Question

我在画布上绘制了一条路径，我希望它出现在ImageView中但是我得到一个NullPointerException，然后显示在堆栈跟踪中：

Process: com.example.abcd, PID: 9233
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.abcd/com.example.abcd.NewTest}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5017)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.example.abcd.NewTest.onCreate(NewTest.java:28)
at android.app.Activity.performCreate(Activity.java:5231)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
... 11 more

我就是这样做的：

public class NewTest extends Activity{
public static ImageView im1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(new NewAnimation(this));

    ImageView im = (ImageView)findViewById(R.id.imageView1);
    im.setImageBitmap(NewAnimation.bitMap); 
}




public static class NewAnimation extends View{

Paint paint;
Context context;
int screenW, screenH;
Path path; 
float X,Y;
public static Bitmap bitMap;
Canvas canvas;


public NewAnimation(Context context) {
    super(context);
    this.context = context;
    paint = new Paint();
    paint.setAntiAlias(true);
    paint.setStyle(Style.STROKE); 
    paint.setStrokeWidth(10); 
    paint.setColor(Color.RED);
    path = new Path();
}


public NewAnimation(Context context, AttributeSet attrs) {
    super(context);
    this.context = context;
    paint = new Paint();
    paint.setAntiAlias(true);
    paint.setStyle(Style.STROKE);
    paint.setStrokeWidth(10);
    paint.setColor(Color.RED);
    path = new Path();

}


@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);

    screenW = w;
    screenH = h;
    X = screenW/4;
    Y = screenH/2;

    path.moveTo(X, Y);

    Bitmap.Config conf = Bitmap.Config.ARGB_8888; 
    bitMap = Bitmap.createBitmap(500, 1000, conf); 
    canvas = new Canvas(bitMap); 
}

@Override
protected void onDraw(Canvas canvas) { 
    super.onDraw(canvas); 

        path.lineTo(X+20, Y);
path.lineTo(X+20, Y+20);

    canvas.drawPath(path, paint); 
    canvas.drawBitmap(bitMap, 500, 500, paint); 

    invalidate();
} 

}
}

是否将位图设置为imageview时出错？这样做的正确方法是什么？

Answer 1

错误在于

setContentView(new NewAnimation(this));
ImageView im = (ImageView)findViewById(R.id.imageView1);

您正在将自定义视图设置为内容视图，然后找到那里不存在的imageView1。所以它获得了空值。

设置

setContentView(R.layout.layout_which_contains_imageView1).
im.setImageBitmap((new NewAnimation(this)).bitMap); //ignore the static warning

在第一个构造函数中移动这三行

Bitmap.Config conf = Bitmap.Config.ARGB_8888;
bitMap = Bitmap.createBitmap(500, 1000, conf);
canvas = new Canvas(bitMap);

接下来在这三个

之下添加这一行

draw(canvas);

此致

Answer 2

您正在设置NPE，因为您正在设置

new NewAnimation(this);

在 setContentView（）中

，然后您将找到imageview ID。您必须已在某些xml中定义了imageview，因此请在setContentView（）中设置该xml布局。

Answer 3

这部分存在问题：

ImageView im = (ImageView)findViewById(R.id.imageView1);

由于你没有调用：setContentView(R.layout.activity_main);，activity_main没有膨胀，findViewbyID返回null。

现在就这样做：

View base = getLayoutInflater().inflate(R.layout.activity_main,null);//I am assuming that imageView1 is in activity_main.xml file
ImageView im = (ImageView)base.findViewById(R.id.imageView1);

更新：删除invalidate()表单onDraw()方法。它创建了递归循环。

将画布设置为图像视图时出现空指针异常

3 个答案: