Map的遍历

1.通过map.entrySet遍历Key和Value



Map<Integer,Integer> map = new HashMap<>();
        map.put(1, 10);
        map.put(2, 8);
for(Map.Entry<Integer, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + " ---- " + entry.getValue() );
}


如果你遍历的Map是一个空的对象(null)，for-each循环将会抛出java.lang.NullPointerException异常

2.通过map.keySet()来遍历key，通过map.values()来遍历Value，适用于只需要map中的Key或者Value的情况



Map<Integer,Integer> map = new HashMap<>();
        map.put(1, 10);
        map.put(2, 8);
        for (Integer i : map.keySet()) {
            System.out.println(i);
        }

        for ( Integer i : map.values()) {
            System.out.println(i);
        }


3.通过Iterator遍历Map



//使用泛型
Map<Integer,Integer> map = new HashMap<>();
        map.put(1, 10);
        map.put(2, 8);
        Iterator<Map.Entry<Integer, Integer>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<Integer, Integer> temp = iterator.next();
            System.out.println(temp.getKey() + " ----- " + temp.getValue());
        }
//不使用泛型
Map<Integer,Integer> map = new HashMap<>();
map.put(1, 10);
map.put(2, 8);
Iterator iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry entry = (Map.Entry) iterator.next();
Integer key = (Integer) entry.getKey();
Integer value = (Integer) entry.getValue();
System.out.println(key + " ----- " + value);
}


4. 通过key找value(效率低)



Map<Integer,Integer> map = new HashMap<>();
        map.put(1, 10);
        map.put(2, 8);
        for ( Integer key : map.keySet()) {
            Integer value = map.get(key);
            System.out.println(key + " ----- " + value);
        }


 List的遍历

1.方式一



List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        for ( Integer i : list) {
            System.out.println(i);
        }


2.方式二



List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        Iterator iterator = list.iterator();
        while(iterator.hasNext()) {
            System.out.println(iterator.next());
        }


3.方式三



List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        for ( int i = 0 ; i < list.size() ; i++ ) {
            System.out.println(list.get(i));
        }