java - ImageView未初始化

Question

我正在测试我正在制作的游戏的碰撞箱系统。问题是我在球类中遇到以下错误:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“android.view.View android.view.Window.findViewById(int)”

它发生在 ball 类的构造函数方法的第一行。有谁知道我做错了什么吗？

主类:

package xander.mazetestapp;

import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.RelativeLayout;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity implements SensorEventListener {
    private TextView text;
    private SensorManager sManager;
    private int a = 300;     //x position
    private int b = 300;     //y position
    int x = 0;
    int y = 0;
    ball playingBall;
    wall mazeWall;
    float show = 1;
    float hide = 0;
    boolean allowedMovement[] = {true, true, true, true};
    int maxX = 0;
    int maxY = 0;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    text = (TextView) findViewById(R.id.info);
    sManager = (SensorManager) getSystemService(SENSOR_SERVICE);

    playingBall = new ball();
    mazeWall = new wall(hide, R.id.wall1);

}

//when this Activity starts
@Override
protected void onResume() {
    super.onResume();
    /*register the sensor listener to listen to the gyroscope sensor, use the
    callbacks defined in this class, and gather the sensor information as quick
    as possible*/
    sManager.registerListener(this, sManager.getDefaultSensor(Sensor.TYPE_ORIENTATION), SensorManager.SENSOR_DELAY_FASTEST);
}

//When this Activity isn't visible anymore
@Override
protected void onStop() {
    //unregister the sensor listener
    sManager.unregisterListener(this);
    super.onStop();
}

@Override
public void onAccuracyChanged(Sensor arg0, int arg1) {
    //Do nothing.
}

public void limitMovement(ball ball, wall wall) {
    float wy = (ball.getWidth() + wall.getWidth()) * (ball.getCenterY() - wall.getCenterY());
    float hx = (ball.getHeight() + wall.getHeight()) * (ball.getCenterX() - wall.getCenterX());

    if (wy > hx) {
        if (wy > -hx) {//top
            allowedMovement[1] = false;
        } else {//left
            allowedMovement[2] = false;
        }
    } else {
        if (wy > -hx) {//right
            allowedMovement[3] = false;
        } else {//bottom
            allowedMovement[0] = false;
        }
    }
}

public boolean intersect(ball ball, wall wall) {
    //top left corner of the ball
    if (ball.getTopLeftX() >= wall.getTopLeftX() && ball.getTopLeftX() <= wall.getTopRightX()) {
        if (ball.getTopLeftY() >= wall.getTopLeftY() && ball.getTopLeftY() <= wall.getBottomLeftY()) {
            limitMovement(ball, wall);
            return true;
        }
    }

    //top rigth corner of the ball
    if (ball.getTopRightX() >= wall.getTopLeftX() && ball.getTopRightX() <= wall.getTopRightX()) {
        if (ball.getTopRightY() >= wall.getTopLeftY() && ball.getTopRightY() <= wall.getBottomLeftY()) {
            limitMovement(ball, wall);
            return true;
        }
    }

    //bottom left corner of the ball
    if (ball.getBottomLeftX() >= wall.getBottomLeftX() && ball.getBottomLeftX() <= wall.getBottomRightX()) {
        if (ball.getBottomLeftY() >= wall.getTopLeftY() && ball.getBottomLeftY() <= wall.getBottomLeftY()) {
            limitMovement(ball, wall);
            return true;
        }
    }

    //bottom rigth corner of the ball
    if (ball.getBottomRightX() >= wall.getBottomLeftX() && ball.getBottomRightX() <= wall.getBottomRightX()) {
        if (ball.getBottomRightY() >= wall.getTopLeftY() && ball.getBottomRightY() <= wall.getBottomLeftY()) {
            limitMovement(ball, wall);
            return true;
        }
    }


    return false;

}


public void move(int x, int y) {
    RelativeLayout.LayoutParams alp = playingBall.getLayoutParams();
    int maxMovementX = Math.abs(x);
    int maxMovenentY = Math.abs(y);
    int stepsTakenX = 0;
    int stepsTakenY = 0;

    while (maxMovementX > stepsTakenX || maxMovenentY > stepsTakenY) {
        //up 0, down 1, right 3, left 2
        if (stepsTakenX < maxMovementX) {
            stepsTakenX = stepsTakenX + 1;
            if (x > 0 && allowedMovement[3] == true) {//right
                playingBall.setCenterX(playingBall.getCenterX() - 1);
                a = a - 1;
            }
            if (x < 0 && allowedMovement[2] == true) {//left
                playingBall.setCenterX(playingBall.getCenterX() + 1);
                a = a + 1;
            }
        }

        if (stepsTakenY < maxMovenentY) {
            stepsTakenY = stepsTakenY + 1;
            if (y > 0 && allowedMovement[1] == true) {//down
                playingBall.setCenterY(playingBall.getCenterY() - 1);
                b = b - 1;
            }
            if (y < 0 && allowedMovement[0] == true) {//up
                playingBall.setCenterY(playingBall.getCenterY() + 1);
                b = b + 1;
            }
        }

    }

    alp.leftMargin = a;
    alp.topMargin = b;
    playingBall.setLayoutParams(alp);
}


@Override
public void onSensorChanged(SensorEvent event) {

    //if sensor is unreliable, return void
    if (event.accuracy == SensorManager.SENSOR_STATUS_UNRELIABLE) {
        return;
    }

    //else it will output the Roll, Pitch and Yawn values
    x = Math.round(event.values[2]) / 3;
    y = Math.round(event.values[1]) / 3;

    if (x > 15) {
        x = 15;
    }
    if (x < -15) {
        x = -15;
    }
    if (y > 15) {
        y = 15;
    }
    if (y < -15) {
        y = -15;
    }


    //kleinere x is naar links
    //kleinere y is naar boven
    //balk1 is boven
    //balk2 is onder
    //balk3 is links
    //balk 4 is rechts


    move(x, y);


    text.setText("Width: " + playingBall.getWidth() +
                    "    Height: " + playingBall.getHeight() +
                    "   B x: " + playingBall.getCenterX() +
                    "   B y: " + playingBall.getCenterY() +
                    "   W  x: " + mazeWall.getCenterX() +
                    "   W  y: " + mazeWall.getCenterY() +
                    "  wall LB x: " + mazeWall.getTopLeftX() +
                    "  wall LB y: " + mazeWall.getTopLeftY() +
                    "Width: " + mazeWall.getWidth() +
                    "    Height: " + mazeWall.getHeight()


    );

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

}

球类:

package xander.mazetestapp;


import android.app.Activity;
import android.support.v7.app.AppCompatActivity;
import android.widget.ImageView;
import android.widget.RelativeLayout;



public class ball extends AppCompatActivity {

private int centerX;
private int centerY;
private int topLeftX;
private int topRightX;
private int bottomLeftX;
private int bottomRightX;
private int topLeftY;
private int topRightY;
private int bottomLeftY;
private int bottomRightY;
private int width;
private int height;
public ImageView ballImage;


public ball(){
   ballImage = (ImageView) findViewById(R.id.ball);

    centerX =(int) ballImage.getX();
    centerY = (int) ballImage.getY();
    width = ballImage.getWidth();
    height = ballImage.getHeight();

    setCorners();
}

private void setCorners() {
    topLeftX=(centerX-(width/2));
    topLeftY=(centerY-(height/2));

    topRightX=(centerX+(width/2));
    topRightY=(centerY-(height/2));

    bottomRightX=(centerX+(width/2));
    bottomRightY=(centerY+(height/2));

    bottomLeftX=(centerX-(width/2));
    bottomLeftY=(centerY+(height/2));
}

public int getWidth(){
    return width;
}

public int getHeight(){
    return height;
}

public int getCenterX(){
    return centerX;
}

public int getCenterY(){
    return centerY;
}

public int getTopLeftX(){
    return topLeftX;
}

public int getTopRightX(){
    return topRightX;
}

public int getBottomLeftX(){
    return bottomLeftX;
}

public int getBottomRightX(){return bottomRightX;}

public int getTopLeftY(){
    return topLeftY;
}

public int getTopRightY(){
    return topRightY;
}

public int getBottomLeftY(){
    return bottomLeftY;
}

public int getBottomRightY(){
    return bottomRightY;
}

public void setCenterX(int x){
    centerX=x;
}

public void setCenterY(int y){
    centerY=y;
}









public RelativeLayout.LayoutParams getLayoutParams(){
    return (RelativeLayout.LayoutParams) ballImage.getLayoutParams();

}

public void setLayoutParams(RelativeLayout.LayoutParams alp){
    ballImage.setLayoutParams(alp);
}

Answer 1

我猜 Hardik 是对的，球类是一个 Activity ，因为您已经使用了扩展 appCompatActivity。所以你必须在onCreate方法中初始化 ImageView 。