Java 8 新特性｜Collectors.joining() 详解
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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 0end
