exit app on button click

Just use this code for your button:

exitButton.setOnClickListener(new OnClickListener() {

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub
        finish();
        System.exit(0);
    }
});

Though it is not a good idea to use an exit button on an Android app. Let the OS decide when to kill your Activity. I should recommend, learn about the Android Activity lifecycle first and then implement any necessary callbacks.

Android's design does not favor exiting an application by choice, but rather manages it by the OS. You can bring up the Home application by its corresponding Intent:

Intent intent = new Intent(Intent.ACTION_MAIN);
intent.addCategory(Intent.CATEGORY_HOME);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(intent);

reference