看看这两段JAVA代码

Question

public class OrderForm{//订单
private List MaterialList = new ArrayList();
private Transport transport;
public OrderForm(){
//从数据取出数据
//实例化transport
//填充MaterialList
}
public double getTotPrice(){
double result = 0;
for(OrderMaterial ma : MaterialList){
result += ma.unitPrice * ma.amount ;
}
result += transport.transPrice;
return result;
}
public String getAddress(){
return transport.Address;
}
private class OrderMaterial{//订货
public double unitPrice;
public double amount;
}
private class Transport{
public String Address;//送货地址
public double transPrice; //运输费用
}
}

public class OrderForm1{//订单
private List MaterialList = new ArrayList();
private Transport transport;
public OrderForm1(){}
public double getTotPrice(){
if(transport==null){
//从数据库取得数据，实例化transport
}
if(MaterialList.size()<1){
//从数据库取得数据，填充MaterialList
}
double result = 0;
for(OrderMaterial ma : MaterialList){
result += ma.unitPrice * ma.amount ;
}
result += transport.transPrice;
return result;
}
public String getAddress(){
if(transport==null){
//从数据库取得数据，实例化transport
}
return transport.Address;
}
}

小弟初学JAVA，在看这两段代码时 不知道有何区别？

Answer 1

前一个是eager initialization，在构造器里就把整个对象的状态都初始化好，无论实际上那些数据有没有被用到；

后一个是lazy initialization，构造对象的时候暂时不把整个对象的状态都初始化完，而是等真的用到的时候再初始化。不过这个实现不是线程安全的……