Change Actionbar button icon

I have this layout with this heart button, so inside the click event of this button I change the icon

IwantedtochangemyhearttherebythisheartwhenIclick

The answer below solved more than one problem

You need to override the onCreateOptionsMenu and onOptionsItemSelected methods

1) Retain the desired MenuItem:

private MenuItem menuCoracao;
...  
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.seu_menu, menu);
    menuCoracao = menu.findItem(R.id.id_do_seu_menu);
    return super.onCreateOptionsMenu(menu);

} 2) Detect the menu click and change the icon:

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    if (item.getItemId() == R.id.id_do_seu_menu) {
        menuCoracao.setIcon(R.drawable.seu_drawable);
    }
    return super.onOptionsItemSelected(item);
}

I've tried using onCreate so

meuCoracao.setIcon(R.drawable.ic_action_favorite_v);

gave this error

E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{praias.android.makerapp.com.praias/praias.android.makerapp.com.praias.MainActivity}: java.lang.NullPointerException