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003-三、Java8 Collectors.joining() 详解

引言

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 com.souyunku.tech.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 com.souyunku.tech.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 &quot;-&quot;,前缀为 &quot;[&quot; , 后缀为 &quot;]&quot; 。那么输出结果为[A-B-C-D]

JoiningExample.java


package com.souyunku.tech.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 com.souyunku.tech.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

Person.java


package com.souyunku.tech.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 com.souyunku.tech.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-- |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]

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标题:003-三、Java8 Collectors.joining() 详解

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