Performance Comparison of Java String Concatenation: '+' Operator vs StringBuilder

When concatenating strings in Java, developers often choose StringBuilder for perceived performance benefits, but the IDE may suggest using the '+' operator. This article investigates the actual performance differences between the two approaches.

1. Simple Concatenation

For a few string literals, using '+' is common. Since JDK 5, the Java compiler automatically transforms '+' concatenations into StringBuilder bytecode. The author created a StringTest class with two methods—one using '+' and the other using StringBuilder —and executed JUnit tests that performed 100,000 concatenations.

/**
 * 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();
}

/**
 * 测试使用+拼接字符串耗时
 */
@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");
}

/**
 * 测试使用StringBuilder拼接字符串耗时
 */
@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");
}

Running the tests produced the following timings:

testStringConcatenation01ByPlus，拼接字符串100000次，花费33秒
testStringConcatenation02ByStringBuilder，拼接字符串100000次，花费36秒

The difference is minimal; both approaches compile to identical bytecode, as confirmed by decompiling StringTest.class with javap -c . Therefore, for simple concatenations, using '+' is shorter and equally fast.

2. Loop Concatenation

When concatenating inside a loop to build a long string, the '+' operator still compiles to StringBuilder , but each iteration creates a new StringBuilder instance, leading to significant overhead. Directly using a single StringBuilder avoids this cost.

/**
 * 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 loop test results were:

testLoopStringConcatenation03ByPlus，拼接字符串10000次，花费463秒
testLoopStringConcatenation04ByStringBuilder，拼接字符串10000次，花费13秒

The performance gap is dramatic, confirming that StringBuilder is the proper choice for repeated concatenations.

Conclusion

For single or few concatenations, using the '+' operator is concise and equally fast.

For concatenations inside loops or when building large strings, prefer StringBuilder to avoid excessive object creation and achieve much better performance.