HashMap
HashM
/* * 常用方法:compute、computeIfAbsent、computeIfPresent、merge * containsKey、containsValue、get、 * put、putAll、clear、remove、isEmpty、size * entrySet(可进行流操作)、keySet、Values(返回map中所有的value组成的集合),三者都是map的view,remove等操作会影响原集合 */ Map<String, String> map = new HashMap<>(); /* 清空map中的数据 */ map.clear(); map.put("key1", "value"); map.put("key2", null); map.put(null, null); System.out.println(map); List<String> list = map.entrySet().stream().filter(entry -> "key1".equals(entry.getValue())) .map(Map.Entry::getValue).collect(Collectors.toList()); /* * compute * 1、key有值,value有值,map中的value更新为新值,返回新value * 2、key为null或者有值,value为null,该key对应的map映射为新的value,返回新value * 3、key不存在,map添加该对key、value,返回新value * 4、如果新value为null,则删除对应的key、value, * If the remapping function returns null, the mapping is removed (or remains absent if initially absent). */ map.compute("key1", (key, value) -> value + " new compute value1"); map.compute("key2", (key, value) -> value + " new compute value2"); map.compute(null, (key, value) -> value + " new compute null"); map.compute("key3", (key, value) -> value + " new compute value3"); map.compute("new key", (key, value) -> { if (value == null) { value = "null value " + 0; } else { value = value + 1; } return value; }); System.out.println(map); map.compute("key2", (key, value) -> null); System.out.println(map); /* * computeIfAbsent * 1、key有值,value有值,map中的值不做改变,返回原value * 2、key为null或者有值,value为null,该key对应的map映射为新的value,返回新value * 3、key不存在,map添加该对key、value,返回新value * 4、新value为null,对原map不起作用, * If the specified key is not already associated with a value (or is mapped to null), * attempts to compute its value using the given mapping function and enters it into this map unless null. * If the mapping function returns null, no mapping is recorded. */ map.clear(); map.put("key1", "value1"); map.put("key2", null); map.put(null, null); System.out.println(map); map.computeIfAbsent("key1", key -> " computeIfAbsent new value1"); map.computeIfAbsent("key2", key -> " computeIfAbsent new value2"); map.computeIfAbsent(null, key -> " computeIfAbsent new value null"); map.computeIfAbsent("key3", key -> " computeIfAbsent new value3"); System.out.println(map); map.computeIfAbsent("key2", key -> null); map.computeIfAbsent("key4", key -> null); System.out.println(map); /* * computeIfPresent * 1、key有值或为null,value有有效值,map中的value更新为新value,如果新value为null,则删除对应key、value * 2、key为null或者有值,value为null,map不更新,返回null * 3、key不存在,map不更新,返回null * If the value for the specified key is present and non-null, * attempts to compute a new mapping given the key and its current mapped value. * If the remapping function returns null, the mapping is removed */ map.clear(); map.put("key1", "value1"); map.put("key2", null); map.put(null, ""); System.out.println(map); map.computeIfPresent("key1", (key, value) -> value + " computeIfPresent new value1"); map.computeIfPresent("key2", (key, value) -> value + " computeIfPresent new value2"); map.computeIfPresent(null, (key, value) -> value + " computeIfPresent new value null"); //如果原value无效,则不更新 map.computeIfPresent("key3", (key, value) -> value + " computeIfPresent new value3"); System.out.println(map); map.computeIfPresent("key2", (key, value) -> null); System.out.println(map); /* * merge * newValue不能为空 * 如果原value为null,直接更新为新value,不用remappingFunction返回的值 * remappingFunction返回null,删除map中对应的key、value * If the specified key is not already associated with a value or is associated with null, * associates it with the given non-null value. * Otherwise, replaces the associated value with the results of the given remapping function, * or removes if the result is null. */ map.clear(); map.put("key1", "value1"); map.put("key2", null); map.put(null, ""); System.out.println(map); map.merge("key1", " merge new value1", (oldValue, newValue) -> oldValue + newValue); map.merge("key2", " merge new value2", (oldValue, newValue) -> "return merge new value2"); map.merge(null, " merge new value null", (oldValue, newValue) -> oldValue + newValue); map.merge("key3", " merge new value3", (oldValue, newValue) -> oldValue + newValue); System.out.println(map); //map.merge("key2", null, (oldValue, newValue) -> newValue); //java.lang.NullPointerException map.merge("key2", "new value2", (oldValue, newValue) -> null); System.out.println(map); System.out.println(); map.clear(); map.put("key1", "value1"); map.put("key2", "value1"); map.put("key3", "value1"); map.put("key4", "value2"); map.put("key5", "value2"); map.put("key6", "value2"); Set<String> keySet = map.keySet(); Collection<String> values = map.values(); System.out.println(map); System.out.println(keySet); System.out.println(values); /* remove value时,找到map中第一个value为对应值得key、value,并将其删除 */ System.out.println(); values.remove("value1"); System.out.println(map); System.out.println(keySet); System.out.println(values); System.out.println(); keySet.remove("key4"); System.out.println(map); System.out.println(keySet); System.out.println(values);
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