致命异常：主要是java.lang.NullPointerException

Question

我收到致命异常/空指针异常。我不知道为什么

如果需要任何其他信息，请告知我们。 为什么我得到Nullpointer异常？ 如果您需要更多信息，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

---

的logcat

03-11 12:46:22.637: E/AndroidRuntime(1769): FATAL EXCEPTION: main
03-11 12:46:22.637: E/AndroidRuntime(1769): Process: com.gunasiri.home, PID: 1769
03-11 12:46:22.637: E/AndroidRuntime(1769): java.lang.RuntimeException: Unable to start
activity ComponentInfo{com.gunasiri.home/com.gunasiri.home.MainActivity}:
java.lang.NullPointerException
03-11 12:46:22.637: E/AndroidRuntime(1769):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at     android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at android.app.ActivityThread.access$700(ActivityThread.java:135)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at android.os.Handler.dispatchMessage(Handler.java:102)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at android.os.Looper.loop(Looper.java:137)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at android.app.ActivityThread.main(ActivityThread.java:4998)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at java.lang.reflect.Method.invokeNative(Native Method)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at java.lang.reflect.Method.invoke(Method.java:515)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at dalvik.system.NativeStart.main(Native Method)
03-11 12:46:22.637: E/AndroidRuntime(1769): Caused by: java.lang.NullPointerException
03-11 12:46:22.637: E/AndroidRuntime(1769):     at com.gunasiri.home.MainActivity.onCreate(MainActivity.java:39)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at android.app.Activity.performCreate(Activity.java:5243)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
03-11 12:46:22.637: E/AndroidRuntime(1769):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
03-11 12:46:22.637: E/AndroidRuntime(1769):     ... 11 more

Answer 1

ImageButtons现在都是null，因为尚未设置正确的内容视图。您需要在OnClickListener方法中设置onFinish()。

所以你的代码是：

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;
    }
}