不兼容的类型，但没有意义（Java）

Question

I'm trying to iterate through a set and I want to save the next element in a temporary variable, but it is giving me a compilation error for incompatible types.

Iterator it = this.iterator();
E next = it.next();

next() returns a generic type E and next is of type E. Why is it giving me the error?

Answer 1

You're using Iterator as a raw type, since you haven't provided a type argument. When you use raw types, any related generic types become their erasure . In other words, any use of a class type variable in methods parameter, return types, etc. become either Object or the upper bound.

You probably want

Iterator<E> it = this.iterator();
E next = it.next();

if this.iterator() has a return type of Iterator<E> .

Answer 2

You are using raw type iterator, use it with Generics, then it will allow to compile

Iterator<E> it = this.iterator();
E next = it.next();

Answer 3

使用泛型或类型转换到对象。