5種字符串拼接方式的效率比拼，你用對(duì)了嗎？

字符串拼接一般使用“+”，但是“+”不能滿足大批量數(shù)據(jù)的處理，Java中有以下五種方法處理字符串拼接，各有優(yōu)缺點(diǎn)，程序開(kāi)發(fā)應(yīng)選擇合適的方法實(shí)現(xiàn)。

1. 加號(hào) “+”
2. String contact() 方法
3. StringUtils.join() 方法
4. StringBuffer append() 方法
5. StringBuilder append() 方法

經(jīng)過(guò)簡(jiǎn)單的程序測(cè)試，從執(zhí)行100次到90萬(wàn)次的時(shí)間開(kāi)銷如下表：

由此可以看出：

1. 方法1 加號(hào) “+” 拼接 和 方法2 String contact() 方法 適用于小數(shù)據(jù)量的操作，代碼簡(jiǎn)潔方便，加號(hào)“+” 更符合我們的編碼和閱讀習(xí)慣；

2. 方法3 StringUtils.join() 方法 適用于將ArrayList轉(zhuǎn)換成字符串，就算90萬(wàn)條數(shù)據(jù)也只需68ms，可以省掉循環(huán)讀取ArrayList的代碼；

3. 方法4 StringBuffer append() 方法 和 方法5 StringBuilder append() 方法 其實(shí)他們的本質(zhì)是一樣的，都是繼承自AbstractStringBuilder，效率最高，大批量的數(shù)據(jù)處理最好選擇這兩種方法。

4. 方法1 加號(hào) “+” 拼接 和 方法2 String contact() 方法 的時(shí)間和空間成本都很高（分析在本文末尾），不能用來(lái)做批量數(shù)據(jù)的處理。

源代碼，供參考

package?cnblogs.twzheng.lab2;

/**
?*?@author?Tan?Wenzheng
?*
?*/
import?java.util.ArrayList;
import?java.util.List;

import?org.apache.commons.lang3.StringUtils;

public?class?TestString?{

????private?static?final?int?max?=?100;

????public?void?testPlus()?{
????????System.out.println(">>>?testPlus()?<<<");

????????String?str?=?"";

????????long?start?=?System.currentTimeMillis();

????????for?(int?i?=?0;?i?????????????str?=?str?+?"a";
????????}

????????long?end?=?System.currentTimeMillis();

????????long?cost?=?end?-?start;

????????System.out.println("???{str?+?\"a\"}?cost="?+?cost?+?"?ms");
????}

????public?void?testConcat()?{
????????System.out.println(">>>?testConcat()?<<<");

????????String?str?=?"";

????????long?start?=?System.currentTimeMillis();

????????for?(int?i?=?0;?i?????????????str?=?str.concat("a");
????????}

????????long?end?=?System.currentTimeMillis();

????????long?cost?=?end?-?start;

????????System.out.println("???{str.concat(\"a\")}?cost="?+?cost?+?"?ms");
????}

????public?void?testJoin()?{
????????System.out.println(">>>?testJoin()?<<<");

????????long?start?=?System.currentTimeMillis();

????????List?list?=?new?ArrayList();

????????for?(int?i?=?0;?i?????????????list.add("a");
????????}

????????long?end1?=?System.currentTimeMillis();
????????long?cost1?=?end1?-?start;

????????StringUtils.join(list,?"");

????????long?end?=?System.currentTimeMillis();
????????long?cost?=?end?-?end1;

????????System.out.println("???{list.add(\"a\")}?cost1="?+?cost1?+?"?ms");
????????System.out.println("???{StringUtils.join(list,?\"\")}?cost="?+?cost
????????????????+?"?ms");
????}

????public?void?testStringBuffer()?{
????????System.out.println(">>>?testStringBuffer()?<<<");

????????long?start?=?System.currentTimeMillis();

????????StringBuffer?strBuffer?=?new?StringBuffer();

????????for?(int?i?=?0;?i?????????????strBuffer.append("a");
????????}
????????strBuffer.toString();

????????long?end?=?System.currentTimeMillis();

????????long?cost?=?end?-?start;

????????System.out.println("???{strBuffer.append(\"a\")}?cost="?+?cost?+?"?ms");
????}

????public?void?testStringBuilder()?{
????????System.out.println(">>>?testStringBuilder()?<<<");

????????long?start?=?System.currentTimeMillis();

????????StringBuilder?strBuilder?=?new?StringBuilder();

????????for?(int?i?=?0;?i?????????????strBuilder.append("a");
????????}
????????strBuilder.toString();

????????long?end?=?System.currentTimeMillis();

????????long?cost?=?end?-?start;

????????System.out
????????????????.println("???{strBuilder.append(\"a\")}?cost="?+?cost?+?"?ms");
????}
}

測(cè)試結(jié)果：

1. 執(zhí)行100次， private static final int max = 100;

>>>?testPlus()?<<<
???{str?+?"a"}?cost=0?ms
>>>?testConcat()?<<<
???{str.concat("a")}?cost=0?ms
>>>?testJoin()?<<<
???{list.add("a")}?cost1=0?ms
???{StringUtils.join(list,?"")}?cost=20?ms
>>>?testStringBuffer()?<<<
???{strBuffer.append("a")}?cost=0?ms
>>>?testStringBuilder()?<<<
???{strBuilder.append("a")}?cost=0?ms

2. 執(zhí)行1000次， private static final int max = 1000;

>>>?testPlus()?<<<
???{str?+?"a"}?cost=10?ms
>>>?testConcat()?<<<
???{str.concat("a")}?cost=0?ms
>>>?testJoin()?<<<
???{list.add("a")}?cost1=0?ms
???{StringUtils.join(list,?"")}?cost=20?ms
>>>?testStringBuffer()?<<<
???{strBuffer.append("a")}?cost=0?ms
>>>?testStringBuilder()?<<<
???{strBuilder.append("a")}?cost=0?ms

3. 執(zhí)行1萬(wàn)次， private static final int max = 10000;

>>>?testPlus()?<<<
???{str?+?"a"}?cost=150?ms
>>>?testConcat()?<<<
???{str.concat("a")}?cost=70?ms
>>>?testJoin()?<<<
???{list.add("a")}?cost1=0?ms
???{StringUtils.join(list,?"")}?cost=30?ms
>>>?testStringBuffer()?<<<
???{strBuffer.append("a")}?cost=0?ms
>>>?testStringBuilder()?<<<
???{strBuilder.append("a")}?cost=0?ms

4. 執(zhí)行10萬(wàn)次， private static final int max = 100000;

>>>?testPlus()?<<<
???{str?+?"a"}?cost=4198?ms
>>>?testConcat()?<<<
???{str.concat("a")}?cost=1862?ms
>>>?testJoin()?<<<
???{list.add("a")}?cost1=21?ms
???{StringUtils.join(list,?"")}?cost=49?ms
>>>?testStringBuffer()?<<<
???{strBuffer.append("a")}?cost=10?ms
>>>?testStringBuilder()?<<<
???{strBuilder.append("a")}?cost=10?ms

5. 執(zhí)行20萬(wàn)次， private static final int max = 200000;

>>>?testPlus()?<<<
???{str?+?"a"}?cost=17196?ms
>>>?testConcat()?<<<
???{str.concat("a")}?cost=7653?ms
>>>?testJoin()?<<<
???{list.add("a")}?cost1=20?ms
???{StringUtils.join(list,?"")}?cost=51?ms
>>>?testStringBuffer()?<<<
???{strBuffer.append("a")}?cost=20?ms
>>>?testStringBuilder()?<<<
???{strBuilder.append("a")}?cost=16?ms

6. 執(zhí)行50萬(wàn)次， private static final int max = 500000;

>>>?testPlus()?<<<
???{str?+?"a"}?cost=124693?ms
>>>?testConcat()?<<<
???{str.concat("a")}?cost=49439?ms
>>>?testJoin()?<<<
???{list.add("a")}?cost1=21?ms
???{StringUtils.join(list,?"")}?cost=50?ms
>>>?testStringBuffer()?<<<
???{strBuffer.append("a")}?cost=20?ms
>>>?testStringBuilder()?<<<
???{strBuilder.append("a")}?cost=10?ms

7. 執(zhí)行90萬(wàn)次， private static final int max = 900000;

>>>?testPlus()?<<<
???{str?+?"a"}?cost=456739?ms
>>>?testConcat()?<<<
???{str.concat("a")}?cost=186252?ms
>>>?testJoin()?<<<
???{list.add("a")}?cost1=20?ms
???{StringUtils.join(list,?"")}?cost=68?ms
>>>?testStringBuffer()?<<<
???{strBuffer.append("a")}?cost=30?ms
>>>?testStringBuilder()?<<<
???{strBuilder.append("a")}?cost=24?ms

查看源代碼，以及簡(jiǎn)單分析

String contact 和 StringBuffer，StringBuilder 的源代碼都可以在Java庫(kù)里找到，有空可以研究研究。

1.其實(shí)每次調(diào)用contact()方法就是一次數(shù)組的拷貝，雖然在內(nèi)存中是處理都是原子性操作，速度非常快，但是，最后的return語(yǔ)句會(huì)創(chuàng)建一個(gè)新String對(duì)象，限制了concat方法的速度。

public?String?concat(String?str)?{
????????int?otherLen?=?str.length();
????????if?(otherLen?==?0)?{
????????????return?this;
????????}
????????int?len?=?value.length;
????????char?buf[]?=?Arrays.copyOf(value,?len?+?otherLen);
????????str.getChars(buf,?len);
????????return?new?String(buf,?true);
????}

2.StringBuffer 和 StringBuilder 的append方法都繼承自AbstractStringBuilder，整個(gè)邏輯都只做字符數(shù)組的加長(zhǎng)，拷貝，到最后也不會(huì)創(chuàng)建新的String對(duì)象，所以速度很快，完成拼接處理后在程序中用strBuffer.toString()來(lái)得到最終的字符串。

/**
?????*?Appends?the?specified?string?to?this?character?sequence.
?????*?

?????*?The?characters?of?the?{@code?String}?argument?are?appended,?in
?????*?order,?increasing?the?length?of?this?sequence?by?the?length?of?the
?????*?argument.?If?{@code?str}?is?{@code?null},?then?the?four
?????*?characters?{@code?"null"}?are?appended.
?????*?

?????*?Let?n?be?the?length?of?this?character?sequence?just?prior?to
?????*?execution?of?the?{@code?append}?method.?Then?the?character?at
?????*?index?k?in?the?new?character?sequence?is?equal?to?the?character
?????*?at?index?k?in?the?old?character?sequence,?if?k?is?less
?????*?than?n;?otherwise,?it?is?equal?to?the?character?at?index
?????*?k-n?in?the?argument?{@code?str}.
?????*
?????*?@param???str???a?string.
?????*?@return??a?reference?to?this?object.
?????*/
????public?AbstractStringBuilder?append(String?str)?{
????????if?(str?==?null)?str?=?"null";
????????int?len?=?str.length();
????????ensureCapacityInternal(count?+?len);
????????str.getChars(0,?len,?value,?count);
????????count?+=?len;
????????return?this;
????}

/**
?????*?This?method?has?the?same?contract?as?ensureCapacity,?but?is
?????*?never?synchronized.
?????*/
????private?void?ensureCapacityInternal(int?minimumCapacity)?{
????????//?overflow-conscious?code
????????if?(minimumCapacity?-?value.length?>?0)
????????????expandCapacity(minimumCapacity);
????}

????/**
?????*?This?implements?the?expansion?semantics?of?ensureCapacity?with?no
?????*?size?check?or?synchronization.
?????*/
????void?expandCapacity(int?minimumCapacity)?{
????????int?newCapacity?=?value.length?*?2?+?2;
????????if?(newCapacity?-?minimumCapacity?0)
????????????newCapacity?=?minimumCapacity;
????????if?(newCapacity?0)?{
????????????if?(minimumCapacity?0)?//?overflow
????????????????throw?new?OutOfMemoryError();
????????????newCapacity?=?Integer.MAX_VALUE;
????????}
????????value?=?Arrays.copyOf(value,?newCapacity);
????}

3.字符串的加號(hào)“+” 方法， 雖然編譯器對(duì)其做了優(yōu)化，使用StringBuilder的append方法進(jìn)行追加，但是每循環(huán)一次都會(huì)創(chuàng)建一個(gè)StringBuilder對(duì)象，且都會(huì)調(diào)用toString方法轉(zhuǎn)換成字符串，所以開(kāi)銷很大。

注：執(zhí)行一次字符串“+”,相當(dāng)于 str = new StringBuilder(str).append("a").toString();

4.本文開(kāi)頭的地方統(tǒng)計(jì)了時(shí)間開(kāi)銷，根據(jù)上述分析再想想空間的開(kāi)銷。常說(shuō)拿空間換時(shí)間，反過(guò)來(lái)是不是拿時(shí)間換到了空間呢，但是在這里，其實(shí)時(shí)間是消耗在了重復(fù)的不必要的工作上（生成新的對(duì)象，toString方法），所以對(duì)大批量數(shù)據(jù)做處理時(shí)，加號(hào)“+” 和 contact 方法絕對(duì)不能用，時(shí)間和空間成本都很高。

來(lái)源：cnblogs.com/twzheng/p/5923642.html

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