Map体系结构

Map父接口

存储一对数据（Key-Value），无序、无下标、键不可重复

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

public class Hello{
    public static void main(String[] args) {
        HashMap<String, String> map = new HashMap<>();

        //put()方法添加键值对
        map.put("cn", "中国");
        map.put("usa", "美国");
        map.put("uk", "英国");
        System.out.println(map);

        //两种遍历集合的方式
        //1.keySet()方法获得键
        for (String key : map.keySet()){

            //get()方法根据键获取值
            System.out.println(key + " " + map.get(key));
        }

        //2.entrySet()方法获得键值对
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println(entry);

            //getKey()和getValue()方法单独获取键和值
            System.out.println(entry.getKey() + " " + entry.getValue());
        }

        System.out.println(map.containsKey("cn"));
        System.out.println(map.containsValue("韩国"));

        //remove()方法删除键值对
        map.remove("usa");
        System.out.println(map);
    }
}

HashMap实现类

运行效率快，线程不安全；允许null作为键或值

存储结构：哈希表（数组+链表或数组+红黑树）

import java.util.HashMap;

public class Hello{
    public static void main(String[] args) {
        HashMap<Test, String> map = new HashMap<>();
        Test a = new Test("xm", 20);
        Test b = new Test("xh", 21);
        Test c = new Test("xw", 22);
        map.put(a, "Asia");
        map.put(b, "Eu");
        map.put(c, "India");
		
        //重写hashCode()和equals()方法以后，只要属性相同，就算重复
        map.put(new Test("xm", 20), "Asia");
        System.out.println(map.toString());
    }
}

class Test{
    String name;
    int age;

    public Test(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return name + "/" + age;
    }
	
    //IDEA右键，可以自动生成只判断属性的重写方法
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Test)) return false;

        Test test = (Test) o;

        if (age != test.age) return false;
        return name != null ? name.equals(test.name) : test.name == null;
    }

    @Override
    public int hashCode() {
        int result = name != null ? name.hashCode() : 0;
        result = 31 * result + age;
        return result;
    }
}

拓展：HashMap源码分析

//如果没有添加任何元素，容量为0；添加一个元素之后，数组长度变成16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

//当链表的长度大于8，且数组长度大于64时，链表转换为红黑树存储
static final int TREEIFY_THRESHOLD = 8;
static final int MIN_TREEIFY_CAPACITY = 64;

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        //每次扩容，容量增大2倍
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold
    }

拓展：HashMap和HashSet的联系

//从源码看，HashSet调用的就是HashMap
public HashSet() {map = new HashMap<>();}

TreeMap实现类

实现了SortedMap接口，可以对键自动排序

存储结构：红黑树

import java.util.TreeMap;

public class Hello{
    public static void main(String[] args) {
        TreeMap<Test, String> map = new TreeMap<>();
        Test a = new Test("xm", 20);
        Test b = new Test("xh", 21);
        Test c = new Test("xw", 22);
        map.put(a, "Asia");
        map.put(b, "Eu");
        map.put(c, "India");

        //重写hashCode()和equals()方法以后，只要键相同，就算重复；后面传入的值会覆盖原先的值
        map.put(new Test("xm", 20), "Eu");
        System.out.println(map.toString());
    }
}

//自定义类，不能自动排序，可以实现Comparab接口和重写compareTo()方法来自定义顺序
class Test implements Comparable<Test>{
    String name;
    int age;

    public Test(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return name + "/" + age;
    }

    @Override
    public int compareTo(Test o) {
        int n1 = this.name.compareTo(o.name);
        int n2 = this.age - o.age;
        return n1 == 0 ? n2 : n1;
    }
}

拓展：TreeMap和TreeSet的联系

TreeSet调用的就是TreeMap