public class Test {
    List<String> list = new ArrayList<>();


    @Before
    public void init(){
        IntStream.range(0, 100000).forEach((index) -> {
            list.add("str" + index);
        });
    }

    @org.junit.Test
    public void test1() {
        String ss = "";
        long startTime = System.currentTimeMillis();
        for (String s : list) {
            ss += s;
        }
        System.out.println(System.currentTimeMillis() - startTime);
    }

    @org.junit.Test
    public void test2() {
        String ss = "";
        long startTime = System.currentTimeMillis();
        for (String s : list) {
            ss=ss.concat(s);
        }
        System.out.println(System.currentTimeMillis() - startTime);
    }

    @org.junit.Test
    public void test3() {
        StringBuilder ss = new StringBuilder();
        long startTime = System.currentTimeMillis();
        for (String s : list) {
            ss.append(s);
        }
        System.out.println(System.currentTimeMillis() - startTime);
    }

    @org.junit.Test
    public void test4() {
        long startTime = System.currentTimeMillis();
        StringUtils.join(list);
        System.out.println(System.currentTimeMillis() - startTime);
    }

    @org.junit.Test
    public void test5() {
        StringBuffer ss = new StringBuffer();
        long startTime = System.currentTimeMillis();
        for (String s : list) {
            ss.append(s);
        }
        System.out.println(System.currentTimeMillis() - startTime);
    }

}

First: 33809

Second: 8851

Third: 6

Type 4: 12

Type 5: 7

Performance: StringBuilder > StringBuffer > stringutils. Join > concat >+
Then from the source level analysis

StringBuilder:

image.png

Each string concatenation is just to extend the internal char array and only produce one final string, so this is the most efficient

StringBuffer:

image.png

Compared with StringBuilder, it only adds an additional synchronized, so it is not much different in the case of single thread

StringUtils.join:

image.png

image.png

It can be seen that the inner part is still implemented by StringBuilder, but there are more separators in each cycle, so it's a little slower, but it's not much, and the time is an order of magnitude

concat:

image.png

It can be seen that each connection will generate a string, so the efficiency is very low

+:

Because it is an overloaded operator, the source code cannot be found, but the result shows that the efficiency is the lowest