Program that should print the first 100 prime numbers, only prints 0 and 1 instead

public class Programma{
    public static void main(String args[]){
        int flag=0;
        for(int i=0;i<100;++i){
            for(int j=1;j<i;++j){
                if((i%j)==0)
                    flag=1;
                    break;
            }
            if (flag==0)
                System.out.print(i + " ");
            flag = 0;
        }
    }
}

Output:

$ java Programma 
0 1 

I really don't understand why.. I've checked the code multiple times and can't find the bug.

You forgot to add the curly brackets to the if:

            if((i%j)==0) {
                flag=1;
                break;
            }

So only the first line was under the condition, the break was executed each time.

Then you have a logical error, start the second loop with 2:

   for(int i=1;i<100;++i){
        for(int j=2;j<i;++j){

Now your code works. To understand why, add an additionally println like

public class Programma{
    public static void main(String args[]){
        int flag=0;
        for(int i=1;i<100;++i){
            for(int j=2;j<i;++j){
                if((i%j)==0){
                    System.out.println("# " + i + "/" + j);
                    flag=1;
                    break;
                }
            }
            if (flag==0)
                System.out.println(i + " ");
            flag = 0;
        }
    }
}

and try it with / without changed start values for the for loop.

This will give you the first 100 prime numbers.

public class Programma {

	public static void main(String[] args) {
		int flag=0,primeCounter=0,start=2;
		while(primeCounter<100){
			for(int j=2;j<=Math.sqrt(start);j++){
				if((start%j)==0){
					flag=1;
					break;
				}
			}
			if (flag==0) {
				System.out.print(start + " ");
				primeCounter++;
			}
			start++;
			flag = 0;
		}
	}
}

In your code, I suspect that there is something wrong with the if here.

if((i%j)==0) { // By forgetting to add braces {} around the code inside the if 
    flag=1;
    break;  // the break statement executes instantly, without them.
}

By making some small improvements in your original code here is the for loop based code segment that prints all prime numbers up to N.

public class Programma {
    // Function to print all prime numbers up to N.
    static void print_primes(int N) {
        int flag;
        // Traverse each number from 2 to N.
        for (int i = 2; i <= N; i++) {
            flag = 1;
            for (int j = 2; j <= i / 2; ++j)  {
                if (i % j == 0) {
                    flag = 0;
                    break;
                }
            }
            if (flag == 1) // flag == 1 means i is prime.
                System.out.print(i + " ");
        }
    }
    // Main prints all prime numbers up to n = 100.
    public static void main(String[] args) {
        print_primes(100);
    }
}

Output:

2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 

You could also write this in fewer lines of code, than using loops, should you go for a Java 8 stream library based solution.

import java.util.stream.*;
import java.util.Arrays;

public class Programma {
    // Check if a number is prime or not.
    static boolean isPrime(int n) {
        return IntStream.rangeClosed(2, n / 2).noneMatch(i -> n % i == 0);
    }
    // Generate all prime numbers until n, and return them as an int[] array.
    static int[] generatePrimes(int n) {
        return IntStream.rangeClosed(2, n).filter(Programma::isPrime).toArray();
    }
    // Main prints all prime numbers up to n = 100.
    public static void main(String[] args) {
        System.out.println(Arrays.toString(generatePrimes(100)));
    }
}

Output (all prime numbers up to n = 100 are printed):

[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]

Here is the answer:

public class Main{
    public static void main(String[] args) {

    int i;
    int num = 0;
    int maxCheck = 100; 
    boolean isPrime = true;
    String primeNumbersFound = ""; 
    for (i = 1; i <= maxCheck; i++) {
        isPrime = CheckPrime(i);
        if (isPrime) {
            primeNumbersFound = primeNumbersFound + i + " ";
        }
    }
    System.out.println("Prime numbers from 1 to " + maxCheck + " are:");
    System.out.println(primeNumbersFound);
}
public static boolean CheckPrime(int numberToCheck) {
    int remainder;
    for (int i = 2; i <= numberToCheck / 2; i++) {
        remainder = numberToCheck % i;
        if (remainder == 0) {
            return false;
        }
    }
    return true;

}

}

Here you just check if the remainder is 0, then numberToCheck is not prime and so you break the loop. Else you just continue the loop. Hope this helps. If you need a more detailed explanation just visit this link.