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.
TestStreamMap.java
package com.favtuts.java8;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class TestStreamMap {
public static void main(String[] args) {
convertListStringToUpperCase();
}
private static void convertListStringToUpperCase() {
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.favtuts.java8;
import java.math.BigDecimal;
public class Staff {
private String name;
private int age;
private BigDecimal salary;
public Staff() {
}
public Staff(String name, int age, BigDecimal salary) {
this.name = name;
this.age = age;
this.salary = salary;
}
@Override
public String toString() {
return "Staff{" +
" name='" + getName() + "'" +
", age='" + getAge() + "'" +
", salary='" + getSalary() + "'" +
"}";
}
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return this.age;
}
public void setAge(int age) {
this.age = age;
}
public BigDecimal getSalary() {
return this.salary;
}
public void setSalary(BigDecimal salary) {
this.salary = salary;
}
}
TestStreamMap.java
package com.favtuts.java8;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class TestStreamMap {
public static void main(String[] args) {
getAllNamesFromStaffList();
}
private static void getAllNamesFromStaffList(){
List<Staff> staff = Arrays.asList(
new Staff("favtuts", 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); //[favtuts, jack, lawrence]
//Java 8
List<String> collect = staff.stream().map(x -> x.getName()).collect(Collectors.toList());
System.out.println(collect); //[favtuts, 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.
StaffPublic.java
package com.favtuts.java8;
public class StaffPublic {
private String name;
private int age;
private String extra;
public StaffPublic(String name, int age, String extra) {
this.name = name;
this.age = age;
this.extra = extra;
}
public StaffPublic() {
}
@Override
public String toString() {
return "StaffPublic{" +
" name='" + getName() + "'" +
", age='" + getAge() + "'" +
", extra='" + getExtra() + "'" +
"}";
}
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return this.age;
}
public void setAge(int age) {
this.age = age;
}
public String getExtra() {
return this.extra;
}
public void setExtra(String extra) {
this.extra = extra;
}
}
BeforeJava8.java
package com.favtuts.java8;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class TestStreamMap {
public static void main(String[] args) {
List<Staff> staff = Arrays.asList(
new Staff("favtuts", 30, new BigDecimal(10000)),
new Staff("jack", 27, new BigDecimal(20000)),
new Staff("lawrence", 33, new BigDecimal(30000))
);
convertStaffToStaffPublicBeforeJava8(staff);
}
private static void convertStaffToStaffPublicBeforeJava8(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 ("favtuts".equals(temp.getName())) {
obj.setExtra("this field is for favtuts only!");
}
result.add(obj);
}
System.out.println(result);
}
}
Output
[
StaffPublic{name='favtuts', age=30, extra='this field is for favtuts only!'},
StaffPublic{name='jack', age=27, extra='null'},
StaffPublic{name='lawrence', age=33, extra='null'}
]3.3 Java 8 example.
NowJava8.java
package com.favtuts.java8;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class TestStreamMap {
public static void main(String[] args) {
List<Staff> staff = Arrays.asList(
new Staff("favtuts", 30, new BigDecimal(10000)),
new Staff("jack", 27, new BigDecimal(20000)),
new Staff("lawrence", 33, new BigDecimal(30000))
);
convertStaffToStaffPublicNowJava8(staff);
}
private static void convertStaffToStaffPublicNowJava8(List<Staff> staff) {
// 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 ("favtuts".equals(temp.getName())) {
obj.setExtra("this field is for favtuts only!");
}
return obj;
}).collect(Collectors.toList());
System.out.println(result);
}
}
Output
[
StaffPublic{name='favtuts', age=30, extra='this field is for favtuts only!'},
StaffPublic{name='jack', age=27, extra='null'},
StaffPublic{name='lawrence', age=33, extra='null'}
]Download Source Code
$ git clone https://github.com/favtuts/java-core-tutorials-examples
$ cd java-basic/java8