Java8 Collectors.joining() 详解
本章节我们来详细讲讲 Java 8 流 ( stream ) 收集器 ( Collectors ) 中的 joining() 方法。该方法会返回一个 Collectors 实例，方便在流收集器上的链式操作。
Collectors.joining() 方法以遭遇元素的顺序拼接元素。我们可以传递可选的拼接字符串、前缀和后缀
joinning() 方法定义
假设我们的流中有四个元素 ["A","B","C","D"]，那么我们就可以按照以下方式来收集它们
joining()
joinning() 无参数方法会返回一个 Collectors 实例，并且以空字符串 ( "" ) 来拼接收集到的所有元素
JoiningExample.java
package cn.twle.util.stream; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class JoiningExample { public static void main(String[] args) { List<String> list = Arrays.asList("A","B","C","D"); String result= list.stream().collect(Collectors.joining()); System.out.println(result); } }
输出结果为 ABCD
joining(CharSequence delimiter)
joining(CharSequence delimiter) 接受一个参数字符串序列作为拼接符，并返回一个 Collectors 实例。假如我们传递的拼接符为 "-" 。那么输出结果为 A-B-C-D
JoiningExample.java
package cn.twle.util.stream; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class JoiningExample { public static void main(String[] args) { List<String> list = Arrays.asList("A","B","C","D"); String result= list.stream().collect(Collectors.joining("-")); System.out.println(result); } }
运行结果为 A-B-C-D
joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix) 方法接受一个字符串序列作为拼接符，并在拼接完成后添加传递的前缀和后缀。假如我们传递的分隔符为 "-"，前缀为 "[" ， 后缀为 "]" 。那么输出结果为 [A-B-C-D]
JoiningExample.java
package cn.twle.util.stream; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class JoiningExample { public static void main(String[] args) { List<String> list = Arrays.asList("A","B","C","D","[","]"); String result= list.stream().collect(Collectors.joining("-")); System.out.println(result); } }
运行结果为 [A-B-C-D]
范例
范例 1 ： 如果流中的数据是字符串
下面的代码演示了如何使用 joinning() 的三种重载方法来拼接字符串
JoiningExampleWithListOfString.java
package cn.twle.util.stream; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class JoiningExampleWithListOfString { public static void main(String[] args) { List<String> list = Arrays.asList("Ram","Shyam","Shiv","Mahesh"); String result= list.stream().collect(Collectors.joining()); System.out.println(result); result= list.stream().collect(Collectors.joining(",")); System.out.println(result); result= list.stream().collect(Collectors.joining("-","[","]")); System.out.println(result); } }
运行结果为
RamShyamShivMahesh Ram,Shyam,Shiv,Mahesh [Ram-Shyam-Shiv-Mahesh]
范例 2: 如果流中的数据是对象
如果流中的数据是对象，下面的代码演示了如何拼接它们。
首先，我们创建一个 Person 类
Person.java
package cn.twle.util.stream; import java.util.ArrayList; import java.util.List; public class Person { private String name; private int age; public Person(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public int getAge() { return age; } public static List<Person> getList() { List<Person> list = new ArrayList<>(); list.add(new Person("Ram", 23)); list.add(new Person("Shyam", 20)); list.add(new Person("Shiv", 25)); list.add(new Person("Mahesh", 30)); return list; } }
然后创建一个 Person 对象流
JoiningExampleWithListOfObject.java
package cn.twle.util.stream; import java.util.List; import java.util.stream.Collectors; public class JoiningExampleWithListOfObject { public static void main(String[] args) { List<Person> list = Person.getList(); System.out.println("--Join person name--"); String result= list.stream().map(p -> p.getName()).collect(Collectors.joining()); System.out.println(result); result= list.stream().map(p -> p.getName()).collect(Collectors.joining("|")); System.out.println(result); result= list.stream().map(p -> p.getName()).collect(Collectors.joining("-","[","]")); System.out.println(result); System.out.println("--Join person age--"); result= list.stream().map(p -> String.valueOf(p.getAge())).collect(Collectors.joining()); System.out.println(result); result= list.stream().map(p -> String.valueOf(p.getAge())).collect(Collectors.joining("|")); System.out.println(result); result= list.stream().map(p -> String.valueOf(p.getAge())).collect(Collectors.joining("-","[","]")); System.out.println(result); System.out.println("--Join person name-age--"); result= list.stream().map(p -> p.getName()+"-" + p.getAge()).collect(Collectors.joining("|")); System.out.println(result); result= list.stream().map(p -> p.getName()+"-" + p.getAge()).collect(Collectors.joining("|","[","]")); System.out.println(result); } }
运行结果为
--Join person name-- RamShyamShivMahesh Ram|Shyam|Shiv|Mahesh [Ram-Shyam-Shiv-Mahesh] --Join person age-- 23202530 23|20|25|30 [23-20-25-30] --Join person name-age-- Ram-23|Shyam-20|Shiv-25|Mahesh-30 [Ram-23|Shyam-20|Shiv-25|Mahesh-30]