How to address Java String instantiation issue reported by Sonarqube

Question

I have used the below statement in my code to declare an empty string.

String temp = new String();

This has led to an issue raised by Sonarqube. So what would be the efficient way to fix this ? Is the below declaration a good way?

String temp = "";

Doesn't Sonar(Qube) provide you with some information as to why they flag that line as 'not recommended' ? — Stultuske, May 13 '15 at 12:16
There is never a good reason to use `new String()`... Yes, `""` is the empty `String` and will use the intern cache. — Elliott Frisch, May 13 '15 at 12:18
Without context, it would be hard to advise a good replacement. But initializing to empty string would be preferable to `new String()` in most cases. — MadConan, May 13 '15 at 12:18
@MadConan In most of the cases, i have nested if else Statements which will decide the value to be assigned to temp. — user1168608, May 13 '15 at 12:21
Would you mind precising the rule key raising the issue ? that could help to improve the documentation of this rule. — benzonico, May 13 '15 at 12:42

score 1 · Answer 1 · answered May 13 '15 at 12:22

Sonar is correct in that you shouldn't be using new String(). Initializing to empty string (String temp = "") is better. But if you do not use the value of empty string in any case, you should not initialize the variable to anything. You should only initialize a variable to a value you intend to use.

This is perfectly, and usually, acceptable:

String temp;

Your conditional logic should cover all cases of assignment.

score 0 · Answer 2 · answered May 13 '15 at 12:18

/**
     * Initializes a newly created {@code String} object so that it represents
     * an empty character sequence.  Note that use of this constructor is
     * unnecessary since Strings are immutable.
     */
    public String() {
        this.value = new char[0];
    }

Above souce code of String class depicts that use of this constructor is unnecessary. Every time you will create new object in heap. Better is to use String pool.

score 0 · Answer 3 · answered May 13 '15 at 12:21

0

The second example is the correct one.

Use

String temp = "";

answered May 13 '15 at 12:21

chokdee

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4
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score 0 · Answer 4 · edited May 23 '17 at 11:52

Yes, Sonar is correct. Using new String() is pretty much never a good idea.

The reason for this is, that the JVM caches Strings, so that you don't need to create a new object on the heap every time (this is why sometimes wrongly comparing strings with == works - They are both referring to the same cached instance on the heap).

Constructing a String yourself will circumvent that built-in cache and can lead to performance problems down the line. If all you need is an empty String just assign it:

String temp = "";

Or, if you just want to declare a String since you want to assign it later, just don't assign anything to it!

String temp;
if(condition()) {
    temp = "hello!";
} else {
    temp = "bye!";
}

If you plan on concatenating to your empty String in a loop though read this question about the attached performance issues and how to handle them.

How to address Java String instantiation issue reported by Sonarqube

4 Answers4