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In Java, how can I instantiate a class with a different type variable as type?

In a Java tutorial , I read that "a [generic] type variable can be any non-primitive type that you specify: any class type, any interface type, any array type, or even a variable of a different type ."

In other words, given this:

class Box<T> {
    private T t;
    public void set(T t) { this.t = t; }
    public T get() { return t; }
}

I can do it as written:

Box<Object> box1 = new Box<>();
Box<Serializable> box2 = new Box<>();
Box<Comparable<Object>> box3 = new Box<>();
Box<char[]> box4 = new Box<>();

But what about a type variable? What does "variable of a different type" mean even at compile time?

// nothing like this works
// Box<Z> box5 = new Box<>();
// Box<<Z>> box5 = new Box<>();
// Box<Z> box5 = new Box<Z>();
// Box<<Z>> box5 = new Box<<Z>>();
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3 answers


I think this refers to the case where it is used inside another generic class, for example:

class SomeGenericClass<T> {
    Box<T> box = new Box<T>();
}

Where T

is another type variable and you can construct an object like this:

SomeGenericClass<Object> someGenericClass = new SomeGenericClass<>();

Where box

, initialized in SomeGenericClass

isBox<Object>



It can also refer to using another shared instance inside your shared class like this:

class Box<T> {
    ArrayList<T> arrayList = new ArrayList<>();
}

And let's build the class like this:

Box<Object> box = new Box<>();

Where will it be ArrayList

insideBox<Object>

ArrayList<Object>

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This means that if you have another class that uses generics, you can set the type to another type variable from that class, so something like this:



public class A<S>{
  private B<S>();

}

public class B<T>{
}
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I would do something like this

Box <T> extends SomeParametrizedClass<T>{
// class body
private List<T> someList;
}

Here you have a parameterized field and a superclass with variable types instead of specially provided types.

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