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初始化和增删改查的实现 1.初始化 Map map = new HashMap(); 2.加元素 map.put(key,value) 3.删除元素 map.remove(key); 4.查元素 map,get(key); 5.清空元素 map.clear() 遍历 1.增强型for循环遍历 使用keySet()遍历 for(String key : map.keySet()) { System,out,println(key+":"+map.get(key)); } 使用entrySet()遍历 for(Map.EntrySet entry : map.entrySet()) { System,out,println(entry.getKey()+":"+entry.getValue()); } 2.迭代器遍历 使用keySet()遍历 Iterator it =map.keySet().Iterator(); while(it.hasNext()) { String key = it.next(); System.out.println(key+":"+map.get(key)); } 使用entrySet()进行遍历 Iterator it = map.entrySet().iterator(); while(it.hasNext()) { Map entry = it.next(); System.out.println(entry.getKey()+":"+entry.getValue()); } 测试代码 import java.util.HashMap; import java.util.Iterator; import java.util.Map; import java.util.Map.Entry;
public class TestMap {
public static void main(String[] args) {
// 初始化，10W次赋值
Map map = new HashMap();
for (int i = 0; i < 100000; i++)
map.put(i, i);
/** 增强for循环，keySet迭代 */
long start = System.currentTimeMillis();
for (Integer key : map.keySet()) {
map.get(key);
}
long end = System.currentTimeMillis();
System.out.println("增强for循环，keySet迭代 -> " + (end - start) + " ms");
/** 增强for循环，entrySet迭代 */
start = System.currentTimeMillis();
for (Entry entry : map.entrySet()) {
entry.getKey();
entry.getValue();
}
end = System.currentTimeMillis();
System.out.println("增强for循环，entrySet迭代 -> " + (end - start) + " ms");
/** 迭代器，keySet迭代 */
start = System.currentTimeMillis();
Iterator iterator = map.keySet().iterator();
Integer key;
while (iterator.hasNext()) {
key = iterator.next();
map.get(key);
}
end = System.currentTimeMillis();
System.out.println("迭代器，keySet迭代 -> " + (end - start) + " ms");
/** 迭代器，entrySet迭代 */
start = System.currentTimeMillis();
Iterator iterator1 = map.entrySet().iterator();
Map.Entry entry;
while (iterator1.hasNext()) {
entry = iterator1.next();
entry.getKey();
entry.getValue();
}
end = System.currentTimeMillis();
System.out.println("迭代器，entrySet迭代 -> " + (end - start) + " ms");
}
} 结果: 增强for循环，keySet迭代 -> 13 ms 增强for循环，entrySet迭代 -> 8 ms 迭代器，keySet迭代 -> 10 ms 迭代器，entrySet迭代 -> 6 ms entrySet比keySet快的原因是:entrySet直接返回了key和value的对应关系对,而keySet需要再去Map中查找一次. 参考:https://www.cnblogs.com/lzq198754/p/5780165.html
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