何时在“警报对话框”中使用此内容或上下文

Question

I am trying to create an alert dialog but in the bracket in the declaration, on the line where it says :

AlertDialog.Builder alertDialog=new AlertDialog.Builder(this).create();

the (this) causes errors. I tried changing it to context, still more errors. HELP?

import android.app.Activity;
import android.app.AlertDialog.Builder;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.app.AlertDialog;

public class userPreferences extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.userpreferences);
    EditText editText=(EditText)findViewById(R.id.editText);
    Button save=(Button)findViewById(R.id.button);


  save.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
          AlertDialog.Builder alertDialog=new AlertDialog.Builder(this).create();
                  alertDialog.setTitle("Alert Dialog");
                  alertDialog.setMessage("Are you sure you want to save?");
                  alertDialog.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                      @Override
                      public void onClick(DialogInterface dialog, int which) {

                      }
                  });
                  alertDialog.setNegativeButton("No", null);
                  alertDialog.show();


      }
  });

}
}

Answer 1

alertDialog should be declared as final outside OnClickListener :

final AlertDialog.Builder alertDialog = new AlertDialog.Builder(this);

save.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {

        alertDialog.setTitle("Alert Dialog");
        alertDialog.setMessage("Are you sure you want to save?");
        alertDialog.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                ...
            }
        });
        alertDialog.setNegativeButton("No", null);
        alertDialog.show();
    }
});

Hope this helps.

Answer 2

this is a reference to the currently running instance. You could use this to pass your Activity (which inherits from Context , thus why we can pass an Activity reference to a Context parameter) to the AlertDialog.Builder , if it's outside of the listener.

When you define an anonymous inner class, like:

new View.OnClickListener() { ... }

The this reference changes to the instance of OnClickListener .

AlertDialog.Builder() expects a Context though! Since it's an anonymous inner class we can simply use userPreferences.this (because userPreferences is still in scope) to get the correct reference.

Answer 3

This inside of the click listener is going to reference the click listener

    save.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
                  AlertDialog.Builder alertDialog=new AlertDialog.Builder(userPrefereces.this).create();
                  alertDialog.setTitle("Alert Dialog");
                  alertDialog.setMessage("Are you sure you want to save?");
                  alertDialog.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                      @Override
                      public void onClick(DialogInterface dialog, int which) {

                      }
                  });
                  alertDialog.setNegativeButton("No", null);
                  alertDialog.show();


      }
  });

but you can get the context of the activity by

userPreferences.this

Answer 4

Create the builder outside onClick method

@Override
protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.userpreferences);
  EditText editText=(EditText)findViewById(R.id.editText);
  Button save=(Button)findViewById(R.id.button);

  final AlertDialog.Builder alertDialog=new AlertDialog.Builder(this).create();

  save.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {

                alertDialog.setTitle("Alert Dialog");
                alertDialog.setMessage("Are you sure you want to save?");
                alertDialog.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {

                    }
                });
                alertDialog.setNegativeButton("No", null);
                alertDialog.show();
    }
  });
}