out.println("===========");
        //2.删除
        map.remove("usa");
        System.out.println("删除之后："+map.size());
        System.out.println(map.toString());
        System.out.println("==============");
        //3.遍历
        //3.1使用KeySet();
        System.out.println("=====使用KeySet()======");
        Set<String> keyset=map.keySet();
        for (String key:keyset
             ) {
           System.out.println(key+"----"+map.get(key));
        }
        //3.2使用entrySet();
        System.out.println("=====使用entrySet()======");
        Set<Map.Entry<String, String>> entries = map.entrySet();
        for (Map.Entry<String,String> entry:entries
             ) {
            System.out.println(entry.getKey()+"----"+entry.getValue());

        }
        //4.判断
        System.out.println("=====判断======");
        System.out.println(map.containsKey("uk"));
        System.out.println(map.containsValue("中国"));
        System.out.println(map.isEmpty());

    }
}

1. HashMap：重点，面试高频问点

允许用null作为key或是value

package com.oop.demo11;

import java.util.HashMap;
import java.util.Map;

/**
 * HashMap集合的使用
 * 存储结构：哈希表（数组+链表+红黑树）
 */
public class Demo13 {
    public static void main(String[] args) {
        //创建集合
        HashMap<Person,String> persons=new HashMap<Person,String>();
        //添加元素
        Person p1 = new Person("孙悟空",100);
        Person p2 = new Person("猪八戒",101);
        Person p3 = new Person("沙和尚",102);

        persons.put(p1,"北京");
        persons.put(p2,"上海");
        persons.put(p3,"杭州");
        System.out.println("元素个数："+persons.size());
        System.out.println(persons.toString());
        System.out.println("============");
        //删除元素
        persons.remove(p1);
        System.out.println("元素个数："+persons.size());
        System.out.println(persons.toString());
        System.out.println("=====使用keySet()遍历=====");
        //遍历
        //使用keySet()遍历
        for (Person key:persons.keySet()
             ) {
            System.out.println(key.toString()+"----"+persons.get(key));
        }
        System.out.println("=====使用entrySet()遍历=====");
        //使用entrySet()遍历
        for (Map.Entry<Person,String> entry:persons.entrySet()
             ) {
            System.out.println(entry.getKey()+"----"+entry.getValue());
        }
        System.out.println("=====判断=====");
        System.out.println(persons.containsKey(p2));
        System.out.println(persons.containsValue("上海"));
        System.out.println(persons.isEmpty());
    }
}


package com.oop.demo11;

public class Person {
    private String name;
    private int age;

    public Person() {
    }

    public Person(String name, int age) {
        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 "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

 

2.TreeMap

实现了SortedMap接口（是Map的子接口），可以对Key自动排序

package com.oop.demo11;

import java.util.Map;
import java.util.TreeMap;

/**
 * TreeMap的使用
 * 存储结构：红黑树
 */
public class Demo14 {
    public static void main(String[] args) {
        //创建集合
        TreeMap<Person, String> treeMap = new TreeMap<>();
        //添加元素
        Person p1 = new Person("孙悟空",100);
        Person p2 = new Person("猪八戒",101);
        Person p3 = new Person("沙和尚",102);

        treeMap.put(p1,"北京");
        treeMap.put(p2,"上海");
        treeMap.put(p3,"杭州");
        System.out.println(treeMap.size());
        System.out.println(treeMap.toString());
        System.out.println("----删除元素----");
        treeMap.remove(p1);
        System.out.println(treeMap.size());
        System.out.println(treeMap.toString());
        System.out.println("----使用keySet----");
        for (Person key:treeMap.keySet()
             ) {
            System.out.println(key+"----"+treeMap.get(key));
        }
        System.out.println("----使用entrySet----");
        for (Map.Entry<Person,String> entry:treeMap.entrySet()
             ) {
            System.out.println(entry.getKey()+"----"+entry.getValue());
        }

        System.out.println("----判断----");
        System.out.println(treeMap.containsKey(p2));
        System.out.println(treeMap.containsValue("杭州"));
        System.out.println(treeMap.isEmpty());
    }
}


package com.oop.demo11;

public class Person implements Comparable<Person> {
    private String name;
    private int age;

    public Person() {
    }

    public Person(String name, int age) {
        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 "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    @Override
    public int compareTo(Person o) {
        int n2=this.age-o.getAge();
        return n2;
    }
}