Java基础教程之Map遍历的5种方式

    HashMap<Integer, String> map = new HashMap<>();
        map.put(1,"ljj");
        map.put(2,"zsy");
        map.put(3,"sxf");
        map.put(4,"wjx");
    }

//Iterator 迭代器遍历
    public static void iterator(Map<Integer, String> map){
        System.out.println("---使用entrySet()迭代器进行遍历map集合---");
        Iterator<Map.Entry<Integer, String>> entryIterator = map.entrySet().iterator();
        while (entryIterator.hasNext()){
            Map.Entry<Integer, String> entry = entryIterator.next();
            System.out.println(entry.getKey()+":"+entry.getValue());
        }
        System.out.println("---使用KeySet()迭代器进行遍历map集合---");
        Iterator<Integer> keySetIterator = map.keySet().iterator();
        while (keySetIterator.hasNext()){
            Integer key = keySetIterator.next();
            System.out.println(key+":"+map.get(key));
        }

    }

 //For Each方式遍历
    public static void forEach_map(Map<Integer,String> map){
        System.out.println("map.forEach(BiConsumer action)方法遍历map");
        map.forEach((key,value) -> System.out.println(key+" : "+value));
        
    }

 //values()方法
    public static void Collection_forEach(Map<Integer,String> map){
        System.out.println("获取map集合的values值集合对象，进行forEach遍历");
        //Map中的values()方法，通过这个方法可以直接获取Map中存储所有值的Collection集合
        Collection<String> values = map.values();
        values.forEach( v -> System.out.println(v));
    }

    public static void for_plus(Map<Integer,String> map){
        System.out.println("增强for循环，Map.Entry<>");
        for (Map.Entry<Integer,String> entry : map.entrySet()){
            String value = entry.getValue();
            System.out.println(value);
        }
        System.out.println("增强for循环，map.keySet");
        for (Integer key : map.keySet()){
            String value = map.get(key);
            System.out.println(value);
        }
    }

//Stream流遍历
    public static void stream_list(Map<Integer,String> map){
        System.out.println("map.entrySet().stream.forEach()遍历—Stream流遍历");
        map.entrySet().stream().forEach((Map.Entry<Integer,String> entry) -> {
            System.out.print(entry.getKey()+":");
            System.out.println(entry.getValue());
        });
        System.out.println("map.keySet().stream.forEach()遍历—Stream流遍历");
        map.keySet().stream().forEach(key -> {
            System.out.println(map.get(key));
        });
    }

Map map = new HashMap<>();
Random random = new Random();
for (int i = 0; i < 1000000; i++) {
    map.put(random.nextInt(1000000), "some value");
}

long start = System.currentTimeMillis();
for (Map.Entry entry : map.entrySet()) {
    // do nothing
}
long end = System.currentTimeMillis();
System.out.println("遍历entrySet耗时：" + (end - start) + "ms");

start = System.currentTimeMillis();
for (Integer key : map.keySet()) {
    // do nothing
}
end = System.currentTimeMillis();
System.out.println("遍历keySet耗时：" + (end - start) + "ms");

start = System.currentTimeMillis();
for (String value : map.values()) {
    // do nothing
}
end = System.currentTimeMillis();
System.out.println("遍历values耗时：" + (end - start) + "ms");