A obj = new A();
SoftRefenrence sr = new SoftReference(obj);
if(sr!=null){
obj = sr.get();
}else{
obj = new A();
sr = new SoftReference(obj);
}
A obj = new A();
WeakReference wr = new WeakReference(obj);
obj = null;
//等待一段時間,obj對象就會被垃圾回收
if (wr.get()==null) {
System.out.println("obj 已經被清除了 ");
} else {
System.out.println("obj 尚未被清除,其資訊是 "+obj.toString());
}