Java HashMap

HashMap是java下的一个类
1. HashMap的创建和使用
HashMap<type, type> varName = new HashMap<>()
type只能是基本数据类型的封装类：Boolean, Character, Byte, Short, Integer, Long, Float, Double, String
HashMap<Integer, Integer> varName = HashMap<>()

2. 插入：put() & putAll()
varName.put(key, value)
putAll()方法，把一个HashMap中的元素加入到另一个HashMap

4. 删除：remove方法

可以单次删除一个元素

varName.remove(key)

5. 遍历：EntrySet() & keySet()

for(keyType key: varName.keySet()) {System.out.println(varName.getValue(key));}

6. 查询get(), containsKey()
varName.get(key)

if (varName.containsKey(key))

import java.util.HashMap;import java.util.Map;public class hMap {public static void main(String[] args) {HashMap<String, Integer> hMap_1 = new HashMap<String, Integer>();// put hMap_1.put("Bob", 90);hMap_1.put("Tom", 95);hMap_1.put("John", 100);System.out.println("original hashmap: " + hMap_1.toString());// removehMap_1.remove("John");System.out.println("after remove(): " + hMap_1.toString());// traveral by keySet()for (String key: hMap_1.keySet()) {System.out.println(key + " " + hMap_1.get(key));}// traveral by entrySet()for (Map.Entry<String, Integer> entry: hMap_1.entrySet()) {System.out.println(entry.getKey() + " = " + entry.getValue().toString());}// inquire: get, containSKeySystem.out.println(hMap_1.get("Tom"));if (!hMap_1.containsKey("UniqueZ_")) {hMap_1.put("UniqueZ_", 100);}}}