无法实例化类……; 没有空的构造函数
当我运行我的PaintView.java ，我在logcat中收到以下错误，即无法实例化类com.example.connectthedots.PaintView ; 没有空的构造函数。
07-22 18:47:43.453: D/dalvikvm(19728): newInstance failed: no () 07-22 18:47:43.453: D/AndroidRuntime(19728): Shutting down VM 07-22 18:47:43.453: W/dalvikvm(19728): threadid=1: thread exiting with uncaught exception (group=0x41bf6da0) 07-22 18:47:43.453: E/AndroidRuntime(19728): FATAL EXCEPTION: main 07-22 18:47:43.453: E/AndroidRuntime(19728): Process: com.example.connectthedots, PID: 19728 07-22 18:47:43.453: E/AndroidRuntime(19728): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.connectthedots/com.example.connectthedots.PaintView}: java.lang.InstantiationException: can't instantiate class com.example.connectthedots.PaintView; no empty constructor 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2231) 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363) 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.app.ActivityThread.access$900(ActivityThread.java:161) 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265) 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.os.Handler.dispatchMessage(Handler.java:102) 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.os.Looper.loop(Looper.java:157) 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.app.ActivityThread.main(ActivityThread.java:5356) 07-22 18:47:43.453: E/AndroidRuntime(19728): at java.lang.reflect.Method.invokeNative(Native Method) 07-22 18:47:43.453: E/AndroidRuntime(19728): at java.lang.reflect.Method.invoke(Method.java:515) 07-22 18:47:43.453: E/AndroidRuntime(19728): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265) 07-22 18:47:43.453: E/AndroidRuntime(19728): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081) 07-22 18:47:43.453: E/AndroidRuntime(19728): at dalvik.system.NativeStart.main(Native Method) 07-22 18:47:43.453: E/AndroidRuntime(19728): Caused by: java.lang.InstantiationException: can't instantiate class com.example.connectthedots.PaintView; no empty constructor 07-22 18:47:43.453: E/AndroidRuntime(19728): at java.lang.Class.newInstanceImpl(Native Method) 07-22 18:47:43.453: E/AndroidRuntime(19728): at java.lang.Class.newInstance(Class.java:1208) 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.app.Instrumentation.newActivity(Instrumentation.java:1079) 07-22 18:47:43.453: E/AndroidRuntime(19728): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2222) 07-22 18:47:43.453: E/AndroidRuntime(19728): ... 11 more
我的PaintView.java看起来像这样：
package com.example.connectthedots; import java.util.ArrayList; import java.util.List; import android.content.Context; import android.content.res.Resources; import android.graphics.Bitmap; import android.graphics.Canvas; import android.graphics.Color; import android.graphics.Paint; import android.graphics.Path; import android.graphics.Point; import android.util.AttributeSet; import android.util.TypedValue; import android.view.MotionEvent; import android.view.View; public class PaintView extends View { private Bitmap mBitmap; private Canvas mCanvas; private Path mPath; private Paint mPaint; private static final int TOUCH_TOLERANCE_DP = 24; private static final int BACKGROUND = 0xFFDDDDDD; private List mPoints = new ArrayList(); private int mLastPointIndex = 0; private int mTouchTolerance; private boolean isPathStarted = false; public PaintView(Context context) { super(context); mCanvas = new Canvas(); mPath = new Path(); mPaint = new Paint(); mPaint.setAntiAlias(true); mPaint.setDither(true); mPaint.setColor(Color.BLACK); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeJoin(Paint.Join.ROUND); mPaint.setStrokeCap(Paint.Cap.ROUND); mPaint.setStrokeWidth(12); mTouchTolerance = dp2px(TOUCH_TOLERANCE_DP); // TODO just test points Point p1 = new Point(20, 20); Point p2 = new Point(100, 100); Point p3 = new Point(200, 250); Point p4 = new Point(280, 400); Point p5 = new Point(350, 600); Point p6 = new Point(400, 500); mPoints.add(p1); mPoints.add(p2); mPoints.add(p3); mPoints.add(p4); mPoints.add(p5); mPoints.add(p6); } public PaintView(Context context, AttributeSet attrs) { super(context, attrs); mCanvas = new Canvas(); mPath = new Path(); mPaint = new Paint(); mPaint.setAntiAlias(true); mPaint.setDither(true); mPaint.setColor(Color.BLACK); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeJoin(Paint.Join.ROUND); mPaint.setStrokeCap(Paint.Cap.ROUND); mPaint.setStrokeWidth(12); mTouchTolerance = dp2px(TOUCH_TOLERANCE_DP); // TODO just test points Point p1 = new Point(20, 20); Point p2 = new Point(100, 100); Point p3 = new Point(200, 250); Point p4 = new Point(280, 400); Point p5 = new Point(350, 600); Point p6 = new Point(400, 500); mPoints.add(p1); mPoints.add(p2); mPoints.add(p3); mPoints.add(p4); mPoints.add(p5); mPoints.add(p6); } public PaintView(Context context, AttributeSet attrs, int defStyle) { super(context, attrs, defStyle); mCanvas = new Canvas(); mPath = new Path(); mPaint = new Paint(); mPaint.setAntiAlias(true); mPaint.setDither(true); mPaint.setColor(Color.BLACK); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeJoin(Paint.Join.ROUND); mPaint.setStrokeCap(Paint.Cap.ROUND); mPaint.setStrokeWidth(12); mTouchTolerance = dp2px(TOUCH_TOLERANCE_DP); } @Override protected void onSizeChanged(int width, int height, int oldWidth, int oldHeight) { super.onSizeChanged(width, height, oldWidth, oldHeight); clear(); } @Override protected void onDraw(Canvas canvas) { canvas.drawColor(BACKGROUND); canvas.drawBitmap(mBitmap, 0, 0, null); canvas.drawPath(mPath, mPaint); // TODO remove if you dont want points to be drawn for (Point point : mPoints) { canvas.drawPoint(point.x, point.y, mPaint); } } @Override public boolean onTouchEvent(MotionEvent event) { float x = event.getX(); float y = event.getY(); switch (event.getAction()) { case MotionEvent.ACTION_DOWN: touch_start(x, y); invalidate(); break; case MotionEvent.ACTION_MOVE: touch_move(x, y); invalidate(); break; case MotionEvent.ACTION_UP: touch_up(x, y); invalidate(); break; } return true; } private void touch_start(float x, float y) { if (checkPoint(x, y, mLastPointIndex)) { mPath.reset(); // user starts from given point so path can beis started isPathStarted = true; } else { // user starts move from point which doen's belongs to mPinst list isPathStarted = false; } } //ADDED WITH LAST EDIT private void touch_move(float x, float y) { // draw line with finger move if (isPathStarted) { mPath.reset(); Point p = mPoints.get(mLastPointIndex); mPath.moveTo(px, py); mPath.lineTo(x, y); } } /** * Draws line. */ private void touch_up(float x, float y) { mPath.reset(); if (checkPoint(x, y, mLastPointIndex + 1) && isPathStarted) { // move finished at valid point so draw whole line // start point Point p = mPoints.get(mLastPointIndex); mPath.moveTo(px, py); // end point p = mPoints.get(mLastPointIndex + 1); mPath.lineTo(px, py); mCanvas.drawPath(mPath, mPaint); mPath.reset(); // increment point index ++mLastPointIndex; isPathStarted = false; } } /** * Sets paint * * @param paint */ public void setPaint(Paint paint) { this.mPaint = paint; } /** * Returns image as bitmap * * @return */ public Bitmap getBitmap() { return mBitmap; } /** * Clears canvas */ public void clear() { mBitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888); mBitmap.eraseColor(BACKGROUND); mCanvas.setBitmap(mBitmap); invalidate(); } /** * Checks if user touch point with some tolerance */ private boolean checkPoint(float x, float y, int pointIndex) { if (pointIndex == mPoints.size()) { // out of bounds return false; } Point point = mPoints.get(pointIndex); //EDIT changed point.y to poin.x in the first if statement if (x > (point.x - mTouchTolerance) && x (point.y - mTouchTolerance) && y < (point.y + mTouchTolerance)) { return true; } } return false; } public List getPoints() { return mPoints; } public void setPoints(List points) { this.mPoints = points; } private int dp2px(int dp) { Resources r = getContext().getResources(); float px = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, r.getDisplayMetrics()); return (int) px; } }
仅供参考：activity_paint_view.xml
这里的错误是什么？
引起：java.lang.InstantiationException：无法实例化类com.example.connectthedots.PaintView; 没有空的构造函数
正如exception所告诉你的那样， PaintView需要一个没有任何参数的构造函数：
public PaintView() { ... }
添加一个空的构造函数：
public PaintView() { }
我不认为简单地添加一个空构造函数是最好的解决方案。 如果要实例化并使用PaintView对象，则应使用已定义的构造函数之一。 如果您查看Android视图文档 ，则没有空构造函数。 由于您有View的子类，因此实现空构造函数并没有多大意义。
相反，我建议您查看用于创建PaintView对象的代码。 现在，你可能有一些东西：
PaintView mPaintView = new PaintView();
相反，您应该使用已经实现的构造函数之一：
PaintView mPaintView = new PaintView(mContext);
其中mContext是此实例中的有效Context对象。 如果您在Activity中执行此操作，则可以将该Activity的当前实例传递给构造函数。