Using "this" in another constructor

Question

Suppose you have a class with this constructor:

public SomeObj(int x, int y) {
    this.x = x;
    this.y = y;
}

All good. But now if you want to clone the object, I want a constructor to accept one argument with an object from that type, so inside the constructor all (necessary) fields can be copied.

public SomeObj(SomeObj objectToClone) { ... }

But now which of the following two ways is better? What are the advantages and disadvantages (performance (byte code), readability...)?

// 1
public SomeObj(SomeObj objectToClone) {
    this.x = objectToClone.x;
    this.y = objectToClone.y;
}

// 2
public SomeObj(SomeObj objectToClone) {
    this(objectToClone.x, objectToClone.y);
}

Jon Skeet · Accepted Answer · 2012-03-08 11:50:36Z

13

I would go with the latter, personally.

Where possible, I try to make exactly one constructor have a "real" body, and make all others delegate to it. That's not always possible - in particular, different constructors may need to delegate to different superconstructors - but it's nice knowing there's one place you can put extra initialization, logging, breakpoints etc which will always get hit.

answered Mar 8, 2012 at 11:50

Jon Skeet

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2 Comments

Peter Lawrey Over a year ago

+1: Having one "real" constructor all the others call means the functionality will be consistent between constructors. Having different code in different constructors can be a nightmare.

MC Emperor Over a year ago

Although it's just a vision on this subject, I would like to accept this answer (even though you already have loads of points and medals). Because this vision gives me a clear view on this one.

Chii · Accepted Answer · 2012-03-08 11:51:08Z

3

Number 2 is better. Why? because you are no longer repeating yourself, setting a member in two different constructors.

There is no performance hit really, unless the miniscule extra indirection of calling this() is going to affect you (and i doubt you can even measure this difference accurately).

answered Mar 8, 2012 at 11:51

Chii

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1 Comment

Chii Over a year ago

@Mudu: i m not sure if the compiler inlines it, but yea, it might!

sch · Accepted Answer · 2012-03-08 11:51:29Z

2

If you use the second option, you don't have to change public SomeObj(SomeObj objectToClone) every time you change the implementation of public SomeObj(int x, int y). So it is better as it allows you to avoid duplicating code.

answered Mar 8, 2012 at 11:51

sch

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Comments

Ilya · Accepted Answer · 2012-03-08 11:51:54Z

1

It's not C++. Override .clone() method from Object class for copy objects. It's true way

answered Mar 8, 2012 at 11:51

Ilya

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3 Comments

Matthias Meid Over a year ago

+1 for the option. But it does not vote for one of the options. From clone, I'd personally invoke the constructor instead of copying the fields again.

Călin Darie Over a year ago

Isn't clone() traditionally used for deep copying? I think it should be overridden in that case only.

MC Emperor Over a year ago

Okay, but the question is not about the clone thing — that was just an example. It's about passing this(...) to another constructor.

Matthias Meid · Accepted Answer · 2012-03-08 12:02:12Z

1

The latter does not build anymore as soon as you add another parameter, let's say z. If you'd take the first approach, you might just forget to copy z. It still builds, but doesn't work properly.

edited Mar 8, 2012 at 12:02

answered Mar 8, 2012 at 11:51

Matthias Meid

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2 Comments

jenaiz Over a year ago

Don't build but the compiler give you and error, but the first.... builds but it doesn't work if you add another parameter.

Matthias Meid Over a year ago

That is what I wanted to point out. I tried to express myself more clearly.

Andrei G · Accepted Answer · 2012-03-08 11:59:30Z

// 1
public SomeObj(SomeObj objectToClone) {
    this.x = objectToClone.x;
    this.y = objectToClone.y;
}

is better.

Your second option would require you to update your constructor each time you add a new parameter. Also, if you have to call that in 100 places with 100 properties to list in each constructor, it's obvious it starts loosing readability.

I can't understand why you would like to pass multiple properties of the same object as parameter instead of passing the object. There's no reason for that.

Compare these two:

public SomeObj(SomeObj objectToClone) {
    this.x = objectToClone.x;
    this.y = objectToClone.y;
    this.z = objectToClone.z;
    this.p = objectToClone.p;
    this.s = objectToClone.s;
    this.a = objectToClone.a;
    this.b = objectToClone.b;
    }

and then call SomeObj(obj) 10 times in your code.

versus SomeObj(obj.x,obj.y,obj.z,obj.p,obj.s,obj.t,obj.a,obj.b); 10 times in your code

Also, with the second option you have no guarantee that the parameters passed are what you want, so you could get something like this: SomeObj(obj.x,obj.y,obj2.z,obj3.p,0,0,-1,null);

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