课程： https://www.imooc.com/video/21569

Map初识

Map接口及其实现类

Map接口通用方法

V put(K key, V value) # 存入一个key-value项

V get(K key) # 根据key值返回value值

V remove(Object key) # 根据key值删除Map中的一个key-value项

boolean containsKey(Object key) # 是否包含指定的key

HashMap的使用

HashMap的构造方法

HashMap()    # 使用默认初始容量(16)和默认加载因子(0.75)构造一个空HashMap   
HashMap(int initialCapacity)    # 使用指定的初始容量和默认加载因子(0.75)构造一个空HashMap
HashMap(int initialCapacity, float loadFactor)    # 使用指定初始容量和指定加载因子构造一个HashMap

HashMap的基本用法

// use the default initial capacity(16) and default loadFactor(0.75)
Map<String, Object> userMap = new HashMap<String, Object>(); 
// put an item(key-value) into the map
userMap.put("zs", new Integer(120));
// get an value from assigned key 
userMap.get("zs")

public class GeneralUsageInMap {
	public static void main(String[] args) {
		// use the default initial capacity(16) and default loadFactor(0.75)
		Map<String, Integer> userMap = new HashMap<String, Integer>(); 
		userMap.put("zs01", 120);
		userMap.put("zs02", 120);
		userMap.put("zs03", 120);
		userMap.put("zs04", 120);
		userMap.put("zs05", 111);
		
		Integer num02 = userMap.get("zs03");
		Integer num01 = userMap.get("zs05"); 
		System.out.println("num02: " + num02); // num02: 120
		System.out.println("num01: " + num01); // num01: 111
	}
}

HashMap的Entry结构

Entry底层实现

static class Entry<K, V> implements Map.Entry<K, V>{
  final K key; 
  V value;
  Entry<K, V> next;
  final int hash;
}

使用案例

public class GeneralUsageInMap {
	public static void main(String[] args) {
		// use the default initial capacity(16) and default loadFactor(0.75)
		Map<String, Integer> userMap = new HashMap<String, Integer>(); 
		userMap.put("zhang1", 2);
		userMap.put("zhang2", 4);
		userMap.put("zhang3", 3);
		userMap.put("zhang4", 1);
		userMap.put("zhang5", 5);
		
		// output the grades of "zhang2"
		int zhang2Grade = userMap.get("zhang2");
		System.out.println("zhang2's grade: " + zhang2Grade); // zhang2's grade: 4
		// default output the map
		System.out.println(userMap); // {zhang2=4, zhang3=3, zhang1=2, zhang4=1, zhang5=5}
		// output the content of the map according to the hash and address of each item of the map
	}
}

利用map.keySet()遍历HashMap

	private static void  outputMapWithKeySet(Map<String, Integer> userMap) {
		for(String key : userMap.keySet()) {
			System.out.println("key: " + key + " - " + "value: " + userMap.get(key));
		}
	}
//	key: zhang2 - value: 4
//	key: zhang3 - value: 3
//	key: zhang1 - value: 2
//	key: zhang4 - value: 1
//	key: zhang5 - value: 5

利用entrySet遍历HashMap

	private static void outputMapWithEntrySet(Map<String, Integer> userMap) {
		for(Map.Entry<String, Integer> entry: userMap.entrySet()) {
			System.out.println("Key: " + entry.getKey() + " - Value: " + entry.getValue());
		}
	}
//	Key: zhang2 - Value: 4
//	Key: zhang3 - Value: 3
//	Key: zhang1 - Value: 2
//	Key: zhang4 - Value: 1
//	Key: zhang5 - Value: 5