如何在MainActivity和没有onCreate的类之间共享变量

public class MainActivity extends AppCompatActivity {
    public static int sharedVariable;
    // ...
}

public class NoCreateClass {
    public void someMethod() {
        MainActivity.sharedVariable = 10;
        // ...
    }
}

public class MainActivity extends AppCompatActivity {
    // ...
    private void goToNoCreateClass() {
        Intent intent = new Intent(this, NoCreateClass.class);
        intent.putExtra("sharedVariable", 10);
        startActivity(intent);
    }
}

public class NoCreateClass extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        int sharedVariable = getIntent().getIntExtra("sharedVariable", 0);
        // ...
    }
}

public interface SharedVariableCallback {
    void onSharedVariableChanged(int sharedVariable);
}

public class MainActivity extends AppCompatActivity implements SharedVariableCallback {
    private int sharedVariable;

    // ...

    private void goToNoCreateClass() {
        NoCreateClass noCreateClass = new NoCreateClass();
        noCreateClass.setSharedVariableCallback(this);
    }

    @Override
    public void onSharedVariableChanged(int sharedVariable) {
        this.sharedVariable = sharedVariable;
        // ...
    }
}

public class NoCreateClass {
    private SharedVariableCallback sharedVariableCallback;

    public void setSharedVariableCallback(SharedVariableCallback callback) {
        this.sharedVariableCallback = callback;
    }

    public void someMethod() {
        int sharedVariable = 10;
        if (sharedVariableCallback != null) {
            sharedVariableCallback.onSharedVariableChanged(sharedVariable);
        }
        // ...
    }
}