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用XMLEncode輸出時候，如果有BigDecimal有時候不好使。
原因是：如果類的變量在定義時候有初始值，而不是null，就必須要重載DefaultPersistenceDelegate的mutatesTo方法。
關于這個說明，在網上這里可以看到：
http://forum.java.sun.com/thread.jspa?threadID=631299&messageID=3642493

有興趣的可以看看：
//setup big decimal delegate.
????????? DefaultPersistenceDelegate bigDecimalDelegate = new DefaultPersistenceDelegate() {
????????????? protected Expression instantiate(Object oldInstance, Encoder out) {
????????????????? BigDecimal decimal = (BigDecimal) oldInstance;
????????????????? return new Expression(oldInstance, oldInstance.getClass(), "new", new Object[] {decimal.toString()});
????????????? }
????????????? //must override this method.
????????????? // see http://forum.java.sun.com/thread.jspa?threadID=631299&messageID=3642493
????????????? protected boolean mutatesTo(Object oldInstance, Object newInstance) {
????????????????? return oldInstance.equals(newInstance);
????????????? }
????????????? //--- Joshua
????????? };

網上的牛人說：

This works for BigDecimal properties that aren't initialized, i.e. null. But if you initialize the property, then this won't work unless you override mutatesTo in addition to instantiate mentioned above:

protected boolean mutatesTo(Object oldInstance, Object newInstance) {
return oldInstance.equals(newInstance);
}

public class MethodDemo {

/**
* @param args
*/
public static void main(String[] args) {
   MethodDemo demo = new MethodDemo();
   Integer i = Integer.valueOf(1);
   demo.add(i);
   System.out.println("i:" + i);
  
   String s = "ss";
   demo.stringchange(s);
   System.out.println("s:" + s);
  
   Person per = new Person();

//基本類型變不了
public void add(int i) {
   i++;
}

/***
* 凡是在引用中出現修改引用的賦值語句，
* 修改都變成無效
* @param i
*/

//想修改引用，不行
public void add(Integer i) {
   int j = i.intValue();
   i = Integer.valueOf(j++);//i的原引用已經丟失了
}

//想修改引用，不行
public void stringchange(String s){
   s = "stringchange";
}

public void setDate(Person per){
   Person per2 = new Person();
   per2.setName("per2Name");
   per = per2;//per的原引用已經丟失了,這個估計很多人會出錯
   per.setName("name");
}

}

class Person {
String name ;

public String getName() {
   return name;
}

public void setName(String name) {
   this.name = name;
}

}