Question

我试图创建一个简单的应用程序，在按下一个按钮时绘制随机圆圈，并在按下另一个按钮时将它们全部从屏幕上清除。目前，当我出于某种原因在模拟器上启动应用程序时，它以自动生成的圆圈开始，然后如果我尝试按下按钮生成另一个圆圈，应用程序崩溃，我的< NullPointerException / p>

c.drawcircle(b.getX(), b.getY(), b.getR(), p)

方法

这是我目前所拥有的：

主：

import java.util.ArrayList;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.LinearLayout;

public class Bubbles extends Activity implements OnClickListener 
{
Button b1,b2;
drawView dv;
LinearLayout frame;

@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_bubbles);
    dv = new drawView(getApplicationContext());
    frame = (LinearLayout)findViewById(R.id.LinearLayout2);
    b1 = (Button)findViewById(R.id.BubbleButton);
    b1.setOnClickListener(this);
    b2 = (Button)findViewById(R.id.ClearButton);
    b2.setOnClickListener(this);
    frame.addView(dv);      
}

@Override
public void onClick(View v)
{
    if(v==b1)
    {
        dv.onDraw(null);
    }   
    if(v==b2)
    {
        dv.clear();           
    }    
    v.postInvalidate();        
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.bubbles, menu);
    return true;
}

}

drawView函数：

import java.util.ArrayList;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.view.View;

public class drawView extends View
{
ArrayList<Bubble> bList = new ArrayList<Bubble>();
Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);

public drawView(Context con) 
{
    super(con);

}

public void onDraw(Canvas c)
{
    Bubble b = new Bubble();
    p.setColor(b.getColor());
    p.setStyle(Paint.Style.FILL);
    bList.add(b);
    c.drawCircle(b.getX(), b.getY(), b.getR(), p);

}

public void clear()
{
    bList.clear();        
}

}

我在这个类中使用arrayList存储我制作的所有气泡，然后使用clear按钮清除arrayList。这是我制作的泡泡课程：

气泡：

import android.graphics.Color;

public class Bubble 
{
int color;
int y;
int x;
int r;

public Bubble ()
{
    color = Color.rgb(rand(0,255), rand(0,255), rand(0,255));
    r = rand(0,255);
    x =rand(0,255);
    y = rand(0,255);

}

public int getColor() {
    return color;
}

public int getY() {
    return y;
}

public int getX() {
    return x;
}

public int getR() {
    return r;
}

/******************************rand()*****************************/
public int rand(int a, int b)
{
    return((int)((b-a+1)*Math.random() + a));
}
}

感谢您的任何输入，这是我第一次搞乱图形，所以我不能100％确定为什么会这样。

Answer 1

你将null传递给你的方法onDraw：

dv.onDraw(null);

我猜你想在那里传递Canvas。

在此方法中，您的Canvas将指向null，因为您将null作为参数传递：

public void onDraw(Canvas c) {
    Bubble b = new Bubble();
    p.setColor(b.getColor());
    p.setStyle(Paint.Style.FILL);
    bList.add(b);
    c.drawCircle(b.getX(), b.getY(), b.getR(), p);  // c == null here!
}

==＆GT; NPE

Answer 2

至于你的随机方法试试这个

Random rng = new Random();

在您的字段声明中。

然后使用。 rng.nextInt（255）;

color = Color.rgb(rng.nextInt(255), rng.nextInt(255), rng.nextInt(255));

至于你的圈子：首先你需要一个更新方法（如果你想要任何动作）其次你应该使用surfaceView绘制，第三，你应该使用a copyOnWriteArrayList，因为你的按钮可能在迭代它时修改列表，而copyonwritearraylist是完美的。在这里：我给你的礼物。

CLASS 1

import java.util.Random;

import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Paint.Style;

public class GrowCircle {
float x, y;int radius;
Paint myp = new Paint();
int colr,colg,colb;
int redvar=1;
int bluevar=5;
int greenvar=2;
int tripper=10;
int change=2;
Random rand = new Random();

public GrowCircle(float x, float y){
    this.x=x;
    this.y=y;
    this.radius=2;
    this.colr=rand.nextInt(254)+1;
    this.colg=rand.nextInt(254)+1;
    this.colb=rand.nextInt(254)+1;

}

public void update(){
    radius+=4;
    tripper+=change;
    if(tripper<=1||tripper>=15){
        change=-change;
    }
    Random col = new Random();
    myp.setColor(Color.argb(255,colr,colg,colb));
    colr+=redvar;
    colg+=greenvar;
    colb+=bluevar;

    if(colr<=5||colr>=250){
        redvar=-redvar;
    }
    if(colg<=5||colg>=250){
        greenvar=-greenvar;
    }
    if(colb<=5||colb>=250){
        bluevar=-bluevar;
    }

}

public void drawThis(Canvas canvas){
    myp.setStrokeWidth(tripper);
    myp.setStyle(Style.STROKE);
    canvas.drawCircle(x, y, radius, myp);
}



}

CLASS2

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.CopyOnWriteArrayList;



import com.gmaninc.acidrain2.R;





import android.content.SharedPreferences;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Handler;
import android.os.SystemClock;
import android.preference.PreferenceManager;
import android.service.wallpaper.WallpaperService;
import android.service.wallpaper.WallpaperService.Engine;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.View;
import android.view.View.OnTouchListener;
public class LiveWallpaperService extends WallpaperService {
     CopyOnWriteArrayList<GrowCircle> gc = new CopyOnWriteArrayList<GrowCircle>();
    private final Handler mHandler = new Handler();

    @Override
    public void onCreate() {
        super.onCreate();
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
    }

    @Override
    public Engine onCreateEngine() {
        return new CubeEngine();
    }

    class CubeEngine extends Engine {

        private final Paint mPaint = new Paint();
        private float mOffset;
        private float mTouchX = -1;
        private float mTouchY = -1;
        private long mStartTime;
        private float mCenterX;
        private float mCenterY;

        private final Runnable mDrawCube = new Runnable() {
            public void run() {
                drawFrame();
            }
        };
        private boolean mVisible;

        CubeEngine() {
            // Create a Paint to draw the lines for our cube
            final Paint paint = mPaint;
            paint.setColor(0xffffffff);
            paint.setAntiAlias(true);
            paint.setStrokeWidth(2);
            paint.setStrokeCap(Paint.Cap.ROUND);
            paint.setStyle(Paint.Style.STROKE);

            mStartTime = SystemClock.elapsedRealtime();
        }

        @Override
        public void onCreate(SurfaceHolder surfaceHolder) {
            super.onCreate(surfaceHolder);

            // By default we don't get touch events, so enable them.
            setTouchEventsEnabled(true);
        }

        @Override
        public void onDestroy() {
            super.onDestroy();
            mHandler.removeCallbacks(mDrawCube);
        }

        @Override
        public void onVisibilityChanged(boolean visible) {
            mVisible = visible;
            if (visible) {
                drawFrame();
            } else {
                mHandler.removeCallbacks(mDrawCube);
            }
        }

        @Override
        public void onSurfaceChanged(SurfaceHolder holder, int format, int width, int height) {
            super.onSurfaceChanged(holder, format, width, height);
            // store the center of the surface, so we can draw the cube in the right spot
            mCenterX = width/2.0f;
            mCenterY = height/2.0f;
            for(GrowCircle circ:gc){
                circ.update();
            }
            drawFrame();
        }

        @Override
        public void onSurfaceCreated(SurfaceHolder holder) {
            super.onSurfaceCreated(holder);
        }

        @Override
        public void onSurfaceDestroyed(SurfaceHolder holder) {
            super.onSurfaceDestroyed(holder);
            mVisible = false;
            mHandler.removeCallbacks(mDrawCube);
        }

        @Override
        public void onOffsetsChanged(float xOffset, float yOffset,
                float xStep, float yStep, int xPixels, int yPixels) {
            mOffset = xOffset;
            drawFrame();
        }

        /*
         * Store the position of the touch event so we can use it for drawing later
         */
        @Override
        public void onTouchEvent(MotionEvent event) {
            if (event.getAction() == MotionEvent.ACTION_DOWN) {
                mTouchX = event.getX();
                mTouchY = event.getY();
            } else {
                mTouchX = -1;
                mTouchY = -1;
            }
            float tx = event.getX();
float ty= event.getY();
            gc.add(new GrowCircle(tx,ty));
            super.onTouchEvent(event);
        }

        /*
         * Draw one frame of the animation. This method gets called repeatedly
         * by posting a delayed Runnable. You can do any drawing you want in
         * here. This example draws a wireframe cube.
         */
        void drawFrame() {
            final SurfaceHolder holder = getSurfaceHolder();

            Canvas c = null;
            try {
                c = holder.lockCanvas();
                if (c != null) {
                    // draw something
                    drawCircs(c);
                    for(GrowCircle circ:gc){
                        if(circ.radius>350){
                            gc.remove(circ);
                        }
                        circ.update();
                        circ.drawThis(c);
                    }
                    drawTouchPoint(c);
                }
            } finally {
                if (c != null) holder.unlockCanvasAndPost(c);
            }

            // Reschedule the next redraw
            mHandler.removeCallbacks(mDrawCube);
            if (mVisible) {
                mHandler.postDelayed(mDrawCube, 1000 / 25);
            }
        }

        /*
         * Draw a wireframe cube by drawing 12 3 dimensional lines between
         * adjacent corners of the cube
         */
        void drawCircs(Canvas c) {
            c.save();
            c.translate(mCenterX, mCenterY);
            c.drawColor(0xff000000);

            c.restore();
        }



        /*
         * Draw a circle around the current touch point, if any.
         */
        void drawTouchPoint(Canvas c) {
            if (mTouchX >=0 && mTouchY >= 0) {
                c.drawCircle(mTouchX, mTouchY, 80, mPaint);
            }
        }

    }
}

清单

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.gmaninc.acidrain2"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="16" />

     <uses-feature
        android:name="android.software.live_wallpaper"
        android:required="true" >
    </uses-feature>

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <service
            android:name="LiveWallpaperService"
            android:enabled="true"
            android:label="Acid Rain 2"
            android:permission="android.permission.BIND_WALLPAPER" >
            <intent-filter>
                <action android:name="android.service.wallpaper.WallpaperService" >
                </action>
            </intent-filter>

            <meta-data
                android:name="android.service.wallpaper"
                android:resource="@xml/mywallpaper" >
            </meta-data>
        </service>

    </application>

</manifest>

这是一个livewallpaper创建圈子onTouch ...随意玩它，你应该在这里找到你需要的东西....

注意：您可能需要更改清单中的包名称才能运行它！

在android中生成随机圆圈

2 个答案: