创建自定义类对象时，Android应用程序崩溃

Question

I'm working on my first android app and I'm having trouble creating an object of a class I have programmed in java using the onCreate function of an activity. The activity starts great when I do not instantiate the object, but when I attempt to create the object the app crashes when switching to the activity. The onCreate function looks like this...

    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_entry);
    // Show the Up button in the action bar.
    setupActionBar();
    ForceTable testTable = new ForceTable();
    Double factor = testTable.returnValue(forceTypes.newtons, forceTypes.newtons);
}

ForceTable is the class I have programmed, its code looks this...

public class ForceTable {

private double[][] forceTable;

protected enum forceTypes {newtons(0), pounds(1), kilopond(2);
    public int num;
    private forceTypes(int num)
    {
        this.num = num;
    }
};

protected final class values{
    private final static double zeroZero = 1.00;
    private final static double zeroOne = 4.44872162;
    private final static double zeroTwo = 9.80665;
    private final static double oneZero = .224808943;
    private final static double oneOne = 1.00;
    private final static double oneTwo = 2.20462262;
    private final static double twoZero = .10197164;
    private final static double twoOne = .45359237;
    private final static double twoTwo = 1.00;
}

public ForceTable()
{
    this.makeTable();
}

private void makeTable()
{
    forceTable[0][0] = values.zeroZero;
    forceTable[0][1] = values.zeroOne;
    forceTable[0][2] = values.zeroTwo;
    forceTable[1][0] = values.oneZero;
    forceTable[1][1] = values.oneOne;
    forceTable[1][2] = values.oneTwo;
    forceTable[2][0] = values.twoZero;
    forceTable[2][1] = values.twoOne;
    forceTable[2][2] = values.twoTwo;       
}

public double returnValue(forceTypes ifYouHave, forceTypes thenYouHave){
    double factor = forceTable[thenYouHave.num][ifYouHave.num];
    return factor;
}

}

It's been a very long time since I have programmed in Java, and since the activity starts fine without instantiating it must be my Java code for ForceTable. Anybody notice something wrong? There's a good chance it's something easy that I'm not brushed up on...

Here is the logcat

08-08 18:08:13.206: E/(9801): : Can't open file for reading 08-08 18:08:13.206: E/(9801): : Can't open file for reading 08-08 18:10:34.045: W/dalvikvm(9801): threadid=1: thread exiting with uncaught exception (group=0x4136b438) 08-08 18:10:34.045: E/AndroidRuntime(9801): FATAL EXCEPTION: main 08-08 18:10:34.045: E/AndroidRuntime(9801): java.lang.RuntimeException: Unable to start activity ComponentInfo{khandy.application.convertible/khandy.application.convertible.EntryActivity}: java.lang.NullPointerException 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110) 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2135) 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.app.ActivityThread.access$700(ActivityThread.java:143) 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1241) 08-08 18:10:34.04 5: E/AndroidRuntime(9801): at android.os.Handler.dispatchMessage(Handler.java:99) 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.os.Looper.loop(Looper.java:137) 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.app.ActivityThread.main(ActivityThread.java:4950) 08-08 18:10:34.045: E/AndroidRuntime(9801): at java.lang.reflect.Method.invokeNative(Native Method) 08-08 18:10:34.045: E/AndroidRuntime(9801): at java.lang.reflect.Method.invoke(Method.java:511) 08-08 18:10:34.045: E/AndroidRuntime(9801): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004) 08-08 18:10:34.045: E/AndroidRuntime(9801): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771) 08-08 18:10:34.045: E/AndroidRuntime(9801): at dalvik.system.NativeStart.main(Native Method) 08-08 18:10:34.045: E/AndroidRuntime(9801): Caused by: java.lang.NullPointerException 08-08 18:10:34.045: E/AndroidRuntime(9801): at khandy.application.convertible.ForceTable.makeTable(ForceTable.java: 34) 08-08 18:10:34.045: E/AndroidRuntime(9801): at khandy.application.convertible.ForceTable.(ForceTable.java:29) 08-08 18:10:34.045: E/AndroidRuntime(9801): at khandy.application.convertible.EntryActivity.onCreate(EntryActivity.java:21) 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.app.Activity.performCreate(Activity.java:5179) 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094) 08-08 18:10:34.045: E/AndroidRuntime(9801): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074) 08-08 18:10:34.045: E/AndroidRuntime(9801): ... 11 more

Answer 1

You probably get a NullPointerException at this line

forceTable[0][0] = values.zeroZero;

because you try to access the array without creating it first. Before accessing it, create it like that

forceTable = new double[][];
forceTable[0][0] = values.zeroZero;

And please ALWAYS post the logcat output! It just makes it so much easier to find the problem.

Answer 2

I think the problem is when your makeTable function is called. can you allocate space for your forceTable array.. I mean do a new for double[][] forceTable ;

Answer 3

You are missing

privatetable = new double[n] [m] ;

You can put it in the constructor of the second class!