在Android中，如何获得一个全局变量的引用，该变量是另一个全局变量的子类？

// 父类
public class MyApplication extends Application {
    private ParentClass globalVariable;

    public ParentClass getGlobalVariable() {
        return globalVariable;
    }

    public void setGlobalVariable(ParentClass globalVariable) {
        this.globalVariable = globalVariable;
    }
}

// 子类
public class ChildClass extends Activity {
    private ParentClass parentVariable;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // 获取全局变量的引用
        MyApplication myApp = (MyApplication) getApplication();
        parentVariable = myApp.getGlobalVariable();

        // 使用全局变量
        parentVariable.doSomething();
    }
}