@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ...
        setContentView(R.layout.activity_A);
        toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        ...
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setDisplayShowHomeEnabled(true);
}

@Override
    public boolean onSupportNavigateUp() {
        onBackPressed();
        return true;
    }

@Override
    public void onBackPressed() {
        super.onBackPressed();
        this.finish();
        Log.d(TAG, "onBackPressed: Finished!!");
    }