按钮的 setOnClickListener 使应用程序崩溃

Question

So, When I try to launch this application it crashes. However, it only crashes when I try to use the setOnClickListener. If I comment out that small section it runs just fine.

public class MainActivity extends ActionBarActivity {


@Override
protected void onCreate(Bundle savedInstanceState) {
    setContentView(R.layout.activity_main);
    final TextView textView = (TextView) findViewById(R.id.textView);
    final Button button = (Button)findViewById(R.id.button);
    super.onCreate(savedInstanceState);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            //textView.setText("Boop!");
        }
    });
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);

OK Working code! I had to move super.onCreate up, and declair the textview inside the findviewbyID section. However, now it works as I intended. Thank you guys for the help! I just wanted to make something very basic for a date. xD you guys are a life saver.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Button button = (Button)findViewById(R.id.button);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            TextView textView = (TextView) findViewById(R.id.textView);
            textView.setText("Boop!");
        }
    });
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

WORKING

Answer 1

The error is coming because you are calling the super.onCreate(savedInstanceState); after the setContentView and then using the variables.

Reason that the error is not coming until you don't write onClicklister: The variables textView and button are already null or in inconsistent state, but when you are writing the click listener, the variables are used and you are facing error.

Answer 2

You declared textView as final :

A variable can be declared final. A final variable may only be assigned to once.

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

so you cannot modify it.

You must get the view inside onclick method:

button.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        TextView textView = (TextView) findViewById(R.id.textView);
        textView.setText("Boop!");
    }
});