2021-05-05 15:15  阅读(5736)
文章分类:Java 8 特性 文章标签:JavaJava 8
©  原文作者:简单教程 原文地址:https://www.twle.cn/

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

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] 
    

来源:https://www.twle.cn

点赞(1)
版权归原创作者所有,任何形式转载请联系作者; Java 技术驿站 >> Java8 Collectors.joining() 详解
上一篇
Java8 IntStream,LongStream,DoubleStream
下一篇
Java8 Runnable Lambda 表达式