it tell me: This method must return a result of type boolean

[rubendelope]

import java.util.Scanner;
public class example{
public static boolean isPrime (int a) {
  Scanner in = new Scanner(System.in);
  a = in.nextInt();
  boolean isItPrime = true;


  for(int j = 2; j<= 50; j++){
    if(a % j == 0){
      return isItPrime;
      
    }
    else if(isItPrime){
     return isItPrime;
    
  }
  System.out.println(isItPrime);

  
  }
  
 

  
  



}
public static void main(String[] args) {
  isPrime();
}
}

Delete this before posting. Please make sure to include the language that you're using in the title, and use the <> button for any code.

 

[Sauron]

since your return statements are inside conditional statements, the compiler can't tell if the function always returns a boolean (which it must, since it's declared as a boolean function).

 

Also that function will never work as you expect it to. It can only return true. Think about what you're writing.

[Avocado Diaboli]

I'll admit up front here that my programming ability is very limited and my knowledge of Java rusty, but at a glance, there's a lot of weird stuff going on here.

1. You're defining a function that requires an argument, that "int a" you're declaring in the parentheses of isPrime. But then you're not passing any value to the function once you call it in the main function and instead using user input from the Scanner to assign a value to the variable "a".
2. If you want to define a function that does everything on its own, prints all the statements and doesn't return anything for further processing, you don't need to defined isPrime as a boolean, but rather a void (meaning it returns no value).
3. You have that for loop that always returns true regardless of what happens. Also, this error specifically happens, because you're using "return" from within a loop and if statement. You should use a helper variable that you set inside the loop and at the very end of the function return that.
4. That for loop only works for numbers between 2 and 50 and there's no error handling for anything outside that range.

Here's a quick example of what you're intending to do with a function that returns a boolean value for further processing. Note that this has absolutely no error handling if someone enters an invalid value. Also, I'm sure any competent programmer could whip up something far more elegant than this.

import java.util.Scanner;

class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int num = scan.nextInt();
		scan.close(); //Closing the scanner avoids potential memory leaks.

		/* We're checking if the value is a prime number with our 
		isPrime() function and then deciding what to print.*/
		if (isPrime(num) == true){
			System.out.println(num + " is a prime number.");
		} else {
			System.out.println(num + " is not a prime number.");
		}
  }

	public static boolean isPrime(int a){
		int i;

		/*Since we're checking for a prime number, we only need to 
		loop through half the value of the desired number. 
		(int) Math.ceil() rounds up the division to the next integer.*/
		int j = (int) Math.ceil(a / 2);
		
		/*We assume every value we're passed is a prime number by default, 
		therefore true. If it proves to be divisible by anything else, 
		we set it to false. This is the helper variable that at the end of 
		the function gets returned.*/
		boolean result = true;

		//If the number is below 2, we can already determine that it's not a prime number.
		if (a < 2){
			result = false;
		} else {
			for (i = 2; i < j; i++){
				if(a % i == 0){
					result = false;
				}
			}
		}
		return result;
	}
}

 

[RockSolid1106]

Take a look at this thread ^ ^