Question

I’m quite new to Java and have come accross to a strange behaviour that I can not explain why this happens or where is the mistake in my code.

Here’s the code:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;

abstract class Shape {
    public abstract void printMe(String no);
}

final class Circle extends Shape {
    @Override
    public void printMe(String no){
        System.out.println("This is Circle no: " + no);
    }
}

final class Square extends Shape {
    @Override
    public void printMe(String no) {
        System.out.println("This is Square no: " + no);
    }
}

final class Triangle extends Shape {
    @Override
    public void printMe(String no) {
        System.out.println("This is Triangle no: " + no);
    }
}

public class Foo {
    private ArrayList<Shape> shapes;

    public Foo(){
        this.shapes   = new ArrayList<Shape>();

        this.shapes.add(new Circle());
        this.shapes.add(new Square());
        this.shapes.add(new Triangle());
    }

    public void printShapes(ArrayList<String> numbers){
        for(String s:numbers){
            Iterator<Shape> iter = this.shapes.iterator();
            Shape shape = iter.next();
            shape.printMe(s);
        }
    }

    public static void main(String[] args) {
        ArrayList<String> numbers = new ArrayList<String>(Arrays.asList("1", "2", "3"));
        Foo foo = new Foo();
        foo.printShapes(numbers);
    }
}

The output I’d expect would be:

This is Circle no: 1
This is Square no: 2
This is Triangle no: 3

However, the output I get is:

This is Circle no: 1
This is Circle no: 2
This is Circle no: 3

What am I doing wrong?

Answer 1

Pull this line out of the loop:

 Iterator<Shape> iter = this.shapes.iterator();