为什么程序意外停止？

Question

I'm writing a very basic program that aims for the text view to display the phrase "Hello" after a button is pressed on the screen, but cannot figure out why every time I run it, it says that the application has stopped unexpectedly.

This is the program I wrote:

public class EtudeActivityActivity extends Activity{
  TextView tvResponse;

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

    final TextView tvResponse = (TextView)  findViewById (R.id.tvResponse);
  }


  public void updateTV(View v) {
    tvResponse.setText("Hello");
  }
}

Also, I inserted an android:onClick = "updateTV" into my main.xml file for the button.

Thanks for any help!

Answer 1

It is because you don't set the tvResponse member variable. Instead you set a new local variable by the same name. So when you call setText() , you are accessing an invalid reference

You need to change

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

to

tvResponse = (TextView)  findViewById (R.id.tvResponse);

to set the member variable, so it has a valid reference later on (when updateTV() is called)

Answer 2

I suspect you've got an instance variable called tvResponse which you haven't shown us - that's what the updateTV method will refer to. That's entirely separate from the local tvResponse variable you've declared inside onCreate . I suspect that if you change the last line of onCreate from a local variable declaration to a simple assignment to the tvResponse variable, it may work. Otherwise, if nothing is assigning a value to the instance tvResponse variable, it will have the default value of null , causing a NullPointerException in updateTV .