Its giving an error because of mismatch return type.

Test.java

interface Interface {
	int add(int x,int y);
}
class MethodReference {
	void sum(int a,int b){
		System.out.println(a+b);
		//return a+b;
	}
	public static void main(String[] args) {
		Interface i = new MethodReference()::sum; 
		i.add(10,20);
	}
}