Question

Hi
I have written this code that with output you can get that .remove() method doesn’t work. a, b, c, and d are some Points Objects that have x and y members.

Here are a and b and c and d values, which in the if statement must be deleted for upper but it doesn’t.

X :59  Y: 143
X :165  Y: 140
X :59  Y: 143
X :165  Y: 140


   System.out.println(upper.toString());
        for(int i =0;i<upper.size();i++)

            if(upper.get(i)==a||upper.get(i)==b||upper.get(i)==c||upper.get(i)==d){
                upper.remove(i);

            }
        for(int i =0;i<lower.size();i++)

            if(lower.get(i)==a||lower.get(i)==b||lower.get(i)==c||lower.get(i)==d){
                upper.remove(i);
            }



        System.out.println(upper.toString());
        System.out.println(lower.toString());


   first println : [X :108  Y: 89, X :165  Y: 140]

   second println: [X :108  Y: 89, X :165  Y: 140]

   third println :  [X :105  Y: 191]

Answer 1

If I’m reading your question right, you’re assuming that == will compare the properties of two objects. It doesn’t, that’s what equals does. == tells you whether two references are to the same object instance, not to equivalent ones.

So for example:

public class Foo {
    public Foo(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @override
    public boolean equals(Object other) {
        Foo otherFoo;

        if (other == null || !(other instanceof Foo)) { // or you might be more restrictive
            return false;
        }

        otherFoo = (Foo)other);
        return otherFoo.x == this.x && otherFoo.y == this.y;
    }

    @override
    public int hashCode() {
        // ...appropriate implementation of hashCode...
    }
}

Foo a = new Foo(0, 0);
Foo b = new Foo(0, 0);
System.out.println(a == b);      // "false"
System.out.println(a.equals(b)); // "true"

Separately: Consider what happens when you have two consequtive matching objects in the ArrayList that you have to remove. Say they’re at indexes 8 and 9 in the list. So when i == 8, you remove the item at index 8, and the one that used to be at 9 is now at 8. But then you increment i in the for loop and continue with the new item at index 9, leaving the second one untouched. If you want to modify the list while you’re looping through it, consider looping backward to avoid that, or using an Iterator.