java - 在 onCreate() 中更改文本时出现空指针异常

Question

我得到一个空指针异常，这是它所在的代码。

    final TextView view = (TextView) findViewById(R.id.textView2);
final Button next = (Button) findViewById(R.id.button1);
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    next.setOnClickListener(new OnClickListener() {

        public void onClick(View v) {
            view.setText("These questions will help me to help you :)");
        }
    });

这是我的日志文件:

24 15:21:45.441: E/AndroidRuntime(376): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.icemountaininc.app/com.icemountaininc.app.MainActivity}: java.lang.NullPointerException
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1660)
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1752)
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.app.ActivityThread.access$1500(ActivityThread.java:123)
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:993)
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.os.Handler.dispatchMessage(Handler.java:99)
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.os.Looper.loop(Looper.java:126)
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.app.ActivityThread.main(ActivityThread.java:3997)
11-24 15:21:45.441: E/AndroidRuntime(376):  at java.lang.reflect.Method.invokeNative(Native Method)
11-24 15:21:45.441: E/AndroidRuntime(376):  at java.lang.reflect.Method.invoke(Method.java:491)
11-24 15:21:45.441: E/AndroidRuntime(376):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
11-24 15:21:45.441: E/AndroidRuntime(376):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
11-24 15:21:45.441: E/AndroidRuntime(376):  at dalvik.system.NativeStart.main(Native Method)
11-24 15:21:45.441: E/AndroidRuntime(376): Caused by: java.lang.NullPointerException
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.app.Activity.findViewById(Activity.java:1741)
11-24 15:21:45.441: E/AndroidRuntime(376):  at com.icemountaininc.app.MainActivity.<init>(MainActivity.java:14)
11-24 15:21:45.441: E/AndroidRuntime(376):  at java.lang.Class.newInstanceImpl(Native Method)
11-24 15:21:45.441: E/AndroidRuntime(376):  at java.lang.Class.newInstance(Class.java:1424)
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.app.Instrumentation.newActivity(Instrumentation.java:1022)
11-24 15:21:45.441: E/AndroidRuntime(376):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
11-24 15:21:45.441: E/AndroidRuntime(376):  ... 11 more

有什么帮助吗？

多年来一直被这个问题困扰!

我用的是android的SDK VERSION 8

谢谢。

Answer 1

实际上你得到了 NullPointerException 因为你的 TextView view 和 Button next 是 NULL。

解决方案:

你必须在 onCreate() 的 setContentView() 方法之后定义 TextView view 和 Button next你的 Activity 。

喜欢:

setContentView(R.layout.activity_main);
final TextView view = (TextView) findViewById(R.id.textView2);
final Button next = (Button) findViewById(R.id.button1);