String.join

小鱼儿 2023-07-19 12:29 1阅读 0赞

> [https://docs.oracle.com/javase/8/docs/api/][https_docs.oracle.com_javase_8_docs_api] 
> 
> [http://www.matools.com/api/java8][http_www.matools.com_api_java8]

--------------------

# 1 使用场景  #

> String集合快速转化为指定分隔符分割的字符串；最后一个元素后面无分隔符；

## 1.1 java8前  ##

@Test
    public void test() {
        //分隔符
        final String SEPARATOR = ",";
        List<String> cities = Arrays.asList("Milan", "London", "New York", "San Francisco");
        StringBuilder csvBuilder = new StringBuilder();
        //拼接 除最后一个字符
        for (int i = 0; i < cities.size() - 1; i++) {
            csvBuilder.append(cities.get(i)).append(SEPARATOR);
        }
        //拼接最后一个
        csvBuilder.append(cities.get(cities.size() - 1));
        //Milan,London,New York,San Francisco
        System.out.println(csvBuilder.toString());
    }

@Test
    public void test() {
        //分隔符
        final String SEPARATOR = ",";
        List<String> cities = Arrays.asList("Milan", "London", "New York", "San Francisco");
        StringBuilder csvBuilder = new StringBuilder();
        for (String city : cities) {
            csvBuilder.append(city).append(SEPARATOR);
        }
        String csv = csvBuilder.toString();
        //Milan,London,New York,San Francisco,
        System.out.println(csv);
        //字符串最后也有分隔符 需要删除
        csv = csv.substring(0, csv.length() - SEPARATOR.length());
        //Milan,London,New York,San Francisco
        System.out.println(csv);
    }

## 1.2 java8后 ##

> String.join 可以简化代码

@Test
    public void test() {
        //分隔符
        final String SEPARATOR = ",";
        List<String> cities = Arrays.asList("Milan", "London", "New York", "San Francisco");
        String joinStr = String.join(SEPARATOR, cities);
        //Milan,London,New York,San Francisco
        System.out.println(joinStr);
    }

> 采用java8的Stream api实现前面效果 
> 
>  *  [https://yuanyu.blog.csdn.net/article/details/105026457][https_yuanyu.blog.csdn.net_article_details_105026457]

@Test
    public void test() {
        //分隔符
        final String SEPARATOR = ",";
        List<String> cities = Arrays.asList("Milan", "London", "New York", "San Francisco");
        String joinStr = cities.stream().collect(Collectors.joining(SEPARATOR));
        //Milan,London,New York,San Francisco
        System.out.println(joinStr);
    }

--------------------

# 2 官方文档 #

> [https://docs.oracle.com/javase/8/docs/api/java/lang/String.html][https_docs.oracle.com_javase_8_docs_api_java_lang_String.html]

<table> 
 <tbody> 
  <tr> 
   <td style="width:148px;"><code>static&nbsp;<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/String.html" rel="nofollow">String</a></code></td> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#join-java.lang.CharSequence-java.lang.CharSequence...-" rel="nofollow">join</a>(<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html" rel="nofollow">CharSequence</a>&nbsp;delimiter,&nbsp;<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html" rel="nofollow">CharSequence</a>...&nbsp;elements)</code> <p>Returns a new String composed of copies of the&nbsp;<code>CharSequence elements</code>&nbsp;joined together with a copy of the specified&nbsp;<code>delimiter</code>.</p> </td> 
  </tr> 
  <tr> 
   <td style="width:148px;"><code>static&nbsp;<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/String.html" rel="nofollow">String</a></code></td> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#join-java.lang.CharSequence-java.lang.Iterable-" rel="nofollow">join</a>(<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html" rel="nofollow">CharSequence</a>&nbsp;delimiter,&nbsp;<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Iterable.html" rel="nofollow">Iterable</a>&lt;? extends&nbsp;<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html" rel="nofollow">CharSequence</a>&gt;&nbsp;elements)</code> <p>Returns a new&nbsp;<code>String</code>&nbsp;composed of copies of the&nbsp;<code>CharSequence elements</code>&nbsp;joined together with a copy of the specified&nbsp;<code>delimiter</code>.</p> </td> 
  </tr> 
 </tbody> 
</table>

//java.lang.String#join(java.lang.CharSequence, java.lang.CharSequence...)
    public static String join(CharSequence delimiter, CharSequence... elements) {
    
    //java.lang.String#join(java.lang.CharSequence, java.lang.Iterable<? extends java.lang.CharSequence>)
    public static String join(CharSequence delimiter,
    		Iterable<? extends CharSequence> elements) {

@Test
    public void testJoin(){
        String message = String.join("-", "Java", "is", "cool");
        // message returned is: "Java-is-cool"
        System.out.println(message);
    }

@Test
    public void testJoin() {
        List<String> strings = new LinkedList<>();
        strings.add("Java");
        strings.add("is");
        strings.add("cool");
        String message = String.join(" ", strings);
        //message returned is: "Java is cool"
        System.out.println(message);
    
        String strMessage = strings.stream().collect(Collectors.joining(" "));
        //Java is cool
        System.out.println(strMessage);
    }

@Test
    public void testJoin(){
        Set<String> strings = new LinkedHashSet<>();
        strings.add("Java");
        strings.add("is");
        strings.add("very");
        strings.add("cool");
        String message = String.join("-", strings);
        //message returned is: "Java-is-very-cool"
        System.out.println(message);
    }

--------------------

# 3 CharSequence接口 #

> String.join传入的参数或者迭代器需要实现CharSequence接口；当前们传入的参数未实现CharSequence接口或者集合的迭代器未实现CharSequence接口程序是会报错的 Xxx cannot be cast to java.lang.CharSequence

//java.lang.String#join(java.lang.CharSequence, java.lang.CharSequence...)
    public static String join(CharSequence delimiter, CharSequence... elements) {
    //java.lang.String#join(java.lang.CharSequence, java.lang.Iterable<? extends java.lang.CharSequence>)
    public static String join(CharSequence delimiter,
    		Iterable<? extends CharSequence> elements) {

//public final class Integer extends Number implements Comparable<Integer> {
    @Test
    public void test() {
        //分隔符
        final String SEPARATOR = ",";
        List<Integer> cities = Arrays.asList(1,2,3,4,5,6);
        //需要强转不然编译器就报错
        //java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.lang.CharSequence
        String joinStr = String.join(SEPARATOR, (CharSequence) cities);
    }

> CharSequence是一个描述字符串结构的接口，在这个接口里面一般有三种常用的子类

![20200331111438578.png][]

// Stirng类
    public final class String
        implements java.io.Serializable, Comparable<String>, CharSequence {
    
    // StringBuffer类
    public final class StringBuffer
       extends AbstractStringBuilder
       implements java.io.Serializable, CharSequence
    
    // StirngBuilder类
    public final class StringBuilder
        extends AbstractStringBuilder
        implements java.io.Serializable, CharSequence

> [https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html][https_docs.oracle.com_javase_8_docs_api_java_lang_CharSequence.html]

<table> 
 <tbody> 
  <tr> 
   <th>Modifier and Type</th> 
   <th>Method and Description</th> 
  </tr> 
  <tr> 
   <td><code>char</code></td> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html#charAt-int-" rel="nofollow">charAt</a>(int&nbsp;index)</code> <p>Returns the&nbsp;<code>char</code>&nbsp;value at the specified index.</p> </td> 
  </tr> 
  <tr> 
   <td><code>default&nbsp;<a href="https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html" rel="nofollow">IntStream</a></code></td> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html#chars--" rel="nofollow">chars</a>()</code> <p>Returns a stream of&nbsp;<code>int</code>&nbsp;zero-extending the&nbsp;<code>char</code>&nbsp;values from this sequence.</p> </td> 
  </tr> 
  <tr> 
   <td><code>default&nbsp;<a href="https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html" rel="nofollow">IntStream</a></code></td> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html#codePoints--" rel="nofollow">codePoints</a>()</code> <p>Returns a stream of code point values from this sequence.</p> </td> 
  </tr> 
  <tr> 
   <td><code>int</code></td> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html#length--" rel="nofollow">length</a>()</code> <p>Returns the length of this character sequence.</p> </td> 
  </tr> 
  <tr> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html" rel="nofollow">CharSequence</a></code></td> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html#subSequence-int-int-" rel="nofollow">subSequence</a>(int&nbsp;start, int&nbsp;end)</code> <p>Returns a&nbsp;<code>CharSequence</code>&nbsp;that is a subsequence of this sequence.</p> </td> 
  </tr> 
  <tr> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/String.html" rel="nofollow">String</a></code></td> 
   <td><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html#toString--" rel="nofollow">toString</a>()</code> <p>Returns a string containing the characters in this sequence in the same order as this sequence.</p> </td> 
  </tr> 
 </tbody> 
</table>

package java.lang;
    public interface CharSequence {
        //
        int length();
        //
        char charAt(int index);
        //
        CharSequence subSequence(int start, int end);
        //
        public String toString();
        //
        public default IntStream chars() {...}
        public default IntStream codePoints() {...}
    }

@Test
    public void test() {
        CharSequence str = "hello world";
        //11
        System.out.println(str.length());
        //h
        System.out.println(str.charAt(0));
        //world
        System.out.println(str.subSequence(6, 11));
        //hello world
        System.out.println(str.toString());
        //104 101 108 108 111 32 119 111 114 108 100 
        str.chars().forEach((item) -> {
            System.out.print(item + " ");
        });
        System.out.println();
        //104 101 108 108 111 32 119 111 114 108 100 
        str.codePoints().forEach((item) -> {
            System.out.print(item + " ");
        });
    }

[https_docs.oracle.com_javase_8_docs_api]: https://docs.oracle.com/javase/8/docs/api/
[http_www.matools.com_api_java8]: http://www.matools.com/api/java8
[https_yuanyu.blog.csdn.net_article_details_105026457]: https://yuanyu.blog.csdn.net/article/details/105026457
[https_docs.oracle.com_javase_8_docs_api_java_lang_String.html]: https://docs.oracle.com/javase/8/docs/api/java/lang/String.html
[20200331111438578.png]: /images/20230528/05c7f9a2d2a7445d9fd5958bc6387e79.png
[https_docs.oracle.com_javase_8_docs_api_java_lang_CharSequence.html]: https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html