How to define integer variables for a generic class in java?

Question

I have a short class:

public class Stack {

private int[] data;
private int Top;

Public Stack(int size) {

   data = new int[size];
   top = -1;

}

public void Push (int value) {

   top++;
   data[top] = value;

}

public int pop() {

   return data[top--];

}

public int top() {

   return data[top];

}

And i'm getting bunch of errors "cannot convert from int to T"... And also getting an error in the array definition of the constructor...

This is my code, i'm a beginner please help me to understand this:

public class Stack <T> {


    private T[] data;
    private T top;

    Public Stack(T size) {

    data = new T[size];// im getting error here "cannot create a generic array of T...
    top = -1; // what should I do with this?

    }

    public void Push (T value) {

    top++; //cannot convert from int to T
    data[top] = value; //cannot convert from int to T

    }

    public T pop() {

    return data[top--]; //cannot convert from int to T

    }

    public T top() {

    return data[top]; //cannot convert from int to T

   }

MAn,your code is incomplete as you haven't shown the main() method! — Am_I_Helpful, Jul 01 '14 at 19:48

score 2 · Accepted Answer · answered Jul 01 '14 at 20:06

2

You did not say why you tried to convert all the "int"s to "T"s, but I can already say:

"public" (visibility of the constructor) should be without the capital.
Your variable "top" must be an int : whatever the type T, the variable top is an index.

You can not create a generic array. Instead of giving a size to the constructor, you should write the constructor to take an array of T.

public class Stack<T> {

    private final T[] data;
    private int top;

    public Stack(final T[] data) {
        this.data = data;
        top = -1;
    }

    public void Push(final T value) {
        top++;
        data[top] = value;
    }

    public T pop() {
        return data[top--];
    }

    public T top() {
        return data[top];
    }
}

edit: I also added "final" to the field "data" because I always declare "final" everything I can.

answered Jul 01 '14 at 20:06

syme

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thanks man. this works. but is there a way I can keep the constructor as it is and still get it to be generic..? maybe with a wrapper class? because i'm sure I should change it's parameters..@syme – Joe Jul 01 '14 at 20:44
I do not understand your new question because it is generic, and you can not create a generic array. – syme Jul 01 '14 at 20:59
public class Stack { private Integer[] data; public Stack(int size) { data = new Integer[size]; } ... } – Joe Jul 01 '14 at 21:23
something like this..@syme – Joe Jul 01 '14 at 21:26
Oh yes ! Sorry I did not thought you wanted to specify your class Stack for Integers only. – syme Jul 02 '14 at 19:01

score 1 · Answer 2 · edited May 23 '17 at 12:29

1

Your top variable is meant to store an index into your array representing the top of the stack. It's not meant to store any actual data; it's just an index. Therefore, when making your Stack class generic, it should not be converted to type T. It should remain an int.

The parameter to your constructor, size, must be of type int also. As for creating generic arrays, please refer to How to: generic array creation.

edited May 23 '17 at 12:29

Community

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answered Jul 01 '14 at 19:49

rgettman

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oh, got you. thanks. and about the rest? i mean, now i still have the prob with the array @rgettman – Joe Jul 01 '14 at 19:50

How to define integer variables for a generic class in java?

2 Answers2