Declaration of return value in generic methods

[Thomas Hermann]

Hello,

I try the following 4 tests about generic methods (where Dog extends Animals extends Object ):

    List<? extends Animal> list1 = testFunc1(); // test1: Compilation OK
    List<? extends Animal> list2 = testFunc2(); // test2: Compilation OK
    List<? extends Animal> list3 = testFunc3(); // test3: Compilation NOK
    List<? extends Animal> list4 = testFunc4(); // test4: Compilation OK ??, why ??

    
	public  List<Dog> testFunc1(){
		return null;
	}
	public <E extends Dog> List<E> testFunc2(){
		return null;
	}	
	public  List<Object> testFunc3(){
		return null;
	}
	public <E extends Object> List<E> testFunc4(){
		return null;
	}	

Here test1 and test2 compile fine, as expected. I also understand that test3 not compiles (Object not extends Animal, so the assignment is not allowed). Now I expect that compilation should fail for test4 also, because upper bound for placeholder E is Object here. Therefore it's possible, that testFunc4 returns (in worst case) a List of Objects, which can't be assigned to list4(Type: List<? extends Animal>). But to my surprise test4 compiles??.

Why does test4 compile?

Thanks,
Thomas

[Stephan van Hulst]

Because the methods returns a List<E>, where E is inferred from the formal type of the variable you're trying to assign the return value to.

In this case, the return type automatically adapts to the type required by the variable.

[Thomas Hermann]

I think, I understand now.

Just as for generic classes the concrete generic type is determined during instantiation, the concrete type for the return value (List<E>) of the generic function is determined by the caller during the assignment.

Thanks to Stephan van Hulst,

Thomas