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

Introduction: IDE may warn about using StringBuilder, StringJoiner, or StringBuffer and suggest replacing them with a single java.lang.String concatenation; this article investigates the performance differences between using the '+' operator and StringBuilder for string concatenation in Java.

Simple concatenation test: a test class StringTest defines methods concatenationStringByPlus and concatenationStringByStringBuilder, each called 100,000 times in JUnit tests. The measured times are 33 seconds for '+' and 36 seconds for StringBuilder, showing negligible difference. Decompiling the class file reveals identical bytecode, confirming equal performance.

/**
     * 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");
    }

Loop concatenation test: another set of JUnit tests concatenates a string 10,000 times inside a loop, once using '+' and once using StringBuilder. The '+' version takes 463 seconds, while the StringBuilder version takes 13 seconds, demonstrating a dramatic performance gap.

/**
     * 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");
    }

Output of the loop tests confirms the large time difference.

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

Conclusion: For single, non-loop concatenations, using '+' is simpler and as fast as StringBuilder; however, in loops, '+' creates a new StringBuilder each iteration, leading to high overhead, so StringBuilder should be used for iterative concatenation.