问一个Producer-Consumer模型问题

[cavinlu]

学java没多久，学操作系统，想模拟进程不同步问题，写了个Producer－Consumer程序：
但结果很怪，consumer老是在下标为－1抛出异常，大家能否帮我看看程序，可以在自己的机器上运行看看：多谢了！

class Goods{
private int[] a; //used as the container
private int index=0; //indicate how many goods are there in the container
private int iput=0; //pointer used to put good to the container
private int iget=0; //pointer used to get good from the container
Goods(){
a=new int[30];
}
public void put(int i){
int nextiput=(iput+1)%30;
if(nextiput==iget){
System.out.println("Sorry the Container is full,you can't put good to the container!");
System.exit(0);
}
else{
int ip=iput;
System.out.println("Successfully stored "+i+" to the number: "+iput+" space");
iput=(iput+1)%30;
a[iput]=i;
index++;
}
}
public void get(){
if(iput==iget){
System.out.println("Sorry the Container is empty,you can't get good from the container!");
System.exit(0);
}
else{
int good=a[iget];
int inx=iget;
iget=(iget-1)%30;
index--;
System.out.println("Successfully get "+good+" from space "+inx);
}

}
}

class Producer extends Thread{
private Goods g;
Producer(Goods g){
this.g=g;
}
public void run(){
for(int i=0; i<10; i++){
g.put(i);
try{
Thread.sleep((int)(Math.random()*100));
}catch(Exception e){}
}
}
}

class Consumer extends Thread{
private Goods g;
Consumer(Goods g){
this.g=g;
}
public void run(){
for(int i=0; i<10; i++){
g.get();
try{Thread.sleep((int)(Math.random()*1000));}
catch(Exception e){}
}
}

}

class main{
public static void main(String[] args){
Goods g=new Goods();
Producer p=new Producer(g);
Consumer c=new Consumer(g);
p.start();
c.start();
}
}

[yuzl32]

public void get(){
if(iput==iget){
System.out.println("Sorry the Container is empty,you can't get good from the container!");
System.exit(0);
}
else{
int good=a[iget];
int inx=iget;
iget=(iget+1)%30; // 更改
index--;
System.out.println("Successfully get "+good+" from space "+inx);
}

}

[cavinlu]

非常感谢楼上的高人！多谢了！
分给到你爽，^_^！

[yuzl32]

不客气,相互交流.

[denny008]

iput=(iput+1)%30;
iget=(iget-1)%30;
分别是什么意思

[ctx442]

怎么用线程做的