将List转变为逗号分隔的字符串

1、使用场景

我们经常在开发过程之中，会遇到需要把返回的Id list转换为一个使用逗号(,)分隔的字符串； 然后提供给后端执行SQL语句的In查询，或者给前端进行展示等等。

2、实现方法

在Java 8中将集合List转变为用逗号分隔的String是非常简单的。 使用String.join()函数，给函数传递一个分隔符合一个迭代器，一个StringJoiner对象会帮助我们完成所有的事情。

List<String> letters= Arrays.asList("A", "B", "C", "D");
String lettersCommaSeparated = String.join(",", letters);
System.out.println(lettersCommaSeparated );

我们采用流的方式来写，仍然能够得到同样的结果

String lettersCommaSeparated = letters.stream().collect(Collectors.joining(","));
System.out.println(lettersCommaSeparated );

Java 7 实现

private static final String SEPARATOR = ",";
public static void main(String[] args) {
  List<String> letters = Arrays.asList("A","B","C","D");
  StringBuilder strBuilder = new StringBuilder();
  for(String letter : letters){
    strBuilder.append(letter);
    strBuilder.append(SEPARATOR);
  }
  String strLetter = strBuilder.toString();
  System.out.println(strLetter);
  //OUTPUT: A,B,C,D,
  //Remove last comma
  strLetter = strLetter.substring(0, strLetter.length() - SEPARATOR.length());
  System.out.println(strLetter);
  //OUTPUT: A,B,C,D
}

3、完整实战代码

public class StringList {
	private static List<String> letters= Arrays.asList("A", "B", "C", "D");
	private static final String SEPARATOR = ",";
	/**
	 * 使用Java8进行返回
	 */
	public static void Java8ListToString() {
		
		String lettersCommaSeparated = String.join(",", letters);
		System.out.println("Java8ListToString: 
"+lettersCommaSeparated );
		
	}
	
	public static void Java8StreamListToString() {
		String lettersCommaSeparated = letters.stream().collect(Collectors.joining(","));
		System.out.println("Java8StreamListToString: 
"+lettersCommaSeparated );
	}
	
	public static void Java7ListToString() {
		 List<String> letters = Arrays.asList("A","B","C","D");
		  StringBuilder strBuilder = new StringBuilder();
		  for(String letter : letters){
		    strBuilder.append(letter);
		    strBuilder.append(SEPARATOR);
		  }
		  String strLetter = strBuilder.toString();
		  System.out.println(strLetter);
		  //OUTPUT: A,B,C,D,
		  //Remove last comma
		  strLetter = strLetter.substring(0, strLetter.length() - SEPARATOR.length());
		  System.out.println("Java7ListToString: 
"+strLetter);
		  //OUTPUT: A,B,C,D
	}
	public static void main(String[] args) {
		Java8ListToString();
		Java8StreamListToString();
		Java7ListToString();
	}
}