多线程为何用while判断条件，而不用if。

一、代码：

 1 package zz.produceandconsumer;
 2 
 3 import java.util.LinkedList;
 4 
 5 public class Storage {
 6     
 7     private final static int MAX_NUM=100;
 8     
 9     private volatile LinkedList<Object> list=new LinkedList<Object>();
10     
11     public void produce(int pNum){
12         synchronized(list){
13             if(list.size()+pNum>MAX_NUM){
14                 System.out.println("当前商品数量："+list.size()+",欲生产数量："+pNum+",超过最大容量："+MAX_NUM+".");
15                 try {
16                     list.wait();
17                 } catch (InterruptedException e) {
18                     e.printStackTrace();
19                 }
20             }
21 //            while(list.size()+pNum>MAX_NUM){
22 //                System.out.println("当前商品数量："+list.size()+",欲生产数量："+pNum+",超过最大容量"+MAX_NUM+".");
23 //                try {
24 //                    list.wait();
25 //                } catch (InterruptedException e) {
26 //                    e.printStackTrace();
27 //                }
28 //            }
29             int i=0;
30             while(i<pNum){
31                 System.out.println("欲生产："+pNum+",list.size："+list.size()+",最大值:"+MAX_NUM+",i:"+i);
32                 list.add(new Object());
33                 i++;
34             }
35             System.out.println("生产了"+pNum+"个商品，仓库中存货"+list.size()+".");
36             list.notifyAll();
37         }
38     }
39     
40     public void consumer(int cNum){
41         synchronized(list){
42 //            if(cNum>list.size()){
43 //                System.out.println("当前商品数量："+list.size()+",欲消费数量："+cNum+",数量不足.");
44 //                try {
45 //                    list.wait();
46 //                } catch (InterruptedException e) {
47 //                    e.printStackTrace();
48 //                }
49 //            }
50             while(cNum>list.size()){
51                 System.out.println("当前商品数量："+list.size()+",欲消费数量："+cNum+",数量不足.");
52                 try {
53                     list.wait();
54                 } catch (InterruptedException e) {
55                     e.printStackTrace();
56                 }
57             }
58             int i=0;
59             if(cNum>list.size()){
60                 System.out.println("error");
61             }
62             while(i<cNum){
63 //                System.out.println("size:"+list.size()+",i:"+i+",cNum:"+cNum);
64                 list.remove();
65                 i++;
66             }
67             System.out.println("消费了"+cNum+"个商品，仓库中存货"+list.size()+".");
68             list.notifyAll();
69         }
70     }
71 
72 }

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