Java8遍历Map、Map转List、List转Map

﹏ヽ暗。殇╰゛Y 2023-10-06 09:59 103阅读 0赞

##### 遍历Map #####

Map<Integer, String> map = new HashMap<>();
    map.put(1, "a");
    map.put(2, "b");
    map.put(3, "c");
    
    // Map.keySet遍历
    for (Integer k : map.keySet()) {
    	System.out.println(k + " ==> " + map.get(k));
    }
    
    map.keySet().forEach(k -> System.out.println(k + " ==> " + map.get(k)));
    
    // Map.entrySet遍历，推荐大容量时使用
    for (Map.Entry<Integer, String> entry : map.entrySet()) {
    	System.out.println(entry.getKey() + " ==> " + entry.getValue());
    }
    
    map.entrySet().forEach(entry -> System.out.println(entry.getKey() + " ==> " + entry.getValue()));
    
    // Iterator遍历
    Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();
    while (it.hasNext()) {
    	Map.Entry<Integer, String> entry = it.next();
    	System.out.println(entry.getKey() + " ==> " + entry.getValue());
    }
    
    map.entrySet().iterator()
    		.forEachRemaining(entry -> System.out.println(entry.getKey() + " ==> " + entry.getValue()));
    
    // 遍历values
    for (String v : map.values()) {
    	System.out.println(v);
    }
    map.values().forEach(System.out::println);
    
    // Java8 Lambda
    map.forEach((k, v) -> System.out.println(k + " ==> " + v));

##### Map转List #####

@Data
    @NoArgsConstructor
    @AllArgsConstructor
    class KeyValue {
    	private Integer key;
    	private String value;
    
    	@Override
    	public String toString() {
    		return key + " ==> " + value;
    	}
    
    }

Map<Integer, String> map = new HashMap<>();
    map.put(1, "a");
    map.put(2, "b");
    map.put(3, "c");
    
    // key 转 List
    List<Integer> keyList = new ArrayList<>(map.keySet());
    List<Integer> keyList2 = map.keySet().stream().collect(Collectors.toList());
    
    keyList.forEach(System.out::println);
    keyList2.forEach(System.out::println);
    
    // value 转 List
    List<String> valueList = new ArrayList<>(map.values());
    List<String> valueList2 = map.values().stream().collect(Collectors.toList());
    
    valueList.forEach(System.out::println);
    valueList2.forEach(System.out::println);
    
    // Iterator转List
    List<KeyValue> keyValueList = new ArrayList<>();
    Iterator<Integer> it = map.keySet().iterator();
    while (it.hasNext()) {
    	Integer k = (Integer) it.next();
    	keyValueList.add(new KeyValue(k, map.get(k)));
    }
    
    keyValueList.forEach(System.out::println);
    
    // Java8 Stream
    List<KeyValue> list = map.entrySet().stream().map(c -> new KeyValue(c.getKey(), c.getValue()))
    		.collect(Collectors.toList());
    list.forEach(System.out::println);

##### List转Map #####

List<KeyValue> list = new ArrayList<>();
    list.add(new KeyValue(1, "a"));
    list.add(new KeyValue(2, "b"));
    list.add(new KeyValue(3, "c"));
    
    // 遍历
    Map<Integer, String> keyValueMap = new HashMap<>();
    for (KeyValue keyValue : list) {
    	keyValueMap.put(keyValue.getKey(), keyValue.getValue());
    }
    keyValueMap.forEach((k, v) -> System.out.println(k + " ==> " + v));
    
    // Java8 Stream
    Map<Integer, String> map = list.stream().collect(Collectors.toMap(KeyValue::getKey, KeyValue::getValue));
    map.forEach((k, v) -> System.out.println(k + " ==> " + v));