如何在Android中声明全局变量？

public class GlobalVariables {
    public static String globalVariable = "This is a global variable";
}

String globalVariable = GlobalVariables.globalVariable;

// 保存全局变量
SharedPreferences sharedPreferences = getSharedPreferences("GlobalVariables", MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putString("globalVariable", "This is a global variable");
editor.apply();

// 读取全局变量
SharedPreferences sharedPreferences = getSharedPreferences("GlobalVariables", MODE_PRIVATE);
String globalVariable = sharedPreferences.getString("globalVariable", "default value");

// 创建自定义的Application类
public class MyApplication extends Application {
    public String globalVariable = "This is a global variable";
}

    android:name=".MyApplication"
    ...>
</application>

MyApplication myApplication = (MyApplication) getApplication();
String globalVariable = myApplication.globalVariable;