Null pointer exception with custom view in layout xml

Question

Here's the custom view code which gives me a null pointer exception in the layout tab of the XML which uses this view:

public class BoardView extends View {

    // Drawables for the board boxes, the playable zone
    public Drawable[][] block = new Drawable[20][10];
    // Drawables for the wall (yes, it's done with tiles)
    public Drawable[] wall = new Drawable[102];
    // Drawable for the background, and boolean for drawing it or not
    Drawable mbg;
    boolean bg = false;
    // Context and canvas to be used along the class
    Context context;
    Canvas c;

    /*************************************************/
    /* Class constructor **************************** */
    /*************************************************/
    /* Defines the context and the canvas *********** */
    /*************************************************/
    public BoardView(Context cont, AttributeSet attrs) {
        super(cont, attrs);
        context = cont;
    }

    /*************************************************/
    /* Initializes drawables for playable boxes ***** */
    /*************************************************/
    /* Must be initialized one by one from the Game * */
    /* activity, passing all the parameters ********* */
    /*************************************************/
    public void initialize(int i, int j, int left, int top, int side) {
        block[i][j] = context.getResources().getDrawable(R.drawable.alpha);
        block[i][j].setBounds(left, top, left + side, top + side);
    }

    /*************************************************/
    /* Draws the board wall ************************* */
    /*************************************************/
    /* Needs the top-left point of the board frame ** */
    /* and the width of the wall ******************** */
    /*************************************************/
    public void createWall(int left, int top, int side) {
        int i = 0, x, y;
        x = left - side / 2;
        y = top;
        // The left wall
        while (i < 40) {
            wall[i] = context.getResources().getDrawable(R.drawable.brick);
            wall[i].setBounds(x, y, x + side / 2, y + side / 2);
            y = y + side / 2;
            i = i + 1;
        }
        x = left + side * 10;
        y = top;
        // The right wall
        while (i < 80) {
            wall[i] = context.getResources().getDrawable(R.drawable.brick);
            wall[i].setBounds(x, y, x + side / 2, y + side / 2);
            y = y + side / 2;
            i = i + 1;
        }
        x = left - side / 2;
        // The floor
        while (i < 102) {
            wall[i] = context.getResources().getDrawable(R.drawable.brick);
            wall[i].setBounds(x, y, x + side / 2, y + side / 2);
            x = x + side / 2;
            i = i + 1;
        }
    }

    /*************************************************/
    /* Draws the board background ******************* */
    /*************************************************/
    /* Needs the top-left point of the board frame ** */
    /* and the width of the wall ******************** */
    /*************************************************/
    public void createBg(int left, int top, int side) {
        // Set board background (if any)
        bg = false;
        int bgn = 1 + (int) (Math.random() * 19);
        switch (bgn) {
        case 1:
            mbg = getResources().getDrawable(R.drawable.bg1);
            break;
        case 2:
            mbg = getResources().getDrawable(R.drawable.bg2);
            break;
        case 3:
            mbg = getResources().getDrawable(R.drawable.bg3);
            break;
        case 4:
            mbg = getResources().getDrawable(R.drawable.bg4);
            break;
        case 5:
            mbg = getResources().getDrawable(R.drawable.bg5);
            break;
        case 6:
            mbg = getResources().getDrawable(R.drawable.bg6);
            break;
        case 7:
            mbg = getResources().getDrawable(R.drawable.bg7);
            break;
        case 8:
            mbg = getResources().getDrawable(R.drawable.bg8);
            break;
        case 9:
            mbg = getResources().getDrawable(R.drawable.bg9);
            break;
        case 10:
            mbg = getResources().getDrawable(R.drawable.bg11);
            break;
        case 11:
            mbg = getResources().getDrawable(R.drawable.bg11);
            break;
        case 12:
            mbg = getResources().getDrawable(R.drawable.bg12);
            break;
        case 13:
            mbg = getResources().getDrawable(R.drawable.bg13);
            break;
        case 14:
            mbg = getResources().getDrawable(R.drawable.bg14);
            break;
        case 15:
            mbg = getResources().getDrawable(R.drawable.bg15);
            break;
        case 16:
            mbg = getResources().getDrawable(R.drawable.bg16);
            break;
        case 17:
            mbg = getResources().getDrawable(R.drawable.bg17);
            break;
        case 18:
            mbg = getResources().getDrawable(R.drawable.bg18);
            break;
        case 19:
            mbg = getResources().getDrawable(R.drawable.bg19);
            break;
        }
        mbg.setBounds((int) (left), (int) (top), (int) (left + side * 10),
                (int) (top + 20 * side));
    }

    /*************************************************/
    /* Draws the board ****************************** */
    /*************************************************/
    /* Draws the walls, the bg and all the boxes **** */
    /*************************************************/
    @Override
    protected void onDraw(Canvas canvas) {
        c = canvas;
        super.onDraw(canvas);
        if (bg)
            mbg.draw(canvas);
        for (int i = 0; i < 102; i++)
            wall[i].draw(c);
        for (int i = 0; i < 20; i++)
            for (int j = 0; j < 10; j++) {
                block[i][j].draw(canvas);
            }
        // Actually draw
        invalidate();
    }

    /*************************************************/
    /* Canvas getter ******************************** */
    /*************************************************/
    public Canvas getCanvas() {
        return c;
    }

    /*************************************************/
    /* Colors a box ********************************* */
    /*************************************************/
    /* Changes the drawable for the indicated box to */
    /* to 'c'. Can also be COLOR_NONE to undraw ***** */
    /*************************************************/
    public void setColor(int i, int j, byte c) {
        Rect rect;
        rect = block[i][j].getBounds();
        switch (c) {
        case Values.COLOR_NONE:
            block[i][j] = context.getResources().getDrawable(R.drawable.alpha);
            ;
            break;
        case Values.COLOR_RED:
            block[i][j] = context.getResources().getDrawable(
                    R.drawable.block_red);
            break;
        case Values.COLOR_GREEN:
            block[i][j] = context.getResources().getDrawable(
                    R.drawable.block_green);
            break;
        case Values.COLOR_BLUE:
            block[i][j] = context.getResources().getDrawable(
                    R.drawable.block_blue);
            break;
        case Values.COLOR_YELLOW:
            block[i][j] = context.getResources().getDrawable(
                    R.drawable.block_yellow);
            break;
        case Values.COLOR_PINK:
            block[i][j] = context.getResources().getDrawable(
                    R.drawable.block_pink);
            break;
        case Values.COLOR_PURPLE:
            block[i][j] = context.getResources().getDrawable(
                    R.drawable.block_purple);
            break;
        case Values.COLOR_WHITE:
            block[i][j] = context.getResources().getDrawable(
                    R.drawable.block_white);
            break;
        }
        block[i][j].setBounds(rect);
    }
}

and here's the error log:

java.lang.NullPointerException
    at com.seavenois.tetris.BoardView.onDraw(BoardView.java:166)
    at android.view.View.draw(View.java:6880)
    at android.view.ViewGroup.drawChild(ViewGroup.java:1646)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
    at android.view.ViewGroup.drawChild(ViewGroup.java:1644)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
    at android.view.ViewGroup.drawChild(ViewGroup.java:1644)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
    at android.view.ViewGroup.drawChild(ViewGroup.java:1644)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
    at android.view.ViewGroup.drawChild(ViewGroup.java:1644)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
    at android.view.View.draw(View.java:6883)
    at android.view.ViewGroup.drawChild(ViewGroup.java:1646)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373)
    at android.view.View.draw(View.java:6883)
    at com.android.layoutlib.bridge.impl.RenderSessionImpl.render(RenderSessionImpl.java:466)
    at com.android.layoutlib.bridge.Bridge.createSession(Bridge.java:320)
    at com.android.ide.common.rendering.LayoutLibrary.createSession(LayoutLibrary.java:325)
    at com.android.ide.eclipse.adt.internal.editors.layout.gle2.RenderService.createRenderSession(RenderService.java:372)
    at com.android.ide.eclipse.adt.internal.editors.layout.gle2.GraphicalEditorPart.renderWithBridge(GraphicalEditorPart.java:1640)
    at com.android.ide.eclipse.adt.internal.editors.layout.gle2.GraphicalEditorPart.recomputeLayout(GraphicalEditorPart.java:1391)
    at com.android.ide.eclipse.adt.internal.editors.layout.gle2.GraphicalEditorPart.activated(GraphicalEditorPart.java:1165)
    at com.android.ide.eclipse.adt.internal.editors.layout.LayoutEditorDelegate.partActivated(LayoutEditorDelegate.java:734)
    at com.android.ide.eclipse.adt.internal.editors.layout.LayoutEditorDelegate.partBroughtToTop(LayoutEditorDelegate.java:744)
    at org.eclipse.ui.internal.PartListenerList$2.run(PartListenerList.java:87)
    at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
    at org.eclipse.core.runtime.Platform.run(Platform.java:888)
    at org.eclipse.ui.internal.PartListenerList.fireEvent(PartListenerList.java:57)
    at org.eclipse.ui.internal.PartListenerList.firePartBroughtToTop(PartListenerList.java:85)
    at org.eclipse.ui.internal.PartService.firePartBroughtToTop(PartService.java:208)
    at org.eclipse.ui.internal.WorkbenchPagePartList.firePartBroughtToTop(WorkbenchPagePartList.java:76)
    at org.eclipse.ui.internal.WorkbenchPagePartList.fireActiveEditorChanged(WorkbenchPagePartList.java:52)
    at org.eclipse.ui.internal.PartList.setActiveEditor(PartList.java:162)
    at org.eclipse.ui.internal.WorkbenchPage.makeActiveEditor(WorkbenchPage.java:1355)
    at org.eclipse.ui.internal.WorkbenchPage.setActivePart(WorkbenchPage.java:3629)
    at org.eclipse.ui.internal.WorkbenchPage.requestActivation(WorkbenchPage.java:3159)
    at org.eclipse.ui.internal.PartPane.requestActivation(PartPane.java:279)
    at org.eclipse.ui.internal.EditorPane.requestActivation(EditorPane.java:98)
    at org.eclipse.ui.internal.PartPane.setFocus(PartPane.java:325)
    at org.eclipse.ui.internal.EditorPane.setFocus(EditorPane.java:127)
    at org.eclipse.ui.internal.PartStack.presentationSelectionChanged(PartStack.java:837)
    at org.eclipse.ui.internal.PartStack.access$1(PartStack.java:823)
    at org.eclipse.ui.internal.PartStack$1.selectPart(PartStack.java:137)
    at org.eclipse.ui.internal.presentations.util.TabbedStackPresentation$1.handleEvent(TabbedStackPresentation.java:133)
    at org.eclipse.ui.internal.presentations.util.AbstractTabFolder.fireEvent(AbstractTabFolder.java:269)
    at org.eclipse.ui.internal.presentations.util.AbstractTabFolder.fireEvent(AbstractTabFolder.java:278)
    at org.eclipse.ui.internal.presentations.defaultpresentation.DefaultTabFolder.access$1(DefaultTabFolder.java:1)
    at org.eclipse.ui.internal.presentations.defaultpresentation.DefaultTabFolder$2.handleEvent(DefaultTabFolder.java:88)
    at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1053)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1077)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1062)
    at org.eclipse.swt.widgets.Widget.notifyListeners(Widget.java:774)
    at org.eclipse.swt.custom.CTabFolder.setSelection(CTabFolder.java:2746)
    at org.eclipse.swt.custom.CTabFolder.onMouse(CTabFolder.java:1433)
    at org.eclipse.swt.custom.CTabFolder$1.handleEvent(CTabFolder.java:257)
    at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1053)
    at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:4165)
    at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3754)
    at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2701)
    at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2665)
    at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2499)
    at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:679)
    at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
    at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:668)
    at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
    at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:123)
    at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
    at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
    at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
    at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:344)
    at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:622)
    at org.eclipse.equinox.launcher.Main.basicRun(Main.java:577)
    at org.eclipse.equinox.launcher.Main.run(Main.java:1410)
    at org.eclipse.equinox.launcher.Main.main(Main.java:1386)

Here is line 166 of BoardView.java:

wall[i].draw(c);

How can I fix this? I got this code from a tetris source code and I want to study this for future reference but I can't view the layout of the game

Answer 1

You are currently not doing any NULL checking in your onDraw().

Having the NULL checks in will help you make more stable code and help you find your problem quicker.

I would add in some NULL checks like below:

protected void onDraw(Canvas canvas) {
    if (canvas != null) {
        c = canvas;
        super.onDraw(canvas);
        if (bg) {
            if (mbg != null) {
                mbg.draw(canvas);
            } else {
                /*
                 * Handle null value
                 * print to logcat this was null
                 */
            }
        }
        for (int i = 0; i < 102; i++) {
            if (wall[i] != null) {
                wall[i].draw(c);
            } else {
                /*
                 * Handle null value
                 * print to logcat this was null
                 */
            }
        }
        for (int i = 0; i < 20; i++) {
            for (int j = 0; j < 10; j++) {
                if (block[i][j] != null) {
                    block[i][j].draw(canvas);
                } else {
                    /*
                     * Handle null value
                     * print to logcat this was null
                     */
                }
            }
        }
        //Actually draw
        invalidate();
    } else {
        /*
         * Handle null value
         * print to logcat this was null
         */
    }

}