JDK 1.8(Java 8)里新增String.join()方法用於字符串連接。本文基於《Java實現String.join()和效率比較》一文，分析和比較四種自定義實現與String.join()方法的效率，並糾正原文的一些錯誤。    

    代碼示例如下：

public class Test {
    public static void main(String[] args) {
        String[] strOri = {"a","b","c","d","e","f","g","h"}; //同new string[]{"a","b","c","d","e","f","g","h"}
        String strRes = ""; int loops = 100000;

        Date date = new Date();

        for(int i = 0; i < loops; i++) {strRes = join1(strOri, ":");} date = recordTime(date, 1);    //1
        for(int i = 0; i < loops; i++) {strRes = join2(strOri, ":");} date = recordTime(date, 2);
        for(int i = 0; i < loops; i++) {strRes = join3(strOri, ":");} date = recordTime(date, 3);
        for(int i = 0; i < loops; i++) {strRes = join4(strOri, ":");} date = recordTime(date, 4);
        for(int i = 0; i < loops; i++) {strRes = join5(strOri, ":");} date = recordTime(date, 5);

        long startTime = System.currentTimeMillis();    //2
        for(int i = 0; i < loops; i++) {strRes = join5(strOri, ":");}
        long endTime = System.currentTimeMillis(); System.out.println("5c:{" + strRes + "} costs " + (endTime-startTime) + "ms");

        startTime = System.nanoTime();
        for(int i = 0; i < loops; i++) {strRes = join5(strOri, ":");}
        endTime = System.nanoTime(); System.out.println("5n:{" + strRes + "} costs " + (endTime-startTime) + "ns");
    }

    private static void recordTime_Wrong(Date date, int no) {
        System.out.println(no + ": costs " + (new Date().getTime()-date.getTime()) + "ms");
        date = new Date();
    }
    private static Date recordTime(Date date, int no) {
        System.out.println(no + ": costs " + (new Date().getTime()-date.getTime()) + "ms");
        return new Date();
    }

    private static String join1(String[] strOri, String delimiter) {
        StringBuffer sb = new StringBuffer();   //3
        for(String s : strOri) {
            sb.append(s+delimiter);   //4
        }
        return sb.toString().substring(0, sb.toString().length()-1);
    }

    private static String join2(String[] strOri, String delimiter) {
        StringBuffer sb = new StringBuffer();
        for(String s : strOri) {
            sb.append(s+delimiter);
        }
        String s = sb.toString();
        return s.substring(0, s.length()-1);
    }

    private static String join3(String[] strOri, String delimiter) {
        StringBuffer sb = new StringBuffer();
        for(int i = 0; i < strOri.length; i++) {
            if (i != strOri.length-1) {
                sb.append(strOri[i]+delimiter);
            } else {
                sb.append(strOri[i]);
            }
        }
        return sb.toString();
    }

    private static String join4(String[] strOri, String delimiter) {
        StringBuilder stringBuilder = new StringBuilder();
        for (int i = 0; i < strOri.length-1; i++) {
            stringBuilder.append(strOri[i]).append(delimiter);
        }
        stringBuilder.append(strOri[strOri.length-1]);
        return stringBuilder.toString();
    }

    private static String join5(String[] strOri, String delimiter) {
        return String.join(delimiter, strOri); //5
    }
}

 

選取三次運行輸出結果如下：

1: costs 930ms
2: costs 902ms
3: costs 637ms
4: costs 230ms
5: costs 364ms
5c:{a:b:c:d:e:f:g:h} costs 413ms
5n:{a:b:c:d:e:f:g:h} costs 286466296ns

 

1: costs 834ms
2: costs 788ms
3: costs 576ms
4: costs 248ms
5: costs 350ms
5c:{a:b:c:d:e:f:g:h} costs 384ms
5n:{a:b:c:d:e:f:g:h} costs 283256112ns

1: costs 774ms
2: costs 728ms
3: costs 605ms
4: costs 297ms
5: costs 417ms
5c:{a:b:c:d:e:f:g:h} costs 280ms
5n:{a:b:c:d:e:f:g:h} costs 279838638ns

 可見，join4()執行最快，其次是join5()。join1()和join2()執行效率接近，前者調用兩次toString()，故效率略低。

    總結如下：
    1. 原文recordTime(即本文recordTime_Wrong)方法中，無法通過"date = new Date()"修改外部的date引用(根因詳見《java中的傳值與傳引用》)。這會導致每次調用recordTime()時，起始時間始終是"Date date = new Date()"獲得的對象(表現為join*耗時遞增)。
    2. 查看Java源碼可知，new Date()其實就是調用System.currentTimeMillis()：

 

1 public Date() {
2     this(System.currentTimeMillis()); //相當於Date(System.currentTimeMillis())
3 }

    可以使用new Date().getTime()獲取當前時間戳(毫秒)。注意，該毫秒數一般以1970-01-01 00:00:00為參考點，但東八區要加上時區，即以1970-01-01 08:00:00為參考時間。此外，通過getTime()獲取毫秒數效率不如System.currentTimeMillis()，后者返回自1970年1月1日0時起的毫秒數。

    System.nanoTime()的計時精度不保證一定高於System.currentTimeMillis()，但可保證數值遞增(后者相減時可能產生負值)。

    若要對代碼進行更准確的計時，可參考《How do I write a correct micro-benchmark in Java?》一文。
    3. StringBuffer對象是線程安全的，其方法都是同步的(synchronized)。臨時變量應使用StringBuilder(效率更高)，避用StringBuffer。
    4. 在循環內部，不要使用append(a+b)的形式，而應改為append(a).append(b)。
    5. String.join()內部使用StringBuilder實現，因此join5()性能接近join4()。當然，String.join()的功能比join4()更多。