关于字符串比较是否相等的有关问题？多谢

import java.util.*;
public class Test_YanZheng {
    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        String str1="0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z";
        String[] str2=str1.split(",");
        int sum=1;
        for(int i=0;i<str2.length;i++)
        {
            if(sum%10!=0)            
                System.out.print(str2[i]+" ");            
            else
            {
                System.out.print(str2[i]);
                System.out.println();
            }
            sum++;
        }
        System.out.println();
//--------------------    
//生成验证码
        Random rand=new Random();
        StringBuilder str=new StringBuilder();
        int index=0;
        System.out.println("验证码为：");
        for(int i=0;i<4;i++)
        {
            index=rand.nextInt(str2.length)-1;
            System.out.print(str2[index]);
            str.append(str2[index]);
        }
        System.out.println();
//--------------------
//检验验证码
        System.out.println("请输入：");
        Scanner scan=new Scanner(System.in);//格式需记忆
        StringBuilder enterStr=new StringBuilder();
        enterStr.append(scan.next());    //接受键盘输入的字符串        
        if(str.equals(enterStr)==true)
            System.out.println("验证码输入正确");
        else
            System.out.println("验证码输入错误");
    }