Java 8 Streams map() 例子

た入场券 2022-06-15 03:22 166阅读 0赞

在java 8，stream().map()允许您将对象转换为别的。复习下面的例子：

## 1. String类型的List集合转大写  ##

1.1 简单的java例子把字符串列表转换为大写的情况.

TestJava8.java

package com.mkyong.java8; import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class TestJava8 { public static void main(String[] args) { List<String> alpha = Arrays.asList("a", "b", "c", "d"); //Before Java8 List<String> alphaUpper = new ArrayList<>(); for (String s : alpha) { alphaUpper.add(s.toUpperCase()); } System.out.println(alpha); //[a, b, c, d] System.out.println(alphaUpper); //[A, B, C, D] // Java 8 List<String> collect = alpha.stream().map(String::toUpperCase).collect(Collectors.toList()); System.out.println(collect); //[A, B, C, D] // Extra, streams apply to any data type. List<Integer> num = Arrays.asList(1,2,3,4,5); List<Integer> collect1 = num.stream().map(n -> n * 2).collect(Collectors.toList()); System.out.println(collect1); //[2, 4, 6, 8, 10] } }

## 2. List of objects -> List of String ##

2.1 从 staff 对象集合中获取所有的 `name` 值.

Staff.java

package com.mkyong.java8; import java.math.BigDecimal; public class Staff { private String name; private int age; private BigDecimal salary; //... }

TestJava8.java

package com.mkyong.java8; import java.math.BigDecimal; import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class TestJava8 { public static void main(String[] args) { List<Staff> staff = Arrays.asList( new Staff("mkyong", 30, new BigDecimal(10000)), new Staff("jack", 27, new BigDecimal(20000)), new Staff("lawrence", 33, new BigDecimal(30000)) ); //Before Java 8 List<String> result = new ArrayList<>(); for (Staff x : staff) { result.add(x.getName()); } System.out.println(result); //[mkyong, jack, lawrence] //Java 8 List<String> collect = staff.stream().map(x -> x.getName()).collect(Collectors.toList()); System.out.println(collect); //[mkyong, jack, lawrence] } }

## 3. List of objects -> List of other objects ##

3.1  这个例子展示如何将  staff  对象的集合转为 `StaffPublic` 对象的集合.

Staff.java

package com.mkyong.java8; import java.math.BigDecimal; public class Staff { private String name; private int age; private BigDecimal salary; //... }

StaffPublic.java

package com.mkyong.java8; public class StaffPublic { private String name; private int age; private String extra; //... }

3.2 Before Java 8.

BeforeJava8.java

package com.mkyong.java8; import java.math.BigDecimal; import java.util.ArrayList; import java.util.Arrays; import java.util.List; public class BeforeJava8 { public static void main(String[] args) { List<Staff> staff = Arrays.asList( new Staff("mkyong", 30, new BigDecimal(10000)), new Staff("jack", 27, new BigDecimal(20000)), new Staff("lawrence", 33, new BigDecimal(30000)) ); List<StaffPublic> result = convertToStaffPublic(staff); System.out.println(result); } private static List<StaffPublic> convertToStaffPublic(List<Staff> staff) { List<StaffPublic> result = new ArrayList<>(); for (Staff temp : staff) { StaffPublic obj = new StaffPublic(); obj.setName(temp.getName()); obj.setAge(temp.getAge()); if ("mkyong".equals(temp.getName())) { obj.setExtra("this field is for mkyong only!"); } result.add(obj); } return result; } }

Output

[
    	StaffPublic{ name='mkyong', age=30, extra='this field is for mkyong only!'},
    	StaffPublic{ name='jack', age=27, extra='null'},
    	StaffPublic{ name='lawrence', age=33, extra='null'}
    ]

3.3 Java 8 例子.

NowJava8.java

package com.mkyong.java8; package com.hostingcompass.web.java8; import java.math.BigDecimal; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class NowJava8 { public static void main(String[] args) { List<Staff> staff = Arrays.asList( new Staff("mkyong", 30, new BigDecimal(10000)), new Staff("jack", 27, new BigDecimal(20000)), new Staff("lawrence", 33, new BigDecimal(30000)) ); // convert inside the map() method directly. List<StaffPublic> result = staff.stream().map(temp -> { StaffPublic obj = new StaffPublic(); obj.setName(temp.getName()); obj.setAge(temp.getAge()); if ("mkyong".equals(temp.getName())) { obj.setExtra("this field is for mkyong only!"); } return obj; }).collect(Collectors.toList()); System.out.println(result); } }

Output

## References ##

1.  [Processing Data with Java SE 8 Streams, Part 1][Processing Data with Java SE 8 Streams_ Part 1]
2.  [Java 8 – Filter a Map examples][Java 8 _ Filter a Map examples]
3.  [Java 8 flatMap example][]
4.  [Collectors JavaDoc][]

[Processing Data with Java SE 8 Streams_ Part 1]: http://www.oracle.com/technetwork/articles/java/ma14-java-se-8-streams-2177646.html
[Java 8 _ Filter a Map examples]: https://www.mkyong.com/java8/java-8-filter-a-map-examples/
[Java 8 flatMap example]: https://www.mkyong.com/java8/java-8-flatmap-example/
[Collectors JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html