等概率划分

//7转为10
 public int rand7(){ 
  int f=(int) (Math.random()*7+1);//1<=x<=7
  return f;
 }
 public int rand7ToRand10(){
  while(true){
   int num=7*(rand7()-1)+rand7();//1-49
   if(num<40){   
    return num%10+1;
   }
  }
 }
 //5转为7
 public int rand5(){ 
  int f=(int) (Math.random()*5+1);//1<=x<=5
  return f;
 }
 public int rand5TorRand7(){
  while(true){
   int num=5*(rand5()-1)+rand5();//1-25
   if(num<19){
    return num%7+1;
   }
  }
 }