C++ References and Java References

Question

//C++ Example

#include 
using namespace std;

int doHello (std::string&);
int main() {
    std::string str1 = \"perry\";
    cout <<         


        

Answer 1

Java does not have pass by reference (which you were using in the C++ code) at all. The references are passed by value. (The values of str and str1 aren't objects at all, they're references - it really helps to keep the two concepts very separate.)

Typically you would use a return value to return a new reference if you need to:

str1 = doHello(str1);

Note that String is slightly different to List etc, because strings are immutable. To modify a collection (well, any mutable collection) you don't need to create a new one, you just modify the object via the original reference:

public static void addHello(List items)
{
    items.add("Hello");
}

You could then call this like so:

List list = new ArrayList();
addHello(list);
System.out.println(list.get(0)); // "Hello"

The difference between mutating an existing object and changing the value of a variable to refer to a different object is crucial. If you want to leave the existing collection alone and create a new one, you'd have to do that explicitly:

public static List withHello(List items)
{
    List newList = new ArrayList(items);
    newList.add("Hello");
    return newList;
}

You'd then call it like this:

List empty = new ArrayList();
List newList = withHello(empty);
System.out.println(empty.size()); // Prints 0
System.out.println(newList.size()); // Prints 1

Does this answer everything you needed?