Question

import java.util.ArrayList;

class Stack {
    private ArrayList stack;
    private int pos;

    Stack() {
        stack = new ArrayList();
        pos = -1;
    }
    int pop() {
        if(pos < 0) {
             System.out.println("Stack underflow.");
             return 0;
        }
        int out = stack.get(pos);
        stack.remove(pos);
        pos--;
        return out;
    }
}

I am trying to write a basic variable length stack and this is a snippet of my code. When I run this, I get an error:

Main.java:16: error: incompatible types
   int out = stack.get(pos);
                       ^
required: int
found:    Object

Why is this being passed as an object?

Answer 1

Currently you are not defining generic types held in your ArrayList called stack so you will get a raw Object return type from ArrayList.get:

You need to replace

ArrayList stack;

with

ArrayList<Integer> stack;

and similarly

stack = new ArrayList();

with

stack = new ArrayList<Integer>(); // new ArrayList<>(); for Java 7

Also have a look at using java.util.Stack