这为什么会导致NullPointerException？

Question

我有一个程序如下，谁能告诉我什么是获得NPE的原因。

public class MainActivity extends Activity { 

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

     secondClass x = new secondClass(); 
     x.sText("Test Spring"); 

    } 

} 



public class secondClass extends MainActivity { 

    public void sText(String s) { 

     TextView text = (TextView) findViewById(R.id.text); 
     text.setText(s); 
    } 

} 

Mainfest

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    ... 

    <TextView 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="@string/hello_world" 
     android:id="@+id/text" 
    /> 

</RelativeLayout> 

logcat的

08-16 01:31:11.320 15032-15032/? E/Trace: error opening trace file: No such file or directory (2) 
08-16 01:31:11.420  836-987/? E/EmbeddedLogger: App crashed! Process: com.example.myapplication 
08-16 01:31:11.420  836-987/? E/EmbeddedLogger: App crashed! Package: com.example.myapplication v1 (1.0) 
08-16 01:31:11.420  836-987/? E/EmbeddedLogger: Application Label: My Application 
08-16 01:31:11.420 15032-15032/? E/AndroidRuntime: FATAL EXCEPTION: main 
     java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException 
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java) 
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java) 
     at android.app.ActivityThread.access$600(ActivityThread.java) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java) 
     at android.os.Handler.dispatchMessage(Handler.java) 
     at android.os.Looper.loop(Looper.java) 
     at android.app.ActivityThread.main(ActivityThread.java) 
     at java.lang.reflect.Method.invokeNative(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java) 
     at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:110) 
     at dalvik.system.NativeStart.main(Native Method) 
     Caused by: java.lang.NullPointerException 
     at android.app.Activity.findViewById(Activity.java) 
     at com.example.myapplication.secondClass.sText(secondClass.java:13) 
     at com.example.myapplication.MainActivity.onCreate(MainActivity.java:14) 
     at android.app.Activity.performCreate(Activity.java) 
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java) 

据我了解，这应该实例化和初始化然后二等对象调用其STEXT（）方法。然后，sText方法应该在TextView中存储一个引用。接下来它调用TextView上的setText（）方法，它将TextView中的文本更改为传递到sText（）中的文本。

logcat说它会抛出一个NullPointerException，但我不完全确定为什么。如果这是一个新秀的错误，那是因为我是一名新秀。谢谢。

Answer 1

您需要在您的secondActivity中覆盖onCreate并且拥有setContentView(R.layout.mylayout)。然后你可以初始化你的textview。

还需要使用启动一个活动意图

 Intent intent = new Intent(MainActivtiy.this,SecondActivity.class); 
    startActivity(intent); 

记住要在清单文件

活动（SecondActivtiy）条目如果你需要把这个字符串传给第二活动使用意图.putExtra

 Intent intent = new Intent(MainActivtiy.this,SecondActivity.class); 
     intent.putExtra("key",mystring); 
     startActivity(intent); 

然后在secondActivity

TextView tv ; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.mysecondlayout); 
    tv = (TextView) findViewByid(R.id.textView1); 
    Bundle extras = getIntent().getExtras(); 
    if (extras != null) { 
    String value = extras.getString("key"); 
    tv.setText(value); 
    //get the value based on the key 
    } 

http://developer.android.com/reference/android/content/Intent.html#putExtra(java.lang.String，android.os.Bundle）

可以设置到活动当前视图herarchy的findViewById。

还需要使用给startActivity意图，而这样做econdClass x = new secondClass()