Java 8 Streams map 示例

在Java 8中stream().map()，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 Java示例将Strings List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.List Object – > List String

2.1 name从staff对象List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]  }  }

3.List Object – > List Other Object

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;  }  }

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);  }  }

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'}
]

参考:

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html

http://www.oracle.com/technetwork/articles/java/ma14-java-se-8-streams-2177646.html

https://www.mkyong.com/java8/java-8-flatmap-example/