Java: toString vs. string concatentation?

Question

Is there a reason to prefer either string concatenation or invoking toString when converting objects to string representations?

Does string concatenation cause the object's toString method to be invoked?

String s;
Properties p = System.getProperties();
s = p.toString();
s = "" + p;

@SotiriosDelimanolis, is that a reason to prefer one over the other? It's not a duplicate question. — Mark Harrison
– Mark Harrison, Commented Oct 29, 2015 at 2:55
p.toString() doesn't involve a new object. "" + p involves a new object. (Though I would imagine the "" could be removed by a smart compiler.) After that, it's all opinion, what looks better. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Oct 29, 2015 at 2:56
so your answer is "there's no reason to prefer one over the other"? — Mark Harrison
– Mark Harrison, Commented Oct 29, 2015 at 2:57

Matt Timmermans · Accepted Answer · 2015-10-29 03:14:19Z

2

p.toString() is better.

When you say s=""+p, the compiler makes something like this:

{
    StringBuilder sb = new StringBuilder();
    sb.append("").append(p.toString());
    s=sb.toString();
}

So, yes, ""+p does mean that p.toString() gets called, but it also adds a lot of extra work.

The best thing that can happen is that the compiler recognizes that it's the same as p.toString() and just calls that instead, but you shouldn't count on that.

answered Oct 29, 2015 at 3:14

Matt Timmermans

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1 Comment

Mike Samuel Over a year ago

I think it does String.valueOf(p), not p.toString() though the former calls the latter when p != null.

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