Java8新特性之Collectors.joining()实例详解
作者：Java小技巧
方法定义
Java 8 流 ( stream ) 收集器 ( Collectors ) 中的 joining() 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。
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
总结
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