Performance Comparison of String Concatenation Using '+' vs StringBuilder in Java

When concatenating strings in Java, developers often wonder whether to use the '+' operator or a StringBuilder . Since JDK 5, the compiler automatically rewrites '+' concatenations into StringBuilder calls, but the performance impact can differ between one‑off concatenations and repeated concatenations inside loops.

Test setup : A StringTest class defines two methods— concatenationStringByPlus and concatenationStringByStringBuilder —each concatenating a prefix, a hyphen, and an integer. JUnit tests invoke each method 100 000 times and record the elapsed time.

/**
 * 使用+拼接字符串
 */
public String concatenationStringByPlus(String prefix, int i) {
    return prefix + "-" + i;
}

/**
 * 使用StringBuilder拼接字符串
 */
public String concatenationStringByStringBuilder(String prefix, int i) {
    return new StringBuilder().append(prefix).append("-").append(i).toString();
}

@Test
public void testStringConcatenation01ByPlus() {
    long startTime = System.currentTimeMillis();
    int count = 100000;
    for (int i = 0; i < count; i++) {
        String str = concatenationStringByPlus("testStringConcatenation01ByStringBuilder:", i);
    }
    long endTime = System.currentTimeMillis();
    System.out.println("testStringConcatenation01ByPlus，拼接字符串" + count + "次，花费" + (endTime - startTime) + "秒");
}

@Test
public void testStringConcatenation02ByStringBuilder() {
    long startTime = System.currentTimeMillis();
    int count = 100000;
    for (int i = 0; i < count; i++) {
        String str = concatenationStringByStringBuilder("testStringConcatenation02ByStringBuilder:", i);
    }
    long endTime = System.currentTimeMillis();
    System.out.println("testStringConcatenation02ByStringBuilder，拼接字符串" + count + "次，花费" + (endTime - startTime) + "秒");
}

The JUnit output shows a negligible difference for single‑pass concatenation (≈33 s vs 36 s), confirming that the compiler’s optimization makes both approaches effectively equivalent.

Loop concatenation test : Two additional tests concatenate a long string 10 000 times, one using '+' inside the loop and the other using a single StringBuilder instance.

/**
 * 循环使用+拼接字符串
 */
@Test
public void testLoopStringConcatenation03ByPlus() {
    long startTime = System.currentTimeMillis();
    int count = 10000;
    String str = "testLoopStringConcatenation03ByPlus:";
    for (int i = 0; i < count; i++) {
        str = str + "-" + i;
    }
    System.out.println(str);
    long endTime = System.currentTimeMillis();
    System.out.println("testLoopStringConcatenation03ByPlus，拼接字符串" + count + "次，花费" + (endTime - startTime) + "秒");
}

/**
 * 测试循环使用StringBuilder拼接字符串耗时
 */
@Test
public void testLoopStringConcatenation04ByStringBuilder() {
    long startTime = System.currentTimeMillis();
    int count = 100000;
    StringBuilder stringBuilder = new StringBuilder("testLoopStringConcatenation04ByStringBuilder:");
    for (int i = 0; i < count; i++) {
        stringBuilder.append("-");
        stringBuilder.append(i);
    }
    String str = stringBuilder.toString();
    System.out.println(str);
    long endTime = System.currentTimeMillis();
    System.out.println("testLoopStringConcatenation04ByStringBuilder，拼接字符串" + count + "次，花费" + (endTime - startTime) + "秒");
}

The results are dramatically different: the '+' loop takes about 463 seconds, while the StringBuilder loop finishes in roughly 13 seconds, demonstrating that repeated '+' concatenation creates many temporary StringBuilder objects and is far slower.

Conclusion :

For simple, one‑off concatenations, using '+' is concise and performs similarly to explicit StringBuilder because of compiler optimization.

For concatenations inside loops or large iterative builds, prefer a manually managed StringBuilder to avoid excessive object creation and achieve significantly better performance.