performance - What's the fastest way to concatenate two Strings in Java?

Question

What's the fastest way to concatenate two Strings in Java?

i.e

String ccyPair = ccy1 + ccy2;

I'm using cyPair as a key in a HashMap and it's called in a very tight loop to retrieve values.

When I profile then this is the bottleneck

java.lang.StringBuilder.append(StringBuilder.java:119)  
java.lang.StringBuilder.(StringBuilder.java:93)

Answer 1

Lots of theory - time for some practice!

private final String s1 = new String("1234567890");
private final String s2 = new String("1234567890");

Using plain for loops of 10,000,000, on a warmed-up 64-bit Hotspot, 1.6.0_22 on Intel Mac OS.

eg

@Test public void testConcatenation() {
    for (int i = 0; i < COUNT; i++) {
        String s3 = s1 + s2;
    }
}

With the following statements in the loops

String s3 = s1 + s2; 

1.33s

String s3 = new StringBuilder(s1).append(s2).toString();

1.28s

String s3 = new StringBuffer(s1).append(s2).toString();

1.92s

String s3 = s1.concat(s2);

0.70s

String s3 = "1234567890" + "1234567890";

0.0s

So concat is the clear winner, unless you have static strings, in which case the compiler will have taken care of you already.