Android中如何声明全局变量

Android中如何声明全局变量？

java
public class MyApplication extends Application {
    private String globalVariable;
    public String getGlobalVariable() {
        return globalVariable;
    }
    public void setGlobalVariable(String globalVariable) {
        this.globalVariable = globalVariable;
    }
}

xml
    android:name=".MyApplication"
    ...
    >
    ...

java
MyApplication myApplication = (MyApplication) getApplication();
myApplication.setGlobalVariable("Hello, World!");
String globalVariable = myApplication.getGlobalVariable();

java
public class GlobalVariable {
    private static GlobalVariable instance;
    private String value;
    private GlobalVariable() {
    }
    public static synchronized GlobalVariable getInstance() {
        if (instance == null) {
            instance = new GlobalVariable();
        }
        return instance;
    }
    public String getValue() {
        return value;
    }
    public void setValue(String value) {
        this.value = value;
    }
}

java
GlobalVariable globalVariable = GlobalVariable.getInstance();
globalVariable.setValue("Hello, World!");
String value = globalVariable.getValue();