Cannot add object of type T to List<T> in java

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Viewed 2014

I have been learning generics in Java. Although I understand the concepts regarding type inference, parameterized classes and methods, I came across a strange scenario while experimenting.

I have implemented a Box class which can be used to hold items of type T. I am using List as an internal data structure for this abstraction. Following is my code:

public class Box<T> {

    private List<T> items;

    public Box(){
        items = new ArrayList<>();
    }

    public <T> void addItemToBox(T t){
        items.add(t);
    }
}

I am getting a compile time error at items.add(t) saying add(T) in List cannot be applied to (T). I am unable to figure out the cause of this error. Also, I don't understand why I can't add an item of type T to a list which is parameterized by T.

generics java
1 Answers

You've made a local redeclaration of generic type variable <T> on your method addItemToBox that shadows the one on class Box<T>.

The <T> on your method is not the same on List<T> items;

Your current code is 100% equivalent to:

public class Box<T> {

    private List<T> items;

    public Box(){
        items = new ArrayList<>();
    }

    public <T2> void addItemToBox(T2 t){
        items.add(t);
    }
}

If you see it that way, it should be clear why your code fails to compile.

Solution: remove the <T> in front of the method declaration

public void addItemToBox(T t)
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