package test1;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Random;
import java.util.Set;
public class CountNumber //对数字进行计数并统计出每个数字出现的次数
{
public static void main(String[] args)
{
Random random = new Random();
HashMap map = new HashMap();
for(int i = 0; i < 50 ; i ++) //随机生成50的数并以map类对象存储
{
int a = random.nextInt();
if((Integer)a == null) //如果该key指向null,说明未出现过,直接存入map中
{
map.put(a, new Integer(1));
}
else //否则出现过,将key对应的value值加1
{
//程序在这行报错 java.lang.NullPointerException
map.put(a,new Integer((Integer)(map.get(a))).intValue() + 1);
}
}
Set set = map.entrySet();
for(Iterator itr = set.iterator();itr.hasNext(); )
{
Map.Entry s = (Map.Entry)itr.next();
String key = (String)s.getKey();
String value = (String)s.getValue();
System.out.print(key + " " + value);
}
}
}