随机生成16位的验证码：

import java.util.Random;

public class Test02 {
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println(getChar(16));

	}
	public static String getChar(int length) {
		 char[] ss = new char[length];
		int i=0;
		while(i<length) {
		    int f = (int) (Math.random()*3);
		    if(f==0)  
		     ss[i] = (char) ('A'+Math.random()*26);
		    else if(f==1)  
		     ss[i] = (char) ('a'+Math.random()*26);
		    else 
		     ss[i] = (char) ('0'+Math.random()*10);    
		    i++;
		 }
			String str=new String(ss);
		 return str;
		}
}

转载：https://blog.csdn.net/quwei7515/article/details/7926105

用StringBuilder随机生成一个16位的密码，可以是大小写字母或数字。

import java.util.Random;

public class Test03 {
	public static void main(String[] args) {
		Random rand = new Random();
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < 16; i++) {
			 int f = (int) (Math.random()*62);
			 if(f<10) {
				 sb.append(f);
			 }else if(f<36) {
				 sb.append((char)(f + 'A'-10));
			 }else {
				 sb.append((char)(f + 'a'-36));
			 }
		}
		
		System.out.println(sb);
	}
}

转载：https://www.cnblogs.com/jeffqing/archive/2012/07/28/2612570.html

自己的方法：第二种：

import java.util.Random;

public class Test04 {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Random rand = new Random();
		StringBuilder sb = new StringBuilder();
		String strAll = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
		for (int i = 0; i < 16; i++) {
			 int f = (int) (Math.random()*62);
			 sb.append(strAll.charAt(f));
		}
		System.out.println(sb);
	}

}