Java8使用StringJoiner看這一篇就夠了

????點(diǎn)擊上方藍(lán)色字體，選擇“標(biāo)星公眾號(hào)”

優(yōu)質(zhì)文章，第一時(shí)間送達(dá)

66套java從入門到精通實(shí)戰(zhàn)課程分享

在開發(fā)過程中，有時(shí)候需要打印集合中的對(duì)象的某個(gè)屬性，為了格式化以前常用StrinigBuffer。

比如下面這種代碼：

Set?catSet?=?new?HashSet<>(2);
?
????@Before
????public?void?init()?{
????????Cat?cat?=?new?Cat();
????????cat.setName("老大");
????????cat.setAge(1);
????????catSet.add(cat);
????????Cat?cat2?=?new?Cat();
????????cat2.setName("老二");
????????cat2.setAge(2);
????????catSet.add(cat2);
?
????}
?
????@Test
????public?void?print1()?{
?
????????//?給出友好提示
????????StringBuilder?stringBuilder?=?new?StringBuilder();
????????stringBuilder.append("[");
????????Iterator?iterator?=?catSet.iterator();
????????int?seq?=?0;
????????while?(iterator.hasNext())?{
????????????seq++;
????????????stringBuilder.append(seq);
????????????stringBuilder.append(":");
????????????stringBuilder.append(iterator.next().getName());
????????????if?(seq?!=?catSet.size())?{
????????????????stringBuilder.append(",");
????????????}
????????}
????????stringBuilder.append("]");
????????Assert.assertEquals("[1:老二,2:老大]",?stringBuilder.toString());
?
????}

最近接觸到Java8提供了StringJoiner，看到一篇不錯(cuò)的文章，翻譯在此，文后給出改進(jìn)方案，一行代碼解決問題！。

--------------------------------------------分割線-------------------------------------------------------------

添加元素

@Test
public?void?whenAddingElements_thenJoinedElements()?{
????StringJoiner?joiner?=?new?StringJoiner(",",?PREFIX,?SUFFIX);
????joiner.add("Red")
??????.add("Green")
??????.add("Blue");
?
????assertEquals(joiner.toString(),?"[Red,Green,Blue]");
}

如果想講List添加到其中：

@Test
public?void?whenAddingListElements_thenJoinedListElements()?{
????List?rgbList?=?new?ArrayList<>();
????rgbList.add("Red");
????rgbList.add("Green");
????rgbList.add("Blue");
?
????StringJoiner?rgbJoiner?=?new?StringJoiner(
??????",",?PREFIX,?SUFFIX);
?
????for?(String?color?:?rgbList)?{
????????rgbJoiner.add(color);
????}
?
????assertEquals(rgbJoiner.toString(),?"[Red,Green,Blue]");
}

構(gòu)造

private?String?PREFIX?=?"[";
private?String?SUFFIX?=?"]";
?
@Test
public?void?whenEmptyJoinerWithoutPrefixSuffix_thenEmptyString()?{
????StringJoiner?joiner?=?new?StringJoiner(",");
??
????assertEquals(0,?joiner.toString().length());
}
?
@Test
public?void?whenEmptyJoinerJoinerWithPrefixSuffix_thenPrefixSuffix()?{
????StringJoiner?joiner?=?new?StringJoiner(
??????",",?PREFIX,?SUFFIX);
??
????assertEquals(joiner.toString(),?PREFIX?+?SUFFIX);
}

?toString()?方法得到當(dāng)前連接對(duì)象的字符串形式。

沒有指定前綴和后綴的Joiner對(duì)象將返回空字符串。

@Test
public?void?whenEmptyJoinerWithEmptyValue_thenDefaultValue()?{
????StringJoiner?joiner?=?new?StringJoiner(",");
????joiner.setEmptyValue("default");
?
????assertEquals(joiner.toString(),?"default");
}
?
@Test
public?void?whenEmptyJoinerWithPrefixSuffixAndEmptyValue_thenDefaultValue()?{
????StringJoiner?joiner?=?new?StringJoiner(",",?PREFIX,?SUFFIX);
????joiner.setEmptyValue("default");
?
????assertEquals(joiner.toString(),?"default");
}

可以調(diào)用joiner.setEmptyValue()函數(shù)，設(shè)置默認(rèn)值。

Joiners組合使用

private?String?PREFIX?=?"[";
private?String?SUFFIX?=?"]";
?
@Test
public?void?whenMergingJoiners_thenReturnMerged()?{
????StringJoiner?rgbJoiner?=?new?StringJoiner(
??????",",?PREFIX,?SUFFIX);
????StringJoiner?cmybJoiner?=?new?StringJoiner(
??????"-",?PREFIX,?SUFFIX);
?
????rgbJoiner.add("Red")
??????.add("Green")
??????.add("Blue");
????cmybJoiner.add("Cyan")
??????.add("Magenta")
??????.add("Yellow")
??????.add("Black");
?
????rgbJoiner.merge(cmybJoiner);
?
????assertEquals(
??????rgbJoiner.toString(),?
??????"[Red,Green,Blue,Cyan-Magenta-Yellow-Black]");
}

兩個(gè)joiner可以通過merge函數(shù)，組合在一起。

利用Stream API

@Test
public?void?whenUsedWithinCollectors_thenJoined()?{
????List?rgbList?=?Arrays.asList("Red",?"Green",?"Blue");
????String?commaSeparatedRGB?=?rgbList.stream()
??????.map(color?->?color.toString())
??????.collect(Collectors.joining(","));
?
????assertEquals(commaSeparatedRGB,?"Red,Green,Blue");
}

英文原文：https://www.baeldung.com/java-string-joiner

--------------------------------------------分割線-------------------------------------------------------------

改進(jìn)

根據(jù)本文的學(xué)習(xí)對(duì)我們的代碼進(jìn)行改進(jìn)：

第一版：

?@Test
????public?void?print()?{
????????StringJoiner?stringJoiner?=?new?StringJoiner(",",?"[",?"]");
????????Iterator?iterator?=?catSet.iterator();
????????int?seq?=?0;
????????while?(iterator.hasNext())?{
????????????seq++;
????????????stringJoiner.add(seq?+?":"?+?iterator.next().getName());
????????}
????????Assert.assertEquals("[1:老二,2:老大]",?stringJoiner.toString());
????}

第二版：

???@Test
????public?void?print2()?{
List?catList?=?new?ArrayList<>(catSet);
????????String?result?=?IntStream.range(0,catList.size()).mapToObj(i->i+":"+?catList.get(i).getName()).collect(Collectors.joining(",",?"[",?"]"));
????????Assert.assertEquals("[1:老二,2:老大]",?result);
????}

一行代碼，很簡(jiǎn)單，很優(yōu)雅！！！

版權(quán)聲明：本文為博主原創(chuàng)文章，遵循 CC 4.0 BY-SA 版權(quán)協(xié)議，轉(zhuǎn)載請(qǐng)附上原文出處鏈接和本聲明。

本文鏈接：

https://blog.csdn.net/w605283073/article/details/89527280

粉絲福利：實(shí)戰(zhàn)springboot+CAS單點(diǎn)登錄系統(tǒng)視頻教程免費(fèi)領(lǐng)取

???

?長按上方微信二維碼?2 秒
即可獲取資料

感謝點(diǎn)贊支持下哈?