Question

这是我第一次创建一个新的android项目。我没有做任何改变。在尝试运行＆＃34; HelloWorld＆＃34;时，我在模拟器中收到此错误。项目

我在设置onclicklisteners时确定了错误。

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);
        counter = 0;
        add = (Button)this.findViewById(R.id.bAdd);
        sub = (Button)this.findViewById(R.id.bSub);
        display = (TextView)this.findViewById(R.id.display);

        add.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                counter = counter+1;
                display.setText("Your number is "+counter);
            }
        });

        if (savedInstanceState == null) {
                    getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                        .commit();
        }
    }
}

这是日志。

05-23 02:50:51.350: D/AndroidRuntime(2443): Shutting down VM
05-23 02:50:51.350: W/dalvikvm(2443): threadid=1: thread exiting with uncaught exception (group=0xb2aafba8)
05-23 02:50:51.380: E/AndroidRuntime(2443): FATAL EXCEPTION: main
05-23 02:50:51.380: E/AndroidRuntime(2443): Process: com.example.helloworld, PID: 2443
05-23 02:50:51.380: E/AndroidRuntime(2443): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.helloworld/com.example.helloworld.MainActivity}: java.lang.NullPointerException
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.os.Looper.loop(Looper.java:136)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at java.lang.reflect.Method.invokeNative(Native Method)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at java.lang.reflect.Method.invoke(Method.java:515)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at dalvik.system.NativeStart.main(Native Method)
05-23 02:50:51.380: E/AndroidRuntime(2443): Caused by: java.lang.NullPointerException
05-23 02:50:51.380: E/AndroidRuntime(2443):     at com.example.helloworld.MainActivity.onCreate(MainActivity.java:31)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.app.Activity.performCreate(Activity.java:5231)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-23 02:50:51.380: E/AndroidRuntime(2443):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-23 02:50:51.380: E/AndroidRuntime(2443):     ... 11 more
05-23 02:50:55.820: I/Process(2443): Sending signal. PID: 2443 SIG: 9

问题出在哪里？我现在该怎么办？

Answer 1

你有一个对MainActivity.java类第31行的引用。

检查您发布的日志：

05-23 02:50:51.380: E/AndroidRuntime(2443): Caused by: java.lang.NullPointerException
05-23 02:50:51.380: E/AndroidRuntime(2443):     at com.example.helloworld.MainActivity.onCreate(MainActivity.java:31)

似乎是您在xml文件中没有的UI元素。看看吧。

Answer 2

您必须在OnCreateView中编写代码，如下一个

@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        counter = 0;
        add = (Button)rootView.findViewById(R.id.bAdd);
        sub = (Button)rootView.findViewById(R.id.bSub);
        display = (TextView)rootView.findViewById(R.id.display);

        add.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                counter = counter+1;
                display.setText("Your number is "+counter);
            }
        });
        return rootView;
    }

Answer 3

第31行是什么？

同时尝试扩展活动，而不是 ActionBarActivity

不幸的是HelloWorld已经停止了

3 个答案: