Java实在一个素数的问题

那我就不客气拉 谢拉
import java.util.*;

public class PrimeNum {

private ArrayList<Integer> primeNumList=new ArrayList<Integer>();
private int amount;
private int testNum;

public PrimeNum(){
primeNumList.add(1);
amount=1;
testNum=1;
}

public PrimeNum(int n){
testNum=n;
if(testNum==1){
primeNumList.add(1);
amount=amount+1;
return;
}
for(int i=2;i<=testNum;i++){

int j;
for(j=2;j<=Math.sqrt(i);j++){
if(i%j==0) break;
}
if(j>Math.sqrt(i)){
primeNumList.add(i);
amount=amount+1;
}
}
}

public int getAmount(){
return this.amount;
}

public void printPrimeNum(){
System.out.println("there are "+amount+" prime numbers less than "+testNum+" :");
for(int i=0;i<primeNumLi