2022-08-07  阅读(452)
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

阅读全文
  • 点赞