怎样让xstream在生成xml时，显示null属性

Question

怎样让xstream在生成xml时，显示null属性。如：
class Admin extends Person {
public String name;
public boolean online;
public Map mailMap = null;

public Admin(String name, boolean online, Map<String, String> mailMap) {
    this.name = name;
    this.online = online;
    this.mailMap = mailMap; 
}

public String toString() {
    return super.toString() + "Admin{" +
        "name=" + name +
        ",online=" + online +
        ",mailMap=" +
        mailMap.toString()      
        + "}";
}

}

Admin admin = new Admin("me", true, null);
XStream xs = new XStream();
System.out.println(xs.toXML(admin));

//运行结果
me
true

//其中没有mailMap属性，我想要下面的效果：
me
true

Answer 1

将new HashMap定义为全局的，每次都是用同一个区域就可以了。