麻煩畫圈到屏幕

Question

我是新來的android編程，並試圖在屏幕上畫一個圓圈。最終我想讓這個圈子四處移動。但現在我一直無法讓圈子畫畫。當我運行的代碼，它給了我一個錯誤：

"Attempt to invoke virtual method 'long android.graphics.Paint.getNativeInstance()' on a null"

我的代碼：

package com... 

import android.content.Context; 
import android.graphics.Bitmap; 
import android.graphics.BitmapFactory; 
import android.graphics.Canvas; 
import android.graphics.Color; 
import android.graphics.Paint; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.View; 

public class MainActivity extends AppCompatActivity { 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(new MyView(this)); 
    } 

    class MyView extends View { 



    private Bitmap b; 
     public Paint p; 

     public MyView(Context context) { 
      super(context); 
      Paint p = new Paint(); 
      p.setColor(Color.GREEN); 
      b = Bitmap.createBitmap(100, 100, Bitmap.Config.ARGB_8888); 
     } 

     @Override 
     protected void onDraw(Canvas canvas) { 
      canvas.drawCircle(50, 50, 10, p); 
     } 
    } 
} 

Answer 1

問題與您的代碼是

public MyView(Context context) { 
      super(context); 
      Paint p = new Paint();// Problem is here this is local declaration 
      p = new Paint() // do like this and remove above line 
      p.setColor(Color.GREEN); 
      b = Bitmap.createBitmap(100, 100, Bitmap.Config.ARGB_8888); 
     }