Java 8 Streams map()

蔚落 2022-06-11 04:15 140阅读 0赞

> 在java8中， stream().map()可以让你转化一个对象成其他的对象。下面我们通过下面的例子来看一下：

*  String集合字符串转为大写

TestJava8.java
    package com.ricky.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]
    
        }
    
    }

*  List of objects -> List of String  
    
    
     *  从staff对象集合中获取所有的名字。

Staff.java
    package com.ricky.java8;
    
    import java.math.BigDecimal;
    
    public class Staff { 
    
        private String name;
        private int age;
        private BigDecimal salary;
        //...
    }
    TestJava8.java
    package com.ricky.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("ricky", 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); //[ricky, jack, lawrence]
    
            //Java 8
            List<String> collect = staff.stream().map(x -> x.getName()).collect(Collectors.toList());
            System.out.println(collect); //[ricky, jack, lawrence]
    
        }
    
    }

*  List of objects -> List of other objects  
    
    
    > 一个例子展示怎么转化staff对象为StaffPublic对象

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

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

*  java8以前的方式

BeforeJava8.java
    package com.ricky.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("ricky", 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 ("ricky".equals(temp.getName())) {
                    obj.setExtra("this field is for ricky only!");
                }
    
                result.add(obj);
            }
    
            return result;
    
        }
    
    }

输出

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

*  java8的方式

NowJava8.java
    package com.ricky.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("ricky", 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 ("ricky".equals(temp.getName())) {
                    obj.setExtra("this field is for ricky only!");
                }
                return obj;
            }).collect(Collectors.toList());
    
            System.out.println(result);
    
        }
    
    }

输出

# 翻译 #

ricky 20170730

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