Pass-by-value (StringBuilder vs String)

Because String is immutable and hence String#concat does not modify the original String instance, it only returns a new String while the original is left unmodified, while StringBuilder is mutable and the change is reflected in the StringBuilder instance passed as parameter.

Looking at the Javadoc for String, one will read that

[...] String objects are immutable [...].

This means concat(String) does not change the String itself, but constructs a new String.

StringBuilders, on the other hand, are mutable. By calling append(CharSequence), the object itself is mutated.

Because when you call speak(name);, inside speak when you do

name = name.concat("4");

it creates a new object because Strings are immutable. When you change the original string it creates a new object,I agree that you are returning it but you are not catching it.

So essentially what you are doing is :

name(new) = name(original) + '4'; // but you should notice that both the names are different objects.

try

String name = "Sam";
name = speak(name);

Of course now I think there is no need to explain why it's working with StringBuilder unless if you don't know that StringBuilder is mutable.