StringBuilder vs String concatenation in toString() in Java

Question

Given the 2 toString() implementations below, which one is preferred:

public String toString(){
    return \"{a:\"+ a + \", b:\" + b + \", c: \"         


        

Answer 1

See the example below:

static final int MAX_ITERATIONS = 50000;
static final int CALC_AVG_EVERY = 10000;

public static void main(String[] args) {
    printBytecodeVersion();
    printJavaVersion();
    case1();//str.concat
    case2();//+=
    case3();//StringBuilder
}

static void case1() {
    System.out.println("[str1.concat(str2)]");
    List savedTimes = new ArrayList();
    long startTimeAll = System.currentTimeMillis();
    String str = "";
    for (int i = 0; i < MAX_ITERATIONS; i++) {
        long startTime = System.currentTimeMillis();
        str = str.concat(UUID.randomUUID() + "---");
        saveTime(savedTimes, startTime);
    }
    System.out.println("Created string of length:" + str.length() + " in " + (System.currentTimeMillis() - startTimeAll) + " ms");
}

static void case2() {
    System.out.println("[str1+=str2]");
    List savedTimes = new ArrayList();
    long startTimeAll = System.currentTimeMillis();
    String str = "";
    for (int i = 0; i < MAX_ITERATIONS; i++) {
        long startTime = System.currentTimeMillis();
        str += UUID.randomUUID() + "---";
        saveTime(savedTimes, startTime);
    }
    System.out.println("Created string of length:" + str.length() + " in " + (System.currentTimeMillis() - startTimeAll) + " ms");
}

static void case3() {
    System.out.println("[str1.append(str2)]");
    List savedTimes = new ArrayList();
    long startTimeAll = System.currentTimeMillis();
    StringBuilder str = new StringBuilder("");
    for (int i = 0; i < MAX_ITERATIONS; i++) {
        long startTime = System.currentTimeMillis();
        str.append(UUID.randomUUID() + "---");
        saveTime(savedTimes, startTime);
    }
    System.out.println("Created string of length:" + str.length() + " in " + (System.currentTimeMillis() - startTimeAll) + " ms");

}

static void saveTime(List executionTimes, long startTime) {
    executionTimes.add(System.currentTimeMillis() - startTime);
    if (executionTimes.size() % CALC_AVG_EVERY == 0) {
        out.println("average time for " + executionTimes.size() + " concatenations: "
                + NumberFormat.getInstance().format(executionTimes.stream().mapToLong(Long::longValue).average().orElseGet(() -> 0))
                + " ms avg");
        executionTimes.clear();
    }
}

Output:

java bytecode version:8
java.version: 1.8.0_144
[str1.concat(str2)]
average time for 10000 concatenations: 0.096 ms avg
average time for 10000 concatenations: 0.185 ms avg
average time for 10000 concatenations: 0.327 ms avg
average time for 10000 concatenations: 0.501 ms avg
average time for 10000 concatenations: 0.656 ms avg
Created string of length:1950000 in 17745 ms
[str1+=str2]
average time for 10000 concatenations: 0.21 ms avg
average time for 10000 concatenations: 0.652 ms avg
average time for 10000 concatenations: 1.129 ms avg
average time for 10000 concatenations: 1.727 ms avg
average time for 10000 concatenations: 2.302 ms avg
Created string of length:1950000 in 60279 ms
[str1.append(str2)]
average time for 10000 concatenations: 0.002 ms avg
average time for 10000 concatenations: 0.002 ms avg
average time for 10000 concatenations: 0.002 ms avg
average time for 10000 concatenations: 0.002 ms avg
average time for 10000 concatenations: 0.002 ms avg
Created string of length:1950000 in 100 ms

As the string length increases, so does the concatenation time.
That is where the StringBuilder is definitely needed.
As you see, the concatenation: UUID.randomUUID()+"---", does not really affect the time.

P.S.: I don't think When to use StringBuilder in Java is really a duplicate of this.
This question talks about toString() which most of the times does not perform concatenations of huge strings.

---

2019 Update

Since java8 times, things have changed a bit. It seems that now(java13), the concatenation time of += is practically the same as str.concat(). However StringBuilder concatenation time is still constant. (Original post above was slightly edited to add more verbose output)

java bytecode version:13
java.version: 13.0.1
[str1.concat(str2)]
average time for 10000 concatenations: 0.047 ms avg
average time for 10000 concatenations: 0.1 ms avg
average time for 10000 concatenations: 0.17 ms avg
average time for 10000 concatenations: 0.255 ms avg
average time for 10000 concatenations: 0.336 ms avg
Created string of length:1950000 in 9147 ms
[str1+=str2]
average time for 10000 concatenations: 0.037 ms avg
average time for 10000 concatenations: 0.097 ms avg
average time for 10000 concatenations: 0.249 ms avg
average time for 10000 concatenations: 0.298 ms avg
average time for 10000 concatenations: 0.326 ms avg
Created string of length:1950000 in 10191 ms
[str1.append(str2)]
average time for 10000 concatenations: 0.001 ms avg
average time for 10000 concatenations: 0.001 ms avg
average time for 10000 concatenations: 0.001 ms avg
average time for 10000 concatenations: 0.001 ms avg
average time for 10000 concatenations: 0.001 ms avg
Created string of length:1950000 in 43 ms

Worth noting also bytecode:8/java.version:13 combination has a good performance benefit compared to bytecode:8/java.version:8