Java8 stream().map()將對象轉換為其他對象

1: 將對象List轉為List

public class user{private String name;private String password;private String address;private String age;} List<String> name= user.stream().map(x -> x.getName()).collect(Collectors.toList());

2: 將List 轉為對象list

List<User> result = staff.stream().map(name-> {User user= new User();user.setName(name);user.setPassword(null);user.setAddress(null);user.setAge(null);return user;}).collect(Collectors.toList());

3:將一個對象List轉為另一個對象List

public class UserInfo {private String name;private String pwd; } // 需指定對應字段 List<UserInfo> collect = userList.stream().map(l -> new UserInfo(l.getName(), l.getPassword())).collect(Collectors.toList());

Java 8 Streams map() 示例

在Java 8中stream().map()，您可以將對象轉換為其他對象。查看以下示例：

1.大寫字符串列表

1.1簡單的Java示例將Strings列表轉換為大寫。

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 Java8List<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 8List<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.對象列表 - >字符串列表

2.1 name從staff對象列表中獲取所有值。

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 8List<String> result = new ArrayList<>();for (Staff x : staff) {result.add(x.getName());}System.out.println(result); //[mkyong, jack, lawrence]//Java 8List<String> collect = staff.stream().map(x -> x.getName()).collect(Collectors.toList());System.out.println(collect); //[mkyong, jack, lawrence]}}

對象列表 - >其他對象列表

3.1此示例說明如何將staff對象列表轉換為對象列表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;}}

輸出

[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'} ]

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);}}

輸出

[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'} ]

總結

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