Java 在android中生成随机圆
我正在尝试制作一个简单的应用程序，当按下一个按钮时，它会随机画出圆圈，当按下另一个按钮时，它会从屏幕上清除所有圆圈。目前，由于某种原因，当我在模拟器上启动应用程序时，它会以自动生成的圆圈开始，然后如果我试图按下按钮生成另一个圆圈，应用程序就会崩溃，我的电脑会出现NullPointerExceptionJava 在android中生成随机圆,java,android,canvas,graphics,nullpointerexception,Java,Android,Canvas,Graphics,Nullpointerexception,我正在尝试制作一个简单的应用程序，当按下一个按钮时，它会随机画出圆圈，当按下另一个按钮时，它会从屏幕上清除所有圆圈。目前，由于某种原因，当我在模拟器上启动应用程序时，它会以自动生成的圆圈开始，然后如果我试图按下按钮生成另一个圆圈，应用程序就会崩溃，我的电脑会出现NullPointerException c.drawcircle(b.getX(), b.getY(), b.getR(), p) 方法 以下是我目前拥有的： 主要内容： drawView： import java.util.Arra
c.drawcircle(b.getX(), b.getY(), b.getR(), p)
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();
}
}
感谢您的输入，这是我第一次处理图形，所以我无法100%确定为什么会发生这种情况。您在此处将null传递给您的onDraw方法：
dv.onDraw(null);
nullpublic 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!
}
==>NPE至于你的随机方法，试试这个
Random rng = new Random();
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);
}
}
import java.util.Random；
导入android.graphics.Canvas；
导入android.graphics.Color；
导入android.graphics.Paint；
导入android.graphics.Paint.Style；
公共阶层成长圈{
浮点x，y；整数半径；
油漆myp=新油漆（）；
内冷，冷，冷；
int-redvar=1；
int-bluevar=5；
int-greenvar=2；
int脱扣器=10；
int-change=2；
Random rand=新的Random（）；
公共循环（浮动x，浮动y）{
这个.x=x；
这个。y=y；
这个。半径=2；
this.colr=rand.nextInt（254）+1；
this.colg=rand.nextInt（254）+1；
this.colb=rand.nextInt（254）+1；
}
公共无效更新（）{
半径+=4；
脱扣器+=更换；
如果（脱扣器=15）{
改变=-改变；
}
随机列=新随机（）；
myp.setColor（Color.argb（255，colr，colg，colb））；
colr+=redvar；
colg+=greenvar；
colb+=bluevar；
如果（colr=250）{
redvar=-redvar；
}
如果（colg=250）{
greenvar=-greenvar；
}
如果（colb=250）{
bluevar=-bluevar；
}
}
公共空白绘图（画布）{
myp.设定行程宽度（脱扣器）；
myp.setStyle（Style.STROKE）；
画布绘制圆（x，y，半径，myp）；
}
}
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);
}
}
}
}
import java.util.ArrayList；
导入java.util.List；
导入java.util.Random；
导入java.util.concurrent.CopyOnWriteArrayList；
进口com.gmaninc.acidrain2.R；
导入android.content.SharedReferences；
导入android.graphics.Bitmap；
导入android.graphics.BitmapFactory；
导入android.graphics.Canvas；
导入android.graphics.Color；
导入android.graphics.Paint；
导入android.os.Handler；
导入android.os.SystemClock；
导入android.preference.PreferenceManager；
导入android.service.wallpar.wallperservice；
导入android.service.wallper.wallperservice.Engine；
导入android.view.MotionEvent；
导入android.view.SurfaceHolder；
导入android.view.view；
导入android.view.view.OnTouchListener；
公共类LiveWallperService扩展了WallperService{
CopyOnWriteArrayList gc=新建CopyOnWriteArrayList（）；
私有最终处理程序mHandler=新处理程序（）；
@凌驾
public void onCreate（）{
super.onCreate（）；
}
@凌驾
公共空间{
super.ondestory（）；
}
@凌驾
公共引擎onCreateEngine（）{
返回新的立方发动机（）；
}
类CubeEngine扩展了引擎{
专用最终油漆mPaint=新油漆（）；
私人浮动货币基金；
私有浮动mTouchX=-1；
私人浮动mTouchY=-1；
私人长时间；
私人浮动mCenterX；
私人浮动麦肯锡；
private final Runnable mDrawCube=new Runnable（）{
公开募捐{
牵引架（）；
}
};
私有布尔mVisible；
立方发动机（）{
//创建一个绘画来绘制立方体的线条
最终油漆=mPaint；
paint.setColor（0xffffffff）；
paint.setAntiAlias（真）；
油漆。设置行程宽度（2）；
油漆固定行程盖（油漆固定行程盖圆形）；
绘制.设置样式（绘制.样式.笔划）；
mStartTime=SystemClock.elapsedRealtime（）；
}
@凌驾
创建时的公共void（SurfaceHolder SurfaceHolder）{
super.onCreate（surfaceHolder）；
//默认情况下，我们不会获取触摸事件，因此请启用它们。
setTouchEventsEnabled（真）；
}
@凌驾
公共空间{
super.ondestory（）；
mHandler.removeCallbacks（mDrawCube）；
}
@凌驾
公共无效onVisibilityChanged（布尔可见）{
mVisible=可见；
如果（可见）{
牵引架（）；
}否则{
mHandler.removeCallbacks（mDrawCube）；
}
}
@凌驾
更改了曲面上的公共空白（曲面文件夹持有者、整型格式、整型宽度、整型高度）{
super.onSurfaceChanged（支架、格式、宽度、高度）；
//存储曲面的中心，以便我们可以在正确的位置绘制立方体
mCenterX=宽度/2.0f；
mCenterY=高度/2.0f；
对于（GrowCirc:gc）{
circ.update（）；
}
牵引架（）；
}
@凌驾
已创建曲面上的公共空白（曲面持有人）{
super.onSurfaceCreated（持有者）；
}
@凌驾
表面上的公共空隙已销毁（表面持有人）{
super.OnSurface（持有人）；
mVisible=false；
mHandler.removeCallbacks（mDrawCube）；
}
@凌驾
公共无效OnOffsetChanged（浮动xOffset、浮动yOffset、，
弗洛亚
color = Color.rgb(rng.nextInt(255), rng.nextInt(255), rng.nextInt(255));
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);
}
}
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);
}
}
}
}
<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>