Question

我正在创建一个显示快速拨号的应用程序，但每当我运行应用程序时，我都会收到消息

java.lang.InstantiationException：无法实例化类：没有空构造函数

我该如何解决这个问题？

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.RadialGradient;
import android.graphics.Shader;
import android.util.AttributeSet;
import android.view.View;

public class Needle extends View {

private Paint linePaint;
private Path linePath;
private Paint needleScrewPaint;

private Matrix matrix;
private int framePerSeconds = 100;
private long animationDuration = 10000;
private long startTime;

public Needle(Context context) {
    super(context);
    matrix = new Matrix();
    this.startTime = System.currentTimeMillis();
    this.postInvalidate();
    init();
}

public Needle(Context context, AttributeSet attrs) {
    super(context, attrs);
    matrix = new Matrix();
    this.startTime = System.currentTimeMillis();
    this.postInvalidate();
    init();
}

public Needle(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
    matrix = new Matrix();
    this.startTime = System.currentTimeMillis();
    this.postInvalidate();
    init();
}

private void init(){

    linePaint = new Paint();
    linePaint.setColor(Color.RED); // Set the color
    linePaint.setStyle(Paint.Style.FILL_AND_STROKE); // set the border and fills the inside of needle
    linePaint.setAntiAlias(true);
    linePaint.setStrokeWidth(5.0f); // width of the border
    linePaint.setShadowLayer(8.0f, 0.1f, 0.1f, Color.GRAY); // Shadow of the needle

    linePath = new Path();
    linePath.moveTo(50.0f, 50.0f);
    linePath.lineTo(130.0f, 40.0f);
    linePath.lineTo(600.0f, 50.0f);
    linePath.lineTo(130.0f, 60.0f);
    linePath.lineTo(50.0f, 50.0f);
    linePath.addCircle(130.0f, 50.0f, 20.0f, Path.Direction.CW);
    linePath.close();

    needleScrewPaint = new Paint();
    needleScrewPaint.setColor(Color.BLACK);
    needleScrewPaint.setAntiAlias(true);
    needleScrewPaint.setShader(new RadialGradient(130.0f, 50.0f, 10.0f,
            Color.DKGRAY, Color.BLACK, Shader.TileMode.CLAMP));
}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);


    long elapsedTime = System.currentTimeMillis() - startTime;

    matrix.postRotate(1.0f, 130.0f, 50.0f); // rotate 10 degree every second
    canvas.concat(matrix);

    canvas.drawPath(linePath, linePaint);

    canvas.drawCircle(130.0f, 50.0f, 16.0f, needleScrewPaint);

    if(elapsedTime < animationDuration){
        this.postInvalidateDelayed(10000 / framePerSeconds);
    }

    //this.postInvalidateOnAnimation();
    invalidate();
}

}

Answer 1

 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{needle.ibizsmart.com.needle/com.needle.views.Needle}: java.lang.InstantiationException: can't instantiate class com.needle.views.Needle; no empty constructor

在您的清单中的某个位置，您有一个<activity>元素，表示com.needle.views.Needle是Activity的子类。

而且，在Java代码的某处，您正在调用startActivity()，Intent指向com.needle.views.Needle。

但是，com.needle.views.Needle不会从Activity继承。它继承自View。您不能将其用作活动。

我们当然欢迎您创建一个从<{1}}继承的继承的另一个类，它使用Activity作为其用户界面的一部分。

Android - java.lang.InstantiationException：无法实例化类：没有空构造函数

1 个答案: