吸血鬼数字

本人不是精通算法，闲来无事写了个求吸血鬼数字的函数，性能有点慢，希望大神批评指正给点意见，怎么修改下能把性能提升。

package com.cnroa.util;

import java.util.Arrays;
import java.util.List;

public class Test {

    public static void main(String[] args){//        Comparator<Object> com=Collator.getInstance(java.util.Locale.CHINA);
//        String[] aa = {"上海","云南","河北","湖南","西藏自治区","陕西","香港特别行政区","黑龙江"};
//        Arrays.sort(aa,com);
//        for(String i : aa){System.out.println(i);}
        xugNumber(6);
//        rejectOne(8918);
    }
    
    public static List<Integer> xugNumber(Integer num){
        if(num%2 != 0){
            System.out.println("需要计算的不是偶位数");
            return null;
        }
        Integer first = getMin(num);
        Integer end = getMax(num);
        for(int i=first;i<=end;i++){
            boolean flag = rejectOne(i);
            if(flag) System.out.println(i);
        }
        return null;
    }
    public static Integer getMin(Integer num){
        StringBuffer sb = new StringBuffer();
        sb.append("1");
        for(int i=0;i<num-1;i++) sb.append("0");
        return Integer.parseInt(sb.toString());
    }
    public static Integer getMax(Integer num){
        StringBuffer sb = new StringBuffer();
        sb.append("1");
        for(int i=0;i<num;i++) sb.append("0");
        return Integer.parseInt(sb.toString())-1;
    }
    public static boolean rejectOne(Integer number){
        Integer half = (number+"").length()/2;
        Integer first = getMin(half);
        Integer end = getMax(half);
        for(int i=first;i<=end;i++){
            for(int j=first;j<=end;j++){
                if(i*j==number){
                    boolean flag = rejectThree(i+"",j+"",number+"");
                    if(flag){
                        System.out.println(i+"---"+j);
                        return true;
                    }
                }
            }
        }
        return false;
    }
    public static boolean rejectTwo(String half,String all){
        boolean flag = true;
        char[] temp = half.toCharArray();
        for(char a : temp){
            if(!all.contains(a+"")) flag = false;
        }
        for(int i=0;i<temp.length-1;i++){
            for(int j=i+1;j<temp.length;j++){
                if(temp[i] == temp[j]) flag = false;
            }
        }
        return flag;
    }
    public static boolean rejectThree(String one,String two,String all){
        boolean flag = false;
        char[] temp1 = one.toCharArray();
        int numOne = 0;
        for(char a : temp1){
            if(all.contains(a+"")) numOne++;
        }
        int numTwo = 0;
        for(char a : temp1){
            if(all.contains(a+"")) numTwo++;
        }
        
        String[] sortSource =all.split("");
        String[] sortTarget =(one+two).split("");
        Arrays.sort(sortSource);
        Arrays.sort(sortTarget);
        if(numOne == all.length()/2 && numTwo == all.length()/2 && arrayToString(sortSource).equals(arrayToString(sortTarget))){
            flag = true;
        }
        return flag;
    }
    public static String arrayToString(String[] s){
        StringBuffer sb = new StringBuffer();
        for(String i : s)    sb.append(i);
        return sb.toString();
    }
}