getHeight() problem

[sapata]

Hey guys,

wondering if you could help me out with the following:
Im trying to create a small class which detects the user's screen resolution and stores it in attributes.

The following code generates a NullPointerException and I can't see why.

Thanks in advance!

Code:

import java.awt.*;

public class ScreenTest {
    private int x;
    private int y;
    private String error;
    private DisplayMode dm;
    
    
    public ScreenTest() {
        x = 0;
        y = 0;  
        error = null;
    }
       
    public int getRefresh(){
        return 	dm.getRefreshRate();
    }
    
    public int returnX(){
        try {
            x = dm.getWidth();
        } catch (HeadlessException hexc) {
            error = "Failed to initialize value for X";
        }
        return x;
    }
    
    public int returnY(){
        try {
            y = dm.getHeight();
        } catch (HeadlessException hexc) {
            error = "Failed to initialize value for Y";
        }
        return y;
    }
    
    public void resetResolution() {
        x = 0;
        y = 0;
    }
    
    public String returnError() {
        return error;
    }
}

Please don't flame me if i am totally stupid ..

[hardwired]

Code:

C:\jexp>java ScreenTest
Exception in thread "main" java.lang.NoSuchMethodError: main

Added a main method and try again.

Code:

C:\jexp>java ScreenTest
Exception in thread "main" java.lang.NullPointerException
        at ScreenTest.returnX(screentest.java:22)
        at ScreenTest.main(screentest.java:49)

Code:

import java.awt.*;

public class ScreenTest {
    private int x;
    private int y;
    private String error;

    // The jvm initializes this to null. If it is not
    // instantiated then all uses of the "dm" variable
    // will cause a NullPointerException because it
    // does not point to an object, viz, it is null.
    private DisplayMode dm;

    public ScreenTest() {
        x = 0;
        y = 0;  
        error = null;
    }

    public int getRefresh(){
        // "dm" is null so you cannot use it here without causing
        // a NullPointerException. If you instantiate it with
        // the new operator, eg,
        // dm = new DisplayMode(width, height, bitDepth, refreshRate);
        // in which you supply proper values for each of the arguments,
        // then the reference/variable "dm" will point to the new
        // object and no longet be null.
        return 	dm.getRefreshRate();
    }

    public int returnX(){
        try {
            x = dm.getWidth();
        } catch (HeadlessException hexc) {
            error = "Failed to initialize value for X";
        }
        return x;
    }

    public int returnY(){
        try {
            y = dm.getHeight();
        } catch (HeadlessException hexc) {
            error = "Failed to initialize value for Y";
        }
        return y;
    }

    public void resetResolution() {
        x = 0;
        y = 0;
    }

    public String returnError() {
        return error;
    }

    public static void main(String[] args) {
        ScreenTest test = new ScreenTest();
        // Threw a NullPointerException in the returnX method.
//        test.returnX();
        // Explore the local environment:
        GraphicsDevice gd =
            GraphicsEnvironment.getLocalGraphicsEnvironment()
                               .getDefaultScreenDevice();
        DisplayMode[] modes = gd.getDisplayModes();
        for(int i = 0; i < modes.length; i++) {
            DisplayMode mode = modes[i];
            query(modes[i]);
        }
    }

    private static void query(DisplayMode mode) {
        int bitDepth = mode.getBitDepth();
        int height = mode.getHeight();
        int refreshRate = mode.getRefreshRate();
        int width = mode.getWidth();
        System.out.printf("bitDepth = %2d  refreshRate = %2d  " +
                          "width = %4d  height = %d%n",
                           bitDepth, refreshRate, width, height);
    }
}