随机产生一个随机数

public class RandomNumUtil {    private static StringBuffer buf = new StringBuffer("a,b,c,d,e,f,g,h,i,g,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,A,B,C,D,E,F,G,H,I,G,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,1,2,3,4,5,6,7,8,9,0");    public static String GenerateActive() {        String[] arr = null;        java.util.Random r;        try{            arr = buf.toString().split(",");            StringBuffer b = new StringBuffer();            int k ;            for(int i=0;i<20;i++){                 r = new java.util.Random();                 k = r.nextInt();                 b.append(String.valueOf(arr[Math.abs(k % 62)]));            }            return b.toString();        }catch(Exception e){            e.printStackTrace();        }finally{            arr = null;            r = null;        }        return null;    }}