Question

In this piece of code i pass a name into a method that modifies the String literal name but not the object itself, when the code exits the method, the object (as identified by the hashcode) is the same, however not the name that is modified in the method.

How should i explain this?

public class ObjectContentsPassByReferenceApp {
    private static void modifyObject(Bus bus) {
        bus.setName("SBS Transit");
    }

    public static void main(String args[]) {
        Bus bus;

        bus = new Bus();
        bus.setName("Trans Island Bus");

        System.out.println("Bus initially set to (hashcode): " + bus);
        System.out.println("Bus name: " + bus.getName());

        modifyObject(bus);

        System.out.println("After calling modifyObject (hashcode): " + bus);
        System.out.println("Bus name: " + bus.getName());
    }
}

class Bus {
    private String name;

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Run results:

Bus initially set to (hashcode): sg.java.concepts.passByReference.Bus@8d2ed0
Bus name: Trans Island Bus
After calling modifyObject (hashcode): sg.java.concepts.passByReference.Bus@8d2ed0
Bus name: SBS Transit

Answer 1

You haven’t overridden hashCode – therefore it will use the implementation in java.lang.Object, which doesn’t vary over the course of an object’s lifetime. You haven’t overridden equals either… which means that a.equals(b) will only return true for Bus a and Bus b if a and b refer to the exact same object – rather than objects with equal name.

Note that the names in your code suggest that Java uses pass by reference. It doesn’t – it always uses pass by value, but those values are always either primitives or references – never actual objects. The same is true for simple assignment etc.

In your code, you’re creating a single Bus object. Think of it as a real life bus. It has a name painted on it, and a serial number embossed on it (the latter being the hash code). When you call the method, that’s telling the method how to get to the Bus object – it’s not creating a new Bus. The method comes and paints over the name with a new one, but that does nothing to the serial number, which is the same as it ever was.

Also note that name is not a string literal – it’s a string variable. Its initial value comes from a string literal, but changing the value later does nothing to the original string object.