请问多线程异常问题

denny009 2006-02-11 02:45:33

import java.util.*;
import java.text.*;

public class AccountManager{
protected CustomerAccount savings;
protected CustomerAccount checking;
public final static int SAVINGS_ACCOUNT=1;
public final static int CHECKING_ACCOUNT=2;
public static void main(String args[])
{
int transfers=1000000;

AccountManager am=new AccountManager(transfers);

}
public AccountManager(int transfers){
savings=new CustomerAccount(SAVINGS_ACCOUNT,1000);
savings=new CustomerAccount(CHECKING_ACCOUNT,1000);
NumberFormat formatter=NumberFormat.getCurrencyInstance(Locale.US);
System.out.println("Total balance before transfers: "+
formatter.format(savings.getbalance()+checking.getbalance()));
transfersManager tm1=new transfersManager(checking,savings,transfers);
transfersManager tm2=new transfersManager(savings,checking,transfers);
Thread t1=new Thread(tm1);
Thread t2=new Thread(tm2);
t1.start();
t2.start();
try{
t1.join();
t2.join();
}catch(Exception e){

};
System.out.println("Total balance after transfers: "+
formatter.format(savings.getbalance()+checking.getbalance()));
}
class transfersManager implements Runnable{
public CustomerAccount fromaccount;
public CustomerAccount toaccount;
protected int transferscount;
public transfersManager(CustomerAccount fromacct,CustomerAccount toacct,int transfers){
fromaccount=fromacct;
toaccount=toacct;
transferscount=transfers;
}

public void run()
{
double balance;
double transfersamount;
for(int i=0;i<transferscount;i++){
synchronized (fromaccount){

balance=fromaccount.getbalance();
transfersamount=(int)balance*Math.random();
balance=balance-transfersamount;
fromaccount.setbalance(balance);
synchronized (toaccount){
balance=toaccount.getbalance();
balance+=transfersamount;
toaccount.setbalance(balance);
}
}
}
}}
class CustomerAccount{
protected int accounttype;
protected double balance;
public CustomerAccount(int type,double bal){
accounttype=type;
balance=bal;
}
public int getaccounttype(){
return accounttype;
}
public double getbalance(){
return balance;
}
public void setbalance(double newbal){
balance=newbal;
}
}
}
这是书上的举例,我运行后为什么出现异常?
Exception in thread "main" java.lang.NullPointerException
at AccountManager.<init>(AccountManager.java:21)
at AccountManager.main(AccountManager.java:13)
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