java如何判断一个数是否是素数(质数)

import java.util.Scanner;
public class TestWork {
 public static void main(String[] args) {
  boolean isPrime = true;
  Scanner sc = new Scanner(System.in);
  System.out.println("请输入一个正整数");
  int num = sc.nextInt();
  if (num > 0) {
   int k = (int) Math.sqrt(num);//k为num的正平方根，取整数
   for (int i = 2; i <= k; i++) {
    if (num % i == 0) {
     isPrime = false;//不是素数
     break;
    }
   }
  }
  if (isPrime) {
   System.out.println(num + "是素数");
  } else {
   System.out.println(num + "不是素数");
  }
 }
}

public class TestWork {
 public static void main(String[] args) {
  boolean isPrime = true;
  int PrimeCount=0;
  for (int i = 3; i <= 1000; i+=2) {
   int k = (int) Math.sqrt(i);//k为num的正平方根，取整数
   isPrime = true;
   for (int j = 2; j <= k; j++) {
    if (i % j == 0) {
     isPrime = false;//不是素数
     break;
    }
   }
   if (isPrime) {
    PrimeCount++;
    System.out.print(i+"\t");
    if(PrimeCount%5==0){
     System.out.println();
    }
   }
  }
 }
}

//1-100之间的质数--------1
public class PrimeNumber {
 public static void main(String[] args) {
  for(int i=2;i<=100;i++) {
   boolean flag=true;
   for(int j=2;j<i;j++) {
    if(i%j == 0) {
     flag=false;
     break;     
    }        
   }
   if(flag) {
    System.out.println("质数：i= "+i);   
   }
  }  
 }
}

//1-100之间的质数--------2
public class PrimeNumber {
 public static void main(String[] args) { 
        for(int i=2;i<=100;i++) {   
            for(int j=2;j<=i;j++) {
                if(i%j==0 && i!=j) {
                    break;     
                }
                if(j==i) {
                    System.out.println("质数：i= "+i);     
                }    
            }
        }
    }
}  

//1-100之间的质数--------3
public class PrimeNumber {
 public static void main(String[] args) {
        for(int i=2;i<=100;i++) {   
            int j=2;
            while(i%j != 0 ) {
                j++;     
            }
            if(j==i) {
                System.out.println("质数：i= "+i);         
            }
        }
    }
}

//1-100之间的质数--------4
public class PrimeNumber {
 public static void main(String[] args) {
        for(int i=2;i<=100;i++) {
            int j=2;
            for(;j<=i/2;j++) {
                if(i%j==0) {
                    break;     
                }   
            }
            if(j==i/2+1) {
                System.out.println("质数：i= "+i);                   
            } 
        }
 } 
} 

public class PrimeNumber {
 public static void main(String[] args) {
  int count=0;
  for(int i=2;i<=100;i++) {
   int j=2;
   for(;j<=(int)Math.sqrt(i);j++) {
    if(i%j==0) {
     break;     
    }   
   }
   if(j==(int)Math.sqrt(i)+1) {
    System.out.println("质数：i= "+i);
    count++;     
   } 
  }
  System.out.println("质数个数：count= "+count);
    }
}

public class PrimeNumber {
 public static void main(String[] args) {
int count1 = 0, count2 = 0;
  for (int i = 2; i <= 100; i++) {   
   for (int j = 2; j <= Math.sqrt(i); j++) {
    if (i % j != 0) {
     //不能被整除累计一次
     count1++;
    }
   }
   // 转为int的类型
   int num2 = (int) Math.sqrt(i);
   //如果累计次数num2 - 1个数相等则为素数
   if (count1 == (num2 - 1) {
    System.out.println("质数: " + i);
    count2++;
   }
   //找到素数后count1再初始化为0
   count1 = 0;
  }
  // 统计素数个数
  System.out.println("质数个数：count2= "+count2);
 }
}