Java 8 Streams map（）示例

以你之姓@ 2023-02-15 05:10 7阅读 0赞

在Java 8中， `stream().map()`允许您将对象转换为其他对象。 查看以下示例：

## 1.大写字符串列表 ##

1.1将字符串列表转换为大写的简单Java示例。

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.对象列表->字符串列表 ##

2.1从`staff`对象列表中获取所有`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.对象列表->其他对象列表 ##

3.1此示例向您展示如何将`staff`对象列表转换为`StaffPublic`对象列表。

Staff.java

StaffPublic.java

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

3.2在Java之前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;
    
        }
    
    }

输出量

[
    	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示例

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);
    
        }
    	
    }

输出量

[
    	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'}
    ]

## 参考文献 ##

1.  [使用Java SE 8流处理数据，第1部分][Java SE 8_1]
2.  [Java 8 –过滤地图示例][Java 8]
3.  [Java 8 flatMap示例][Java 8 flatMap]
4.  [收藏者JavaDoc][JavaDoc]

标签： [java8][] [地图][Link 1] [流][Link 2]

> 翻译自: [https://mkyong.com/java8/java-8-streams-map-examples/][https_mkyong.com_java8_java-8-streams-map-examples]

[Java SE 8_1]: http://www.oracle.com/technetwork/articles/java/ma14-java-se-8-streams-2177646.html
[Java 8]: https://www.mkyong.com/java8/java-8-filter-a-map-examples/
[Java 8 flatMap]: https://www.mkyong.com/java8/java-8-flatmap-example/
[JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html
[java8]: https://mkyong.com/tag/java8/
[Link 1]: https://mkyong.com/tag/map/
[Link 2]: https://mkyong.com/tag/stream/
[https_mkyong.com_java8_java-8-streams-map-examples]: https://mkyong.com/java8/java-8-streams-map-examples/