如何从 Java 中的泛型类型中获取方法？

Question

I have a problem like below:

List<T> someList;

for(T element : someList){
    System.out.println(element.getName()); //cannot resolve method 'getName'
}

how to get method from generic type?

my T is a class where I have many parameters with setters and getters,

can someone tell me how to get some params from T class?

thanks for any help

Answer 1

T needs to be upper-bounded to a type which has the getName() method.

For example:

interface HasGetName {
  String getName();
}

Then add your upper-bound using T extends HasGetName :

// If T is a class-level type variable:
class YourClass<T extends HasGetName> {
  List<T> someList;

  void print() {
    for(T element : someList){
      System.out.println(element.getName());
    }
  }
}

// If T is a method-class-level type variable:
class YourClass {
  <T extends HasGetName> void print(List<T> someList) {
    for(T element : someList){
      System.out.println(element.getName());
    }
  }
}

Answer 2

The safest way would be something like:

List<T> someList;

for(T element : someList){
    if (element instanceof SpecializedElement)
        System.out.println(((SpecializedElement)element).getName());
}

Java 15 introduces something cool List someList;

for(T element : someList){
    if (element instanceof SpecializedElement s)
        System.out.println(s.getName());
}

You don't need to do any casting with instanceof + class + any var name