无法实例化类......; 没有空的构造函数

Question

When I run my PaintView.java , I receive the following error in logcat, namely can't instantiate class com.example.connectthedots.PaintView ; no empty constructor.

07-22 18:47:43.453: D/dalvikvm(19728): newInstance failed: no <init>()
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

My PaintView.java looks like this:

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<Point> mPoints = new ArrayList<Point>();
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(p.x, p.y);
        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(p.x, p.y);
        // end point
        p = mPoints.get(mLastPointIndex + 1);
        mPath.lineTo(p.x, p.y);
        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.x + mTouchTolerance)) {
        if (y > (point.y - mTouchTolerance) && y < (point.y + mTouchTolerance)) {
            return true;
        }
    }
    return false;
}

public List<Point> getPoints() {
    return mPoints;
}

public void setPoints(List<Point> 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;
}
}

FYI: activity_paint_view.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".PaintView" >

    <com.example.connectthedots.PaintView
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</RelativeLayout>

What is the mistake here?

Answer 1

Caused by: java.lang.InstantiationException: can't instantiate class com.example.connectthedots.PaintView; no empty constructor

As the exception tells you, PaintView requires a constructor without any parameters :

public PaintView() {
...
}

Answer 2

add an empty Constructor:

    public PaintView() {

}

Answer 3

I don't think simply adding an empty constructor is the best solution here. If you want to instantiate and use a PaintView object, you should to use one of the already defined constructors. If you take a look at the Android View Documentation , there is no empty constructor. Since you have a subclass of View , it doesn't really make sense to implement an empty constructor.

Instead, I would recommend you take a look at the code you are using to create the PaintView object. Right now, you probably have some thing like:

PaintView mPaintView = new PaintView();

Instead, you should use one of the already implemented constructors:

PaintView mPaintView = new PaintView(mContext);

Where mContext is a valid Context object in this instance. If you are doing this in an Activity , you could just pass the current instance of that Activity to the constructor.