Map之computeIfAbsent

Absent /ˈæbsənt , æbˈsent/ ab相反s存在ent…的

从map中获取key对应的value,如果value不存在就用提供的Function创建一个新的value,然后存入map,最后返回

优化前

Map<String, Set<Pet>> statistics = new HashMap<>();
Set<Pet> pets = statistics.get(threadName);
if (pets == null) {pets = new HashSet<>();
}

优化后

// 根据threadName获取pets,如果pets==null,则创建一个HashSet,然后将新建的HashSet存入map,最后返回新建的HashSet
Map<String, Set<Pet>> statistics = new HashMap<>();
Set<Pet> pets = statistics.computeIfAbsent(threadName, k -> new HashSet<>());

源码

default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {Objects.requireNonNull(mappingFunction);V v;// 如果v有值,则直接返回v,如果v不存在,就创建新的v,并将新的v存入map,然后返回新vif ((v = get(key)) == null) {V newValue;if ((newValue = mappingFunction.apply(key)) != null) {// 将新v存入mapput(key, newValue);// 返回新vreturn newValue;}}return v;
}