相关疑难解决方法(0)

此代码在Java 8中编译,但无法在Java 7中编译:

class Map<K,V> {
    static <K,V> Map<K,V> empty() {return null;}
    Map<K,V> put(K k, V v) {return null;}
    V get(K k) {return null;}
}

class A {
    static void f(Map<Integer,String> m){}
    public static void main(String[] args) {
        f(Map.empty());
    }
}

它不会推断出Map返回的具体类型Map.empty():

$ javac7 A.java
A.java:10: error: method f in class A cannot be applied to given types;
        f(Map.empty());
        ^
  required: Map<Integer,String>
  found: Map<Object,Object>
  reason: actual argument Map<Object,Object> cannot be converted to Map<Integer,String> by method invocation …

假设你有一个List数字.在该值List可以是类型的Integer,Double等等.当你声明这样的List有可能使用一个通配符(声明它?)或不用一个通配符.

final List<Number> numberList = Arrays.asList(1, 2, 3D);
final List<? extends Number> wildcardList = Arrays.asList(1, 2, 3D);

所以,现在我想stream在List和collect它所有的Map使用Collectors.toMap(显然下面的代码只是为了说明问题的例子).让我们开始流式传输numberList:

final List<Number> numberList = Arrays.asList(1, 2, 3D, 4D);

numberList.stream().collect(Collectors.toMap(
        // Here I can invoke "number.intValue()" - the object ("number") is treated as a Number
        number -> Integer.valueOf(number.intValue()),
        number -> number));

但是,我不能对以下内容做同样的操作wildcardList:

final List<? extends Number> wildCardList …