Question

Consider following situation:

I write a java program for an embedded(modem) device. The SDK doesn’t offer Cloneable.
Therefore I have in class GsmSignalStrength the method clone(), but it’s not
from Object.clone(), it is “my” implementation.
I would like to know is this useful and correct or should I use a copy constructor in GsmSignalStrength, like in the comment?
I don’t see the advantage of a copy constructor, because I will have to create another GsmSignalStrength object in my StateMachine.

Here is my code…

File 1 “Status.java”:

class Status
{
    Status(GsmSignalStrength gsmSignalStrength)
    {
        initClass(gsmSignalStrength);
    }

    GsmSignalStrength gsmSignalStrength;

    private void initClass(GsmSignalStrength gsmSignalStrength)
    {
        this.gsmSignalStrength = gsmSignalStrength;
    }
}

File 2 “GsmSignalStrength”:

class GsmSignalStrength
{
    GsmSignalStrength(byte signalStrength)
    {
        initClass(signalStrength);
    }

    GsmSignalStrength()
    {
        initClass(100);
    }

    byte value;

    public void copyTo(GsmSignalStrength destination)
    {
        destination.value = this.value;
    }

    public GsmSignalStrength clone()
    {
        GsmSignalStrength clonedValue = new GsmSignalStrength();

        this.copyTo(clonedValue);

        return clonedValue;
    }

    /* With copy constructor
        GsmSignalStrength(GsmSignalStrength gsmSignalStrength)
        {
            gsmSignalStrength.value = value;
        }
    */

    private void initClass(byte signalStrength)
    {
        this.value = signalStrength;
    }
}

File 5 “GsmModemHandler”:

class GsmModemHandler
{
    GsmModemHandler()
    {
        initClass();
    }

    private GsmSignalStrength m_gsmSignalStrength;

    GsmSignalStrength getGsmSignalStrength()
    {

        ...bla...

        return m_gsmSignalStrength;
    }

    private void initClass()
    {
        m_gsmSignalStrength = new GsmSignalStrength();
    }
}

File 4 “StateMachine”:

class StateMachine
{
    StateMachine(GsmModemHandler gsmModemHandler)
    {
        initClass(gsmModemHandler);
    }

    private GsmModemHandler m_gsmModemHandler;

    void doSomething()
    {
        GsmSignalStrength gsmSignalStrength = m_gsmModemHandler.getGsmSignalStrength();
        Status xy = new Status(gsmSignalStrength.clone());

        /* With copy constructor

            GsmSignalStrength gsmSignalStrengthCopy = new GsmSignalStrength(gsmSignalStrength);
            Status xy = new Status(gsmSignalStrengthCopy);
        */
    }

    private void initClass(GsmModemHandler gsmModemHandler)
    {
        m_gsmModemHandler = gsmModemHandler;
    }
}

Answer 1

What you have had in that comment,

GsmSignalStrength(GsmSignalStrength gsmSignalStrength)
    {
        gsmSignalStrength = this;
    }

is not a copy constructor. The line

gsmSignalStrength = this;

has no effect at all because all it does is move a method-local reference to point to this.

A functioning “copy constructor” would be something like

GsmSignalStrength(GsmSignalStrength gsmSignalStrength)
    {
        this.value = gsmSignalStrength.value;
    }

the point being to create an independent object with the same member variables as the original.

Also, unlike in C++, writing

object1 = object2;

does not invoke a copy constructor. All it does is copies the reference — you end up with two references pointing to the same object.
If you want to create a new object you have to invoke the constructor explicitly:

object1 = new GsmSignalStrength( object2 );

---

As of whether it is “useless”, that depends on your application. A copy constructor, a clone() method, and your copyTo method all accomplish the same basic task. You can create a new object with the constructor, or with clone(), or create a new object and use copyTo on it — pick one way to do what you want and stick with it.