Stuck with Java Generic Classes

Question

I have an assignment and I'm stuck. The assignment is to write a generic class for this method:

public static void main(String[] args) {
    ValueStore<Object> myStore1 = new ValueStore<Object>(); 
    myStore1.set("Test"); 
    myStore1.get();

   ///
    ValueStore<Object> myStore2 = new ValueStore<Object>(); 
    myStore2.set(myStore1); 
    myStore1 = myStore2.get(); 
}

I've come this far.

public class ValueStore<T> {
    private T x;

    public void set(T x) {
        System.out.println(x);
    }

    public T get () {
        return x;
    }
}

I am able to print out mystore.set "test", but not the myStore2.set. And I don't understand why my teacher passed in a reference variable as a argument. When I do that I get ValueStore@15db9742 in the console. Or maybe thats the point?

Can someone explain why it says myStore2.set(myStore1); myStore1 = myStore2.get(), what it should print and the logic behind it?

Thank you in advance. And sorry if my text is all messy. First time here.

Answer 1

I have commented a bit more to explain. The main point is that you can give a type to your ValueStore (in this example, String). This makes the typesystem aware that when you call get() on the valuestore, it gets a string in return. This is actually the entire point of generics. If you simply put object, only you know that the get method will return a String so you would have to cast it (as in the second example).

public static void main(String[] args) {
    // Type your store with String, which is what generics is about.
    ValueStore<String> myStore1 = new ValueStore<String>(); 

    // Store a string in it.
    myStore1.set("Test");
    // Get the object, and the typesystem will tell you it's a string so you can print it.
    System.out.println(myStore1.get());

   ///
    ValueStore<Object> myStore2 = new ValueStore<Object>(); 
    // Store your store.
    myStore2.set(myStore1); 
    // The type system only knows this will return an Object class, as defined above.
    // So you cast it (because you know better).
    myStore1 = (ValueStore<String>) myStore2.get(); 
    System.out.println(myStore1.get());
}

public class ValueStore<T> {
    private T x;

    public void set(T x) {
        this.x = x;
    }

    public T get () {
        return x;
    }
}

This code prints the following:

test
test

Answer 2

I think currently you are just missing a line from your set() method, as in

public void set(T x) {
    System.out.println(x);
    this.x = x;
}

So that you would actually store the object.