致命异常:main java.lang.NullPointerException

Question

我得到一个致命的异常/空指针异常。我不知道为什么

如果需要任何额外的信息，请让我知道。为什么我得到一个空指针异常?如果你需要更多的信息，请告诉我。

---

JAVA

public class MainActivity extends Activity {    
    private ImageButton car;
    private ImageButton foot;
    private ImageButton bus;
    private ImageButton train;
    private ImageButton carlift;
    protected void onCreate(Bundle savedInstanceState) {
        setContentView(R.layout.startpic);
    // display the logo during 5 secondes,
    new CountDownTimer(5000, 1000) {
        @Override
    public void onTick(long millisUntilFinished) {
    }
    @Override
    public void onFinish() {
        // set the new Content of your activity
        MainActivity.this.setContentView(R.layout.activity_main);
    }
        }.start();
    car = (ImageButton) findViewById(R.id.imageButton1);
    foot = (ImageButton) findViewById(R.id.imageButton2);
    bus = (ImageButton) findViewById(R.id.imageButton3);
    train = (ImageButton) findViewById(R.id.imageButton4);
    carlift = (ImageButton) findViewById(R.id.imageButton5);
    car.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
            setContentView(R.layout.car);
            }
    });
    foot.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
            setContentView(R.layout.foot);
       }
    });
    bus.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
            setContentView(R.layout.bus);
        }
    });
    train.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
        setContentView(R.layout.train);
        }
    });
    carlift.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
        setContentView(R.layout.carlift);
        }
    });
    super.onCreate(savedInstanceState);
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
    }
}

---

这是我的Logcat

---

<标题> LOGCAT h1> div class="one_answers">

ImageButtons现在都是null，因为正确的内容视图还没有设置。

需要在onFinish()方法中设置OnClickListener s。所以你的代码应该是:

public class MainActivity extends Activity {    
    private ImageButton car;
    private ImageButton foot;
    private ImageButton bus;
    private ImageButton train;
    private ImageButton carlift;
    protected void onCreate(Bundle savedInstanceState) {
        setContentView(R.layout.startpic);
    // display the logo during 5 secondes,
    new CountDownTimer(5000, 1000) {
        @Override
    public void onTick(long millisUntilFinished) {
    }
    @Override
    public void onFinish() {
        // set the new Content of your activity
        MainActivity.this.setContentView(R.layout.activity_main);

    car = (ImageButton) findViewById(R.id.imageButton1);
    foot = (ImageButton) findViewById(R.id.imageButton2);
    bus = (ImageButton) findViewById(R.id.imageButton3);
    train = (ImageButton) findViewById(R.id.imageButton4);
    carlift = (ImageButton) findViewById(R.id.imageButton5);
    car.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
            setContentView(R.layout.car);
            }
    });
    foot.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
            setContentView(R.layout.foot);
       }
    });
    bus.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
            setContentView(R.layout.bus);
        }
    });
    train.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
        setContentView(R.layout.train);
        }
    });
    carlift.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
        setContentView(R.layout.carlift);
        }
    });
    }
        }.start();
super.onCreate(savedInstanceState);
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
    }
}