Question

您好我正试图通过在整个屏幕上设置ontouchlistener来开始一项新活动。但是，当我点击android模拟器中的屏幕时，我得到了一个实例化问题，ComponentInfo特别是nullpointerexception。我的问题可能是我从第二堂课中删除了saveinstantstate == null

public class LoadScreen extends Activity {

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_load_screen);

    View view=(FrameLayout) findViewById(R.id.container);

    view.setOnTouchListener(new OnTouchListener(){

        @Override
        public boolean onTouch(View v, MotionEvent event) {
            startActivity(new Intent(v.getContext(),SignUp.class));
            return true;
        }

    });

}




}

public class SignUp extends Activity {

public Button signup=(Button) findViewById(R.id.CompleteRegistration);

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

    signup.setOnClickListener(new OnClickListener(){

        @Override
        public void onClick(View v) {
            startActivity(new Intent(v.getContext(),MainActivity.class));

        }

    });
}


}

07-23 01:42:26.200: E/AndroidRuntime(1138): FATAL EXCEPTION: main
 07-23 01:42:26.200: E/AndroidRuntime(1138): Process: com.example.demoww, PID: 1138
07-23 01:42:26.200: E/AndroidRuntime(1138): java.lang.RuntimeException: Unable to     instantiate activity ComponentInfo{com.example.demoww/com.example.demoww.SignUp}:     java.lang.NullPointerException
07-23 01:42:26.200: E/AndroidRuntime(1138):     at     android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2121)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.os.Handler.dispatchMessage(Handler.java:102)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.os.Looper.loop(Looper.java:136)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.app.ActivityThread.main(ActivityThread.java:5017)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at java.lang.reflect.Method.invokeNative(Native Method)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at java.lang.reflect.Method.invoke(Method.java:515)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at dalvik.system.NativeStart.main(Native Method)
 07-23 01:42:26.200: E/AndroidRuntime(1138): Caused by: java.lang.NullPointerException
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.app.Activity.findViewById(Activity.java:1884)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at com.example.demoww.SignUp.<init>(SignUp.java:15)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at java.lang.Class.newInstanceImpl(Native Method)
 07-23 01:42:26.200: E/AndroidRuntime(1138):    at java.lang.Class.newInstance(Class.java:1208)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
07-23 01:42:26.200: E/AndroidRuntime(1138):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2112)
07-23 01:42:26.200: E/AndroidRuntime(1138):     ... 11 more

Answer 1

您应该移动此代码

Button signup=(Button) findViewById(R.id.CompleteRegistration);

在 SignUp活动

中onCreate(..)之后的setContentView(R.layout.activity_sign_up);下

更正代码

public class SignUp extends Activity {

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

Button signup=(Button) findViewById(R.id.CompleteRegistration);

signup.setOnClickListener(new OnClickListener(){

    @Override
    public void onClick(View v) {
        startActivity(new Intent(v.getContext(),MainActivity.class));

    }

  });
  }

 }

Answer 2

移动

Button signup=(Button) findViewById(R.id.CompleteRegistration);

低于setContentView()行。

Answer 3

除了将on find按钮命令移入onCreate（...）方法的建议之外，我想问题可能是R.layout.activity_sign_up布局中的CompleteRegistration按钮的错误定义以及R.layout.activity_load_screen布局中的内容FrameLayout！

请检查这些事情。如果这些是正确的，你应该发送这部分代码来找出问题。

Answer 4

移动此行

Button signup=(Button) findViewById(R.id.CompleteRegistration);到

onCreate(...){
....
setContentView(layoutId);
Button signup=(Button) findViewById(R.id.CompleteRegistration);
}

因为当注册是对象创建时，则下一步是其字段在指定
时，会初始化为默认值

Button signup=(Button)findViewById(R.id.CompleteRegistration);

Activity类调用中的overriden方法查找当前的视图由处理的XML中的id属性标识 onCreate（）。否则返回null，因此您遇到了NPE

public View findViewById(int id) {
        return getWindow().findViewById(id);
}

如何使用ontouchlistener启动新活动

4 个答案:

更正代码