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Java 8 Streams map() examples

摘要: In Java 8, stream().map() lets you convert an object to something else. Review the following examples :

In Java 8, stream().map() lets you convert an object to something else. Review the following examples :

1. A List of Strings to Uppercase

1.1 Simple Java example to convert a list of Strings to upper case.

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 of objects -> List of String

2.1 Get all the name values from a list of the staff objects.

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 of objects -> List of other objects

3.1 This example shows you how to convert a list of staff objects into a list of StaffPublic objects.

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 Before Java 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;

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 example.

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'}

References

  1. Processing Data with Java SE 8 Streams, Part 1
  2. Java 8 – Filter a Map examples
  3. Java 8 flatMap example
  4. Collectors JavaDoc

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