Map中经常被忽略但又非常好用的方法
1. 简介map是我们日常开发中常会的集合类之一, 但是我们除了常用的get和put之外,其他的方法好像很少会用到,接下来我们就介绍一下几个经常被忽略但又很好用的方法.
2. Quick Start
2.1 数据准备
创建一个map对象, 并声明几个用于测试的user对象
Map<Integer, User> hashMap = Maps.newHashMap();User zhangsan = new User(1, "张三");User lisi = new User(2, "李四");User zhangtieniu = new User(3, "张铁牛");2.2 重温put
// hashmap put (添加/更新元素)@Testpublic void put() { User test = hashMap.put(null, null); User user = hashMap.put(1, null); User user1 = hashMap.put(1, zhangsan); User user2 = hashMap.put(1, lisi); User user3 = hashMap.put(null, zhangsan); User user4 = hashMap.put(null, lisi); User user5 = hashMap.get(null); log.info("map: {}", hashMap); log.info("user: {}, user1: {}, user2: {}, user3: {}, user4: {}, user5: {}", user, user1, user2, user3, user4, user5); //map: {null=User(id=2, name=李四), 1=User(id=2, name=李四)} //user: null, user1: null, user2: User(id=1, name=张三), user3: null, user4: User(id=1, name=张三), user5: User(id=2, name=李四)}
[*]key和value可以为null (hashmap 和 linkedhashmap)
[*]使用null可以正常的覆盖和获取元素
[*]put可以直接新增or覆盖已有的元素
[*]put方法返回对应key的oldValue，如果没有oldValue则返回null
2.3 getOrDefault
// getOrDefault(Object key, V defaultValue) (获取/返回默认值)@Testpublic void getOrDefault() { hashMap.put(1, zhangsan); hashMap.put(2, null); final User user1 = hashMap.get(1); final User user2 = hashMap.getOrDefault(2, lisi); final User user3 = hashMap.getOrDefault(3, zhangtieniu); log.info("map: {}", hashMap); //map: {1=User(id=1, name=张三), 2=null} log.info("user1: {}, user2: {}, user3: {}", user1, user2, user3); //user1: User(id=1, name=张三), user2: null, user3: User(id=3, name=张铁牛)}
[*]当map中没有对应的key时, 返回对应的defaultValue
注意: 如果map中存在对应的key, 但是对应的value == null时, 返回的是null, 而不是defaultValue
源码如下:
default V getOrDefault(Object key, V defaultValue) { V v; return (((v = get(key)) != null) || containsKey(key)) ? v : defaultValue;}2.4 putIfAbsent
// putIfAbsent(K key, V value) (如果不存在则添加)@Testpublic void putIfAbsent() { hashMap.put(1, null); User user1 = hashMap.putIfAbsent(1, zhangsan); User user2 = hashMap.putIfAbsent(2, lisi); User user3 = hashMap.putIfAbsent(2, zhangtieniu); log.info("map: {}", hashMap); log.info("user1: {}, user2: {}, user3: {}", user1, user2, user3); //map: {1=User(id=1, name=张三), 2=User(id=2, name=李四)} //user1: null, user2: null, user3: User(id=2, name=李四)}
[*]如果指定的key对应的value不为null时(oldValue != null) : 不覆盖 & 返回oldValue
[*]当指定key的value不存在时(oldValue == null) : 添加元素 & 返回oldValue
可以理解为 当指定key的value不存在时, 才去put, 否则不添加
源码如下:
default V putIfAbsent(K key, V value) { V v = get(key); if (v == null) { v = put(key, value); } return v;}2.5 compute
// compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) (计算)@Testpublic void compute() { hashMap.put(1, zhangsan); User user1 = hashMap.compute(1, (k, oldValue) -> lisi); log.info("map: {}, user1: {}", hashMap, user1); //map: {1=User(id=2, name=李四)}, user1: User(id=2, name=李四) User user2 = hashMap.compute(1, (k, oldValue) -> null); log.info("map: {}, user2: {}", hashMap, user2); //map: {}, user2: null}
[*]remappingFunction返回值 != null : 覆盖oldValue & 返回newValue
[*]remappingFunction返回值 == null : 删除对应元素 & 返回null
可以理解为 使用remappingFunction的返回值覆盖对应key的旧值, 当remappingFunction返回值为null时, 会直接将当前元素移除掉
源码如下:
default V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) { Objects.requireNonNull(remappingFunction); V oldValue = get(key); V newValue = remappingFunction.apply(key, oldValue); if (newValue == null) { // delete mapping if (oldValue != null || containsKey(key)) { // something to remove remove(key); return null; } else { // nothing to do. Leave things as they were. return null; } } else { // add or replace old mapping put(key, newValue); return newValue; }}2.6 computeIfAbsent
// computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) (不存在则计算)@Testpublic void computeIfAbsent() { User user = hashMap.computeIfAbsent(1, k -> zhangsan); User user1 = hashMap.computeIfAbsent(1, k -> lisi); User user2 = hashMap.computeIfAbsent(2, k -> null); log.info("map: {}, user：{}, user1: {}, user2:{}", hashMap, user, user1, user2); //map: {1=User(id=1, name=张三)}, user：User(id=1, name=张三), user1: User(id=1, name=张三), user2:null}
[*]oldValue != null : 不覆盖 & 返回oldValue
[*]oldValue == null && mappingFunction返回值 != null: 添加元素 & 返回newValue
[*]oldValue == null && mappingFunction返回值 == null: 不覆盖 & 返回null
可以理解为 当指定key的value不存在时, 才使用mappingFunction的返回值覆盖对应key的旧值, 如果key对应value存在或者mappingFunction的返回值为null时, 则不覆盖
源码如下:
default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) { Objects.requireNonNull(mappingFunction); V v; if ((v = get(key)) == null) { V newValue; if ((newValue = mappingFunction.apply(key)) != null) { put(key, newValue); return newValue; } } return v;}2.7 computeIfPresent
// computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) (存在则计算)@Testpublic void computeIfPresent() { hashMap.put(1, zhangsan); User user1 = hashMap.computeIfPresent(1, (k,oldValue) -> lisi); User user2 = hashMap.computeIfPresent(3, (k,oldValue) -> zhangtieniu); log.info("map: {}, user1: {}， user2: {}", hashMap, user1, user2); //map: {1=User(id=2, name=李四)}, user1: User(id=2, name=李四)， user2: null User user3 = hashMap.computeIfPresent(1, (k,oldValue) -> null); log.info("map: {}, user3:{}", hashMap, user3); //map: {}, user3:null}
[*]oldValue == null : 不覆盖&返回null
[*]oldValue != null && remappingFunction返回值 == null : 移除元素&返回null
[*]oldValue != null && remappingFunction返回值 != null : 覆盖元素&返回newValue
可以理解为 当key对应的value存在时, 才使用remappingFunction的返回值覆盖对应key的旧值, 如果key对应的value不存在或者remappingFunction的返回值为null时, 则不覆盖
源码如下:
default V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) { Objects.requireNonNull(remappingFunction); V oldValue; if ((oldValue = get(key)) != null) { V newValue = remappingFunction.apply(key, oldValue); if (newValue != null) { put(key, newValue); return newValue; } else { remove(key); return null; } } else { return null; }}2.8 replace
// replace(K key, V value)// replace(K key, V oldValue, V newValue) (替换)@Testpublic void replace() { hashMap.put(1, zhangsan); hashMap.put(2, lisi); hashMap.replace(1, zhangtieniu); hashMap.replace(2, null); hashMap.replace(3, zhangtieniu); hashMap.replace(2, null, zhangtieniu); log.info("map: {}", hashMap); //map: {1=User(id=3, name=张铁牛), 2=User(id=3, name=张铁牛)}}
[*]替换指定key的value值
[*]可以将对应的value设置为null
[*]对应的key不存在时不会添加新元素
[*]replace(K key, V oldValue, V newValue)方法多了一层判断, 当key对应的value与oldValue相等时, 才会替换newValue
可以理解为 替换指定key的value值
源码如下:
default V replace(K key, V value) { V curValue; if (((curValue = get(key)) != null) || containsKey(key)) { curValue = put(key, value); } return curValue;}default boolean replace(K key, V oldValue, V newValue) { Object curValue = get(key); if (!Objects.equals(curValue, oldValue) || (curValue == null && !containsKey(key))) { return false; } put(key, newValue); return true;}2.9 merge
// merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) (融合)@Testpublic void merge() { hashMap.put(1, zhangsan); User user1 = hashMap.merge(1, lisi, (oldValue, defaultValue) -> zhangtieniu); User user2 = hashMap.merge(2, lisi, (oldValue, defaultValue) -> zhangtieniu); log.info("map: {}, user1: {}， user2: {}", hashMap, user1, user2); //map: {1=User(id=3, name=张铁牛), 2=User(id=2, name=李四)}, user1: User(id=3, name=张铁牛)， user2: User(id=2, name=李四)}
[*]oldValue == null : 使用传进来的 value 作为newValue, oldValue != null: 使用remappingFunction的返回值作为newValue
[*]newValue == null : 移除元素 & 返回newValue
[*]newValue != null : 覆盖元素 & 返回newValue
可以理解为 融合三个值 分别为:
[*]key对应的value(oldValue)
[*]merge方法的第二个参数value (可以理解为oldValue的defaultValue)
[*]merge方法的第三个参数remappingFunction方法的返回值
融合逻辑为:如果key对应的value不存在时, 使用merge方法的第二个参数value作为newValue, 如果key对应的value存在时,使用remappingFunction的返回值作为newValue, 如果newValue不为null则覆盖元素, 为null则移除元素
源码如下:
default V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) { Objects.requireNonNull(remappingFunction); Objects.requireNonNull(value); V oldValue = get(key); V newValue = (oldValue == null) ? value : remappingFunction.apply(oldValue, value); if(newValue == null) { remove(key); } else { put(key, newValue); } return newValue; }}2.10 foreach
// foreach java8 新增@Testpublic void foreach() { hashMap.put(1, zhangsan); hashMap.put(2, lisi); hashMap.forEach((key, value) -> log.info("key: {}, value: {}", key, value)); //key: 1, value: User(id=1, name=张三) //key: 2, value: User(id=2, name=李四)}3. 总结
方法名称方法参数方法描述方法特点putkey, value添加元素hashmap/linkedhashmap: key, value 都可以为nullgetOrDefaultkey, defaultValue获取元素当map中没有对应的key时, 返回defaultValueputIfAbsentkey, value当不存在时添加元素这里不存在指的是: key对应的旧值为nullcomputekey, BiFunction<key, oldValue, newValue> remappingFunction重新计算key对应的value使用remappingFunction的返回值替换key的旧值computeIfAbsentkey, Function<key, value> mappingFunction当不存在时计算这里不存在指的是:key对应的旧值为null, 与putIfAbsent方法逻辑类似computeIfPresentkey, BiFunction<key, oldValue, newValue> remappingFunction当存在时计算这里存在指的是: key对应的旧值!=nullreplacekey, value替换元素替换指定key的value, 不会添加元素mergekey, value, BiFunction<oldValue, value, newValue> remappingFunction融合融合key的旧值, 默认值, remappingFunction的返回值作为新值foreachBiConsumer<key, value>遍历java8新加的遍历方式
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