What is the main difference between pass-by-reference and pass-by-value?

Other languages use pass-by-reference or pass-by-pointer. But in Java no matter what type of argument you pass the corresponding parameter (primitive variable or object reference) will get a copy of that data, which is exactly how pass-by-value (i.e. copy-by-value) works. In Java, if a calling method passes a reference of an object as an argument to the called method then the passedin reference gets copied first and then passed to the called method. Both the original reference that was passed-in and the copied reference will be pointing to the same object. So no matter which reference you use, you will be always modifying the same original object, which is how the pass-by-reference works as well.
ref d Pass-by-value for primitive variables vs Object references
public void first(){
int i= 10;
int x = second(i);
//At this point
//value of i is still 10
//value of x is 11
}
public int second(int k) {
k++;
return k ;
}
i = 10
k = 10
k = 11
Copy of I stores i
copies I
acts on k
ref
public void first(){
Car c = new Car("red")
//At this point
//color is Red
second(c);
//At this point
//color is Blue
}
public void second(Car d)
{
d.setColor(blue);
//color is blue
}
Car object
String color = red
ref c
copy of c
Primitive variables Object references modifies the original object through copied reference modifies the copy k but not the original.
Changes color = blue
If your method call involves inter-process (e.g. between two JVMs) communication, then the reference of the calling method has a different address space to the called method sitting in a separate process (i.e. separate Java – Fundamentals JVM). Hence inter-process communication involves calling method passing objects as arguments to called method by-value in a serialized form, which can adversely affect performance due to marshaling and unmarshaling cost.

pass-by-reference returns a address of the value.
pass-by-value returns a value.