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");    }