jdk8的特性stream().map()

转:

https://blog.csdn.net/sanchan/article/details/70753645

java8的optional的使用：

http://www.jdon.com/idea/java/using-optional-effectively-in-java-8.html

http://www.runoob.com/java/java8-optional-class.html

Optional 类是一个可以为null的容器对象。如果值存在则isPresent()方法会返回true，调用get()方法会返回该对象。

Optional 是个容器：它可以保存类型T的值，或者仅仅保存null。Optional提供很多有用的方法，这样我们就不用显式进行空值检测。

Optional 类的引入很好的解决空指针异常。

类声明

以下是一个 java.util.Optional<T> 类的声明：

public final class Optional<T> extends Object

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在Java 8中stream().map()，您可以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简单的Java示例将Strings列表转换为大写。   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 name从staff对象列表中获取所有值。   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]       }   }   对象列表 - >其他对象列表 3.1此示例说明如何将staff对象列表转换为对象列表StaffPublic。   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 Java 8之前。   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   [     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'} ] 1 2 3 4 5 3.3 Java 8的例子。   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);       }   }     [     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'} ]