Java两个乒乓球队比赛名单问题(判断素数)

import java.util.ArrayList;
public class pingpang {
  String a,b,c;
  public static void main(String[] args) {
  String[] op = { "x", "y", "z" };
  ArrayList<pingpang> arrayList=new ArrayList<pingpang>();
  for (int i = 0; i < 3; i++)
  for (int j = 0; j < 3; j++)
  for (int k = 0; k < 3; k++) {
  pingpang a=new pingpang(op[i],op[j],op[k]);
   if(!a.a.equals(a.b)&&!a.b.equals(a.c)&&!a.a.equals("x")
   &&!a.c.equals("x")&&!a.c.equals("z")){
   arrayList.add(a);
   }
  }
  for(Object a:arrayList){
  System.out.println(a);
  }
  }
  public pingpang(String a, String b, String c) {
  super();
  this.a = a;
  this.b = b;
  this.c = c;
  }
  @Override
  public String toString() {
  // TODO Auto-generated method stub
  return "a的对手是"+a+","+"b的对手是"+b+","+"c的对手是"+c+"\n";
  }
}