Java 8 Streams map() 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.mycareerrepublic.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.mycareerrepublic.java8;

import java.math.BigDecimal;

public class Staff {

    private String name;
    private int age;
    private BigDecimal salary;
 //...
}

TestJava8.java

package com.mycareerrepublic.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("mycareerrepublic", 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); //[mycareerrepublic, jack, lawrence]

        //Java 8
        List<String> collect = staff.stream().map(x -> x.getName()).collect(Collectors.toList());
        System.out.println(collect); //[mycareerrepublic, 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.mycareerrepublic.java8;

import java.math.BigDecimal;

public class Staff {

    private String name;
    private int age;
    private BigDecimal salary;
 //...
}

StaffPublic.java

package com.mycareerrepublic.java8;

public class StaffPublic {

    private String name;
    private int age;
    private String extra;
    //...
}

3.2 Before Java 8.

BeforeJava8.java

package com.mycareerrepublic.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("mycareerrepublic", 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 ("mycareerrepublic".equals(temp.getName())) {
                obj.setExtra("this field is for mycareerrepubliconly!");
            }

            result.add(obj);
        }

        return result;

    }

}

Output

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

3.3 Java 8 example.

NowJava8.java

package com.mycareerrepublic.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("mycareerrepublic", 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 ("mycareerrepublic".equals(temp.getName())) {
                obj.setExtra("this field is for mycareerrepubliconly!");
            }
            return obj;
        }).collect(Collectors.toList());

        System.out.println(result);

    }

}

Output

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

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About Mariano

I'm Ethan Mariano a software engineer by profession and reader/writter by passion.I have good understanding and knowledge of AngularJS, Database, javascript, web development, digital marketing and exploring other technologies related to Software development.