String.join

https://docs.oracle.com/javase/8/docs/api/ 
http://www.matools.com/api/java8
1シーンの使用 
Stringコレクションは、指定された区切り文字分割の文字列に急速に変換されます.最後の要素の後ろに区切り記号がありません.
1.1 java 8前 

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

@Test
public void test() {
    //   
    final String SEPARATOR = ",";
    List 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);
    //               
    csv = csv.substring(0, csv.length() - SEPARATOR.length());
    //Milan,London,New York,San Francisco
    System.out.println(csv);
}

1.2 java 8後
String.joinはコードを簡略化できます

@Test
public void test() {
    //   
    final String SEPARATOR = ",";
    List 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);
}

Java 8のStream apiで前の効果を実現 

https://yuanyu.blog.csdn.net/article/details/105026457

@Test
public void test() {
    //   
    final String SEPARATOR = ",";
    List 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公式文書
https://docs.oracle.com/javase/8/docs/api/java/lang/String.html  static String join(CharSequence delimiter, CharSequence... elements) Returns a new String composed of copies of the  CharSequence elements  joined together with a copy of the specified  delimiter . static String join(CharSequence delimiter, Iterable extends CharSequence> elements) Returns a new  String  composed of copies of the  CharSequence elements  joined together with a copy of the specified  delimiter .

//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 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 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インタフェース
String.joinが入力したパラメータまたは反復器はCharSequenceインタフェースを実現する必要がある.現在入力されているパラメータがCharSequenceインタフェースを実装していないか、またはセットの反復器がCharSequenceインタフェースを実装していないプログラムは、エラーを報告します. 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 {
@Test
public void test() {
    //   
    final String SEPARATOR = ",";
    List cities = Arrays.asList(1,2,3,4,5,6);
    //            
    //java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.lang.CharSequence
    String joinStr = String.join(SEPARATOR, (CharSequence) cities);
}

CharSequenceは文字列構造を記述するインタフェースで、このインタフェースには一般的に3つの一般的なサブクラスがあります.

// Stirng 
public final class String
    implements java.io.Serializable, Comparable, CharSequence {

// StringBuffer 
public final class StringBuffer
   extends AbstractStringBuilder
   implements java.io.Serializable, CharSequence

// StirngBuilder 
public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence

https://docs.oracle.com/javase/8/docs/api/java/lang/CharSequence.html
Modifier and Type
Method and Description char charAt(int index) Returns the  char  value at the specified index. default IntStream chars() Returns a stream of  int  zero-extending the  char  values from this sequence. default IntStream codePoints() Returns a stream of code point values from this sequence. int length() Returns the length of this character sequence. CharSequence subSequence(int start, int end) Returns a  CharSequence  that is a subsequence of this sequence. String toString() Returns a string containing the characters in this sequence in the same order as this sequence.

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 + " ");
    });
}