[JAVA] ArrayList to String 변환

1. ArrayList.toString()

ArrayList<String> testArrayList = new ArrayList<>();
testArrayList.add("Json");
testArrayList.add("Test");
testArrayList.add("ArrayListToString");
String strToString = testArrayList.toString();
System.out.println("convertArrayListToString: strToString = " + strToString);

결과 : 변환 실패

결과
convertArrayListToString: strToString = [Json, Test, ArrayListToString]

 

2. StringBuilder append()

ArrayList<String> testArrayList = new ArrayList<>();
testArrayList.add("Json");
testArrayList.add("Test");
testArrayList.add("ArrayListToString");

StringBuilder stringBuilder = new StringBuilder();
for (String item : testArrayList) {
    stringBuilder.append(item);
}
String strAppend = stringBuilder.toString();
System.out.println("convertArrayListToString: strAppend = " + strAppend);

결과 : 성공

결과
convertArrayListToString: strAppend = JsonTestArrayListToString

 

3. String.join()

ArrayList<String> testArrayList = new ArrayList<>();
testArrayList.add("Json");
testArrayList.add("Test");
testArrayList.add("ArrayListToString");
String strJoin = String.join("", testArrayList);
System.out.println("convertArrayListToString: strJoin = " + strJoin);

결과 : 성공

결과
convertArrayListToString: strJoin = JsonTestArrayListToString

 

전체 소스

import java.text.SimpleDateFormat;
import java.util.ArrayList;

class ArrayListToString {

    public static void main(String[] args) {

        ArrayList<String> testArrayList = new ArrayList<>();
        testArrayList.add("Json");
        testArrayList.add("Test");
        testArrayList.add("ArrayListToString");

        convertArrayListToString(testArrayList);
        convertArrayListToStringAppend(testArrayList);
        convertArrayListToStringJoin(testArrayList);
    }

    static void convertArrayListToString(ArrayList<String> strings) {
        String strToString = strings.toString();
        System.out.println("convertArrayListToString: strToString = " + strToString);
    }

    static void convertArrayListToStringAppend(ArrayList<String> strings) {
        StringBuilder stringBuilder = new StringBuilder();
        for (String item : strings) {
            stringBuilder.append(item);
        }
        String strAppend = stringBuilder.toString();
        System.out.println("convertArrayListToString: strAppend = " + strAppend);
    }
    static void convertArrayListToStringJoin(ArrayList<String> strings) {
        String strJoin = String.join("", strings);
        System.out.println("convertArrayListToString: strJoin = " + strJoin);
    }
}

결과
convertArrayListToString: strToString = [Json, Test, ArrayListToString]
convertArrayListToString: strAppend = JsonTestArrayListToString
convertArrayListToString: strJoin = JsonTestArrayListToString

 

성능 테스트 

1. StringBuilder append()

   - 100000 반복 : 45ms

   - 10000000 반복 : 1423ms

2. String.join()

   - 100000 반복 : 47ms

   - 10000000 반복 : 1415ms

* H/W 성능에 따라 결과를 다를 수 있지만 속도 = append() >= join() 결과는 변하지 않을 것으로 예상.

 

성능 결론

String.join() 코드는 아래와 같음.

결국 append() 코드와 동일한 코드임.

public static String join(CharSequence delimiter, Iterable<? extends CharSequence> elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        StringJoiner joiner = new StringJoiner(delimiter);
        for (CharSequence cs: elements) {
            joiner.add(cs);
        }
        return joiner.toString();
    }