How to set values to object attributes inside an array in java

Question

I keep getting this error:

Exception in thread "main" java.lang.NullPointerException
    at Circle.main(Circle.java:35)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

I'm trying to set the attributes for the first circle object, but I guess I can't do this in java this way?

public class Circle {
    private double x,
                   y,
                   radius;

    public static void main(String args[]) {
        // Testing Circles
        System.out.println("\nTESTING CIRCLES:");
        Circle[] circles = new Circle[2];
        Circle circle1 = circles[0]; // Circle 1

        // Setting X
        circle1.setX(20);  // <- Why does this not work and how is this properly done.
    }

    public void setX(double xVal) {
        x = xVal;
    }
}

Robby Cornelissen · Accepted Answer · 2014-07-10 01:33:31Z

3

You have not initialized your Circle object, just declared an empty array that is fit to hold circle objects. At the time of declaration, all objects in the array are null. You need to initialize the objects in the array before you're able to call methods on them.

Circle[] circles = new Circle[2];
circles[0] = new Circle();
circles[0].setX(20);

Or you can do initialization and declaration in one line as follows:

Circle[] circles = new Circle[] { new Circle(), new Circle() };
circles[0].setX(20);

edited Jul 10, 2014 at 1:33

answered Jul 10, 2014 at 1:17

Robby Cornelissen

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2 Comments

johnchen902 Over a year ago

The latter is wrong because you can't have both a dimension expression and an array initializer. It should be Circle[] circles = new Circle[] { new Circle(), new Circle() };, or better, Circle[] circles = { new Circle(), new Circle() };

Robby Cornelissen Over a year ago

@johnchen902 My bad. Correcting my answer.

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