Question

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java String concatenation

Sources tell us that concat is implemented as follows:

   public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
        char buf[] = Arrays.copyOf(value, len + otherLen);
        str.getChars(buf, len);
        return new String(buf, true);
    }

Does + implementation differ when it comes to Strings? How? Is there a performance difference between + and concat. When should one be chosen over another?

Answer 1

This is a test I just made:

I created a class with those 3 instructions:

    String s1 = "foo";
    String s2 = "bar";
    String s3 = s1 + s2;

Then I took the generated .class file and I decompiled using JAD decompiler.
This is how the code show up in the regenerated source:

    String s = "foo";
    String s1 = "bar";
    String s2 = (new StringBuilder()).append(s).append(s1).toString();

So: this is the difference between + and concat.

I guess concat() is always better than StringBuilder, because it requires less objects to be created. You may chose StringBuilder if you want to append string repeatedly in a loop; in this case concat may create a new String each time, while StringBuilder may just expand the internal buffer. But, if StringBuilder is best in this last scenario, we can say that still concat() is better than +, in loops.