关于map函数的方法参考，当键的类型为String时编译错误

Question

Context :

I want to use the function computeIfAbsent on a Map . But, I get a compilation error when I use

• method reference and the key is a String .

I get no compilation error when I use

• method reference and the key is an Integer .
• lambda and the key is a String .

Illustration :

The following statements are legal :

Map<Integer, List<Long>> map = new HashMap<>();
Integer key = Integer.valueOf(0);
Long value = Long.valueOf(2);
map.computeIfAbsent(key, ArrayList::new).add(value); // No compilation error

The following statements are illegal :

Map<String, List<Long>> map = new HashMap<>();
String key = "myKey";
Long value = Long.valueOf(2);
map.computeIfAbsent(key, ArrayList::new).add(value); // Compilation error: The type ArrayList does not define ArrayList(String) that is applicable here

The following statements are legal :

Map<String, List<Long>> map = new HashMap<>();
String key = "myKey";
Long value = Long.valueOf(2);
map.computeIfAbsent(key, x -> new ArrayList<>()).add(value); // No compilation error

Question : I don't get why a String as key is that special when combined with method reference. Any idea?

Answer 1

When you call ArrayList::new instead of x -> new ArrayList<>() it's equals to call x -> new ArrayList<>(x) .

Method computeIfAbsent requires lambda expression with one lambda parameter as second input argument, or a reference to method that consumes one argument of String type.

Your error

Compilation error: The type ArrayList does not define ArrayList(String) that is applicable here

is talking: you trying to call constructor with one String argument . Because, as I told above, lambda x -> someObject.method(x) is equal to someObject::method . Or lambda x -> new SomeClass(x) is equal to SomeClass::new .

You can't use method (constructor) reference here, because here required method (constructor) that consumes one parameter, or a lambda expression. If there was lambda without any parameters, you will be able to call empty constructor.