Question

我是菜鸟，我需要帮助用if，else语句开发一个简单的测验应用程序。我知道我的代码有些问题，但我不知道如何解决它们。即使它没有显示任何错误，当我尝试打开名为“FirstImageLogo”的活动时，应用程序崩溃。 ImageButton应该打开它。在“FirstImageLogo”里面我放了一个TextView和一个Button，用户应该写出正确的答案。如果他这样做，那么按钮应该打开一个新的活动;如果他没有，那么按钮应该打开一个Toast消息。在这种情况下，用户应在TextView中写下的消息是“Facebook”。不幸的是我没有编程知识:( 无论如何，这是我的“FirstImageLogo.class”代码：

private EditText inputtxt;
private Button btnNext;

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

    inputtxt = (EditText) findViewById(R.id.text1);

    btnNext.setOnClickListener(new View.OnClickListener() 
        {

            @Override
            public void onClick(View ContentView) {
                // TODO Auto-generated method stub
                String name;

                  name=inputtxt.getText().toString();

                 if (name.equalsIgnoreCase("facebook"))
                  {

                        Intent intent = new Intent (FirstImageLogo.this, GoodJob.class);
                        startActivity(intent);

                  }
                 else
                  {  Toast.makeText(getApplicationContext(), "Sorry, wrong answer. Try Again!", Toast.LENGTH_SHORT).show();
                  }     
            }

            });

    setupActionBar();
}


/**
 * Set up the {@link android.app.ActionBar}, if the API is available.
 */
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
private void setupActionBar() {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        getActionBar().setDisplayHomeAsUpEnabled(true);
    }
}

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

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    case android.R.id.home:
        // This ID represents the Home or Up button. In the case of this
        // activity, the Up button is shown. Use NavUtils to allow users
        // to navigate up one level in the application structure. For
        // more details, see the Navigation pattern on Android Design:
        //
        // http://developer.android.com/design/patterns/navigation.html#up-vs-back
        //
        NavUtils.navigateUpFromSameTask(this);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

}

这是日志文件：

01-17 21:35:39.365: D/dalvikvm(19074): GC_FOR_ALLOC freed 68K, 1% free 17135K/17236K, paused 14ms, total 14ms
01-17 21:35:39.905: D/dalvikvm(19074): GC_FOR_ALLOC freed 21K, 1% free 17547K/17604K, paused 7ms, total 7ms
01-17 21:35:39.915: D/AndroidRuntime(19074): Shutting down VM
01-17 21:35:39.915: W/dalvikvm(19074): threadid=1: thread exiting with uncaught exception (group=0x41596ba8)
01-17 21:35:39.915: E/AndroidRuntime(19074): FATAL EXCEPTION: main
01-17 21:35:39.915: E/AndroidRuntime(19074): Process: com.example.appquiz, PID: 19074
01-17 21:35:39.915: E/AndroidRuntime(19074): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.appquiz/com.example.appquiz.FirstImageLogo}: java.lang.NullPointerException
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.os.Handler.dispatchMessage(Handler.java:102)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.os.Looper.loop(Looper.java:136)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.app.ActivityThread.main(ActivityThread.java:5017)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at java.lang.reflect.Method.invokeNative(Native Method)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at java.lang.reflect.Method.invoke(Method.java:515)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at dalvik.system.NativeStart.main(Native Method)
01-17 21:35:39.915: E/AndroidRuntime(19074): Caused by: java.lang.NullPointerException
01-17 21:35:39.915: E/AndroidRuntime(19074):    at com.example.appquiz.FirstImageLogo.onCreate(FirstImageLogo.java:28)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.app.Activity.performCreate(Activity.java:5231)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
01-17 21:35:39.915: E/AndroidRuntime(19074):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
01-17 21:35:39.915: E/AndroidRuntime(19074):    ... 11 more

非常感谢您的时间。

Answer 1

您永远不会实例化您的btnNext对象。

因此，在执行btnNext.setOnClickListener时，它会抛出NullPointerException。

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

    inputtxt = (EditText) findViewById(R.id.text1);
    btnNext = (Button) findViewById(R.id.btnNext);//add this line

Answer 2

问题出在这里

btnNext.setOnClickListener(new View.OnClickListener()

btnNext是null，因为您尚未对其进行初始化。

    setContentView(R.layout.activity_first_image_logo);

    inputtxt = (EditText) findViewById(R.id.text1);
    btnNext = (Button) findViewById(R.id.idOfButton);  // you need this

    btnNext.setOnClickListener(new View.OnClickListener()

Answer 3

您从未初始化btnNext。使用以下

btnNext=(Button) findViewById(R.id.id_of_button);

然后使用setOnClickListener

Android：使用if else语句的测验应用程序

3 个答案: