定制开发Java 8 新特性｜Collectors.joining() 详解

方法定义

Java 8 流 ( ) 收集器 ( Collectors ) 中的 joining() 定制开发方法会返回一个 Collectors 实例，定制开发方便在流收集器上的链式操作。

Collectors.joining() 定制开发方法以遭遇元素的顺序拼接元素。定制开发我们可以传递可选的拼接字符串、前缀和后缀。

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。

无参方法

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

package com.sjh.test.java8;
 
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
 
public class JoiningTest1 {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("1","2","3","4");
        String result= list.stream().collect(Collectors.joining());
        System.out.println(result);
    }
}

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.JoiningTest1
1234
 
Process finished with exit code 0

单个参数

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 1-2-3-4

package com.sjh.test.java8;
 
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
 
public class JoiningTest2 {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("1","2","3","4");
        String result= list.stream().collect(Collectors.joining("-"));
        System.out.println(result);
    }
}

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.JoiningTest2
1-2-3-4
 
Process finished with exit code 0

多个参数

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 [1-2-3-4]

package com.sjh.test.java8;
 
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
 
public class JoiningTest3 {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("1","2","3","4");
        String result= list.stream().collect(Collectors.joining("-", "[", "]"));
        System.out.println(result);
    }
}

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.JoiningTest3
[1-2-3-4]
 
Process finished with exit code 0

如果流中的数据是字符串

使用 joinning() 的三种重载方法来拼接字符串

package com.sjh.test.java8;
 
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
 
public class JoniningDemo1 {
 
    public static void main(String[] args) {
        List<String> list = Arrays.asList("JayLa","LayMan","KangKang","XinYI");
        String resultStr = list.stream().collect(Collectors.joining());
        System.out.println(resultStr);
        resultStr= list.stream().collect(Collectors.joining(","));
        System.out.println(resultStr);
        resultStr= list.stream().collect(Collectors.joining("-","[","]"));
        System.out.println(resultStr);
    }
}

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.JoniningDemo1
JayLaLayManKangKangXinYI
JayLa,LayMan,KangKang,XinYI
[JayLa-LayMan-KangKang-XinYI]
 
Process finished with exit code 0

如果流中的数据是对象

首先，我们创建一个 Person 类

package com.sjh.test.java8.bean;
 
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("JayLa", 23));
        list.add(new Person("LayMan", 20));
        list.add(new Person("KangKang", 25));
        list.add(new Person("XinYi", 18));
        return list;
    }
}

然后创建一个 Person 对象流

package com.sjh.test.java8;
 
import com.sjh.test.java8.bean.Person;
 
import java.util.List;
import java.util.stream.Collectors;
 
public class JoiningDemo2 {
    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：

/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/...com.sjh.test.java8.JoiningDemo2
 
--Join person name--
JayLaLayManKangKangXinYi
JayLa|LayMan|KangKang|XinYi
[JayLa-LayMan-KangKang-XinYi]
 
 --Join person age--
23202518
23|20|25|18
[23-20-25-18]
 
 --Join person name-age--
JayLa-23|LayMan-20|KangKang-25|XinYi-18
[JayLa-23|LayMan-20|KangKang-25|XinYi-18]
 
Process finished with exit code 0

end