Java 为什么我的应用程序会崩溃？

有人能指出这段代码中的错误吗。我正在学习编码，我正试图运行我看到的教程中的第一个应用程序，直到我发现这条消息“不幸的是，新波士顿已经停止”。这是我每次安装后试图打开它时收到的消息。我甚至在三部不同的手机和模拟器上试过，同样的事情发生了。我查看了我的代码，发现任何一行代码都没有错误。然后我试着调试它。下面是完整的代码和logcat报告

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.os.Build;
public class MainActivity extends ActionBarActivity {

int counter;
Button add,sub;
TextView display;

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
        add = (Button) findViewById(R.id.bAdd);
        add = (Button) findViewById(R.id.sSub);
        display = (TextView) findViewById(R.id.tvDisplat);
        add.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                counter++;
                display.setText("Your total is" + counter);

            }
        });
          sub.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                counter--;
                display.setText("Your total is" + counter);

            }
        });
    }
}


@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;
}

@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();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }
}

}

Logcat输出

07-18 21:28:21.047: E/AndroidRuntime(10601): FATAL EXCEPTION: main
07-18 21:28:21.047: E/AndroidRuntime(10601): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.thenewbostone/com.thenewbostone.MainActivity}: java.lang.NullPointerException
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.app.ActivityThread.access$2300(ActivityThread.java:125)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.os.Handler.dispatchMessage(Handler.java:99)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.os.Looper.loop(Looper.java:123)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.app.ActivityThread.main(ActivityThread.java:4627)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at java.lang.reflect.Method.invokeNative(Native Method)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at java.lang.reflect.Method.invoke(Method.java:521)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:871)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at dalvik.system.NativeStart.main(Native Method)
07-18 21:28:21.047: E/AndroidRuntime(10601): Caused by: java.lang.NullPointerException
07-18 21:28:21.047: E/AndroidRuntime(10601):    at com.thenewbostone.MainActivity.onCreate(MainActivity.java:34)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
07-18 21:28:21.047: E/AndroidRuntime(10601):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
07-18 21:28:21.047: E/AndroidRuntime(10601):    ... 11 more

---

你只是有这个问题，子应该在第二行，而不是添加

add = (Button) findViewById(R.id.bAdd);
add = (Button) findViewById(R.id.sSub);

所以，它在这条线上崩溃了

sub.setOnClickListener(new View.OnClickListener() {

---

原因sub未初始化。

请在Railag发现输入错误后发布片段和活动的xml：