0

我在使用这个 textView 时遇到问题，应用程序意外停止，这是 xml 文件：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<Button
    android:id="@+button/btnAnterior"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_alignParentLeft="true"
    android:layout_marginLeft="46dp"
    android:text="Anterior" />

<Button
    android:id="@+button/btnProxima"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_alignParentRight="true"
    android:layout_marginRight="45dp"
    android:text="Próxima" />

<ScrollView
    android:id="@+id/scrollView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_above="@+id/button2"
    android:layout_alignParentLeft="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentTop="true" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <TextView
    android:id="@+id/tvPiada"
    android:layout_width="match_parent"
    android:layout_height="400dp"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true" />

    </LinearLayout>
</ScrollView>

这是.java：

// Buttons
Button btnProxima = (Button) findViewById(R.button.btnProxima);
Button btnAnterior = (Button) findViewById(R.button.btnAnterior);   
int piadaN;

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

 // Strings
    final String piada1 = ("Piada1");
    final String piada2 = ("Piada2");

    /*piada2 = ("Piada2");
    String piada3 = new String();
    piada3 = ("Piada3");
    String piada4 = new String();
    piada4 = ("Piada4");*/

    final TextView tvPiada = (TextView) findViewById(R.id.tvPiada);

    // Evento dos botões
    btnProxima.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            piadaN = piadaN + 1;

            switch (piadaN) {
            case 1:
                tvPiada.setText(piada1);
                break;
            case 2:
                tvPiada.setText(piada2);
                break;
            }               
        }
    });

}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.activity_main, menu);
    return true;
}

这是我运行应用程序时的 logcat

11-13 19:00:47.843: E/AndroidRuntime(585): FATAL EXCEPTION: main
11-13 19:00:47.843: E/AndroidRuntime(585): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.piadas/com.example.piadas.MainActivity}: java.lang.NullPointerException
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1879)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.access$600(ActivityThread.java:122)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.os.Handler.dispatchMessage(Handler.java:99)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.os.Looper.loop(Looper.java:137)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.main(ActivityThread.java:4340)
11-13 19:00:47.843: E/AndroidRuntime(585):  at java.lang.reflect.Method.invokeNative(Native Method)
11-13 19:00:47.843: E/AndroidRuntime(585):  at java.lang.reflect.Method.invoke(Method.java:511)
11-13 19:00:47.843: E/AndroidRuntime(585):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
11-13 19:00:47.843: E/AndroidRuntime(585):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
11-13 19:00:47.843: E/AndroidRuntime(585):  at dalvik.system.NativeStart.main(Native Method)
11-13 19:00:47.843: E/AndroidRuntime(585): Caused by: java.lang.NullPointerException
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.Activity.findViewById(Activity.java:1794)
11-13 19:00:47.843: E/AndroidRuntime(585):  at com.example.piadas.MainActivity.<init>(MainActivity.java:13)
11-13 19:00:47.843: E/AndroidRuntime(585):  at java.lang.Class.newInstanceImpl(Native Method)
11-13 19:00:47.843: E/AndroidRuntime(585):  at java.lang.Class.newInstance(Class.java:1319)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
11-13 19:00:47.843: E/AndroidRuntime(585):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1870)
11-13 19:00:47.843: E/AndroidRuntime(585):  ... 11 more
4

2 回答 2

1

在我看来，您收到此错误是因为您试图在声明布局之前声明按钮。将按钮声明移动到您的onCreate()方法内部，并将它们放在您的setContentView(R.layout.main)

所以你需要Button btnProxima = (Button)findViewById(R.button.btnProxima);在使用你的setContentView

于 2012-11-13T19:20:43.117 回答
1

如果没有上下文或活动布局，则无法使用，请在afterfindViewById()中调用此方法：onCreate()setContentView()

// Buttons
Button btnProxima;
Button btnAnterior;   
int piadaN;

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

    btnProxima = (Button) findViewById(R.button.btnProxima);
    btnAnterior = (Button) findViewById(R.button.btnAnterior);
于 2012-11-13T19:21:48.347 回答