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

Developers often see IDE warnings suggesting the use of the '+' operator for string concatenation, prompting a comparison between '+' and StringBuilder in Java.

From JDK 5 onward, the compiler automatically transforms '+' concatenations into StringBuilder calls. To verify performance, a JUnit test concatenates two strings 100,000 times using both approaches.

/**
 * 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 benchmark results were 33 seconds for the '+' version and 36 seconds for the StringBuilder version, indicating negligible difference for single concatenations.

When concatenating inside a loop, the performance gap widens dramatically. Two additional JUnit tests concatenate a long string 10,000 times using both methods.

/**
 * 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 = 10000;
    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 loop test showed 463 seconds for the '+' version versus only 13 seconds for the StringBuilder version, a stark performance difference.

**Conclusion:** For simple, non‑loop concatenations, using the '+' operator yields concise code with comparable performance, while for repeated or loop concatenations, StringBuilder is significantly more efficient and should be preferred.