could someone help me and explain the difference in the result between the two codes?

Question

It would be something like: when I use String to instantiate it passes the reference and when I do not use it, it passes the value. am I sure?

Code 1:

public class Program {
public static void main(String[] args) {
A a = new A("1");
A c = a;
a.a1 = new String("3");
System.out.print(a.a1);
System.out.println(c.a1);
}
}
class A{
public String a1;
public A(String a1){
this.a1=a1;
}
}
//Result: 33

Code 2:

public class Program2{

public static void main(String[] args) {
A a = new A("1");
A c = a;
a = new A("3");
System.out.print(a.a1);
System.out.print(c.a1);
}

}
class A{
public String a1;
public A(String a1){
this.a1=a1;
}
}
//Result:31

Answer 1

When you do - a.a1 = new String("3"); - you're changing the field a1 in the object a, which is public.
And in - a = new A("3"); - you're creating a new object and using a to refer to that new object you just created.

Answer 2

I understand perfectly well! In the first code it changes the value a1 that is public and used both by a and by c, so the result of a and c is the same. In the second code c remains with the value of the first object created.Thanks a lot!

Answer 3

Yes because you created a new object for the variable a and didn't change c so c is still refering the old object.