测试数字在Java中是否为质数

Question

Test if the number is prime or not:

This is my code:

public static boolean isPrime(int x){
      if(x<=1) 
         return false;
      else if(x==2) 
         return true;
      else {
         for(int i=2; i<=x/2; i++){                        
             if(x%i==0) 
                return false;
         }
         return true;
      }
   }

My question is: the last statement 'return true', if the number is a prime, then no false return, but if the number is not a prime, then during the for loop, there will be a false return. And when the loop is finished, the program execute the statement below for loop--'return true'. So I am wondering if the returned true will cover the returned false that happened during for loop. (Although when I am testing this code, the code works well for testing a number is prime or not, but I don't know why)

Answer 1

在return false （或任何return语句）之后，该方法将不执行任何其他操作而return true ，因此在这种情况下不会达到return true ，因此您的代码可以正常工作。

Answer 2

Why do you start the iteration on zero?

Consider what would happen when i = 1 :

if(x % i == 0)    // i = 1, so x % i will be zero
    return false;

Even more, consider what would happen when i = 0 :

if(x % i == 0)    // how would you define x % 0!?

Start the iteration on 2:

// ...
for(int i = 2; i <= x / 2; i++) {
    if(x % i == 0)
        return false;
}
return true;
// ...

Answer 3

return语句使方法退出，因此，如果执行“ return false”，则将永远不会执行返回false和return true的方法退出。

Answer 4

return false ends the method. When we say return , that means "stop what you're doing and give this value back to the method that called us."

Cases where this doesn't happen

• try - finally blocks, any return statement or exception thrown in the finally block of a try block will override the returned value.
• System.exit , this method never returns, so any statements after an exit call will not be executed.

Answer 5

if(x%i==0) return false;

if this condition comes true then it will return false and will not execute anything after that

and when above condition will be false it wil return true.