[Solved]Why output of this two code is different?

Question

//code 1
class Program{
static String str;
public static void main(String[] args){
System.out.println(str+"hello");
}
}
//Output
nullhello
//code 2
class Program{
public static void main(String[] args){
String str;
System.out.println(str+"hello");
}
}
//Output
Error(variable str might not have been initialized)

Answer 1

because
method variables do not have default values and
class variables have default values