Check out example codes for "android java close app". It will help you in understanding the concepts better.

Code Example 1

@Override
    public void onBackPressed() {
        AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);
        alertDialogBuilder.setTitle("Exit Application?");
        alertDialogBuilder
                .setMessage("Click yes to exit!")
                .setCancelable(false)
                .setPositiveButton("Yes",
                        new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog, int id) {
                                moveTaskToBack(true);
                                android.os.Process.killProcess(android.os.Process.myPid());
                                System.exit(1);
                            }
                        })

                .setNegativeButton("No", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {

                        dialog.cancel();
                    }
                });

        AlertDialog alertDialog = alertDialogBuilder.create();
        alertDialog.show();
    }

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