检查列表是否包含对象-Java

Question

I am writing the image processing program, and I have a problem with a list.

I have a list with Points.

     class Point1{
        private int x;
        private int y;

        Point1(int x,int y)
        {
            this.x = x;
            this.y = y;
        }

       int getX(){ return this.x; }
       int getY() {return this.y; }
    }

    ArrayList<Point1> list = new ArrayList();

And now, I create new Point, for example new Point(4,3); I want to check if there is a point in my list which has the same coordinates. The problem is that

    list.contains(Object a) 

is checking if if on my list there is a particular object. It will work if I put

    Point1 first = new Point(1,1);  
    list.add(first);
    list.contains(first) // and this is true

but:

    Point second = new Point(2,2);
    list.add(second);
    list.contains(new Point(2,2)); <- false

how can I check it?

Answer 1

You need to override equals on your Point class. Your Point class must be responsible for determining whether another Point object is "equal" to the current object. An ArrayList will call equals to determine if the object passed in to contains is "equal" to any item in the list.

If you don't override equals , then Point will inherit equals from Object , which will simply see if it's the same exact object. That's why your first code "works", because you are re-using first . It also explains why your second code doesn't "work", because you used a different object.

Also, if you override equals , it's best to override hashCode also (and vice versa).

Answer 2

List determines whether two objects are equal by the default equals() method extended from Object . The default equals() method is just determine whether the two object has the same domain value (I think it is the memory address). And it can be printed by toString() . Thus is you wanna to determine "equals" only by the values in this object are the same, you need to override equals() . you can code as below:
@Override public boolean equals(Object other) { if(this == other) return true; if(other == null) return false; if(getClass() != other.getClass()) return false;
Point1 test = (Point1)other; if(this.x == test.getX() && this.y == test.getY()) return true; return false; }
And you can get the result you want: