Java 8 has made comparator more powerful using Lambda Expression. It has introduced many new APIs for Comparator. Let’s go through some new feature of Comparator using Lambda expression.
Lets create a very simple class called Employee:
Lets create a very simple class called Employee:
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package org.arpit.java2blog; public class Employee { private String employeeName; private int age; public Employee(String employeeName,int age) { this.employeeName=employeeName; this.age=age; } public String getEmployeeName() { return employeeName; } public void setEmployeeName(String employeeName) { this.employeeName= employeeName; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String toString() { return getEmployeeName()+" : "+getAge(); } } |
Sort Employee list by name in classic way:
We will sort employee list in classic way
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Collections.sort(employeeList,new Comparator() { public int compare(Employee e1,Employee e2) { return e1.getName().compareTo(e2.getName()); } }); |
Create EmployeeMain.java
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package org.arpit.java2blog; import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; import java.util.List; public class EmployeeMain { public static void main(String[] args) { List employeeList=getEmployees(); System.out.println("Before Sorting: "); System.out.println(employeeList); // in classic way Collections.sort(employeeList,new Comparator() { public int compare(Employee e1,Employee e2) { return e1.getName().compareTo(e2.getName()); } }); System.out.println("After Sorting: "); System.out.println(employeeList); } public static List getEmployees() { List employeesList=new ArrayList(); Employee e1=new Employee("John", 35); Employee e2=new Employee("Adam", 22); Employee e3=new Employee("Arpit", 28); Employee e4=new Employee("John", 30); Employee e5=new Employee("Grace", 38); Employee e6=new Employee("Arpit", 25); employeesList.add(e1); employeesList.add(e2); employeesList.add(e3); employeesList.add(e4); employeesList.add(e5); employeesList.add(e6); return employeesList; } } |
Run the above program and you will get following output:
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Before Sorting: [John : 35, Adam : 22, Arpit : 28, John : 30, Grace : 38, Arpit : 25] After Sorting: [Adam : 22, Arpit : 28, Arpit : 25, Grace : 38, John : 35, John : 30] |
Sort Employee list by name using lambda expression:
We will sort Employee list by name using lambda expression, It will reduce complex Comparator syntax to simple line of code
Create EmployeeMain.java
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package org.arpit.java2blog; import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; import java.util.List; public class EmployeeMain { public static void main(String[] args) { List employeeList=getEmployees(); System.out.println("Before Sorting: "); System.out.println(employeeList); // using lambda expression Collections.sort(employeeList,(Employee e1,Employee e2)->e1.getEmployeeName().compareTo(e2.getEmployeeName())); System.out.println("After Sorting: "); System.out.println(employeeList); } public static List getEmployees() { List employeesList=new ArrayList(); Employee e1=new Employee("John", 35); Employee e2=new Employee("Adam", 22); Employee e3=new Employee("Arpit", 28); Employee e4=new Employee("John", 30); Employee e5=new Employee("Grace", 38); Employee e6=new Employee("Arpit", 25); employeesList.add(e1); employeesList.add(e2); employeesList.add(e3); employeesList.add(e4); employeesList.add(e5); employeesList.add(e6); return employeesList; } } |
Run the above program and you will get following output:
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Before Sorting: [John : 35, Adam : 22, Arpit : 28, John : 30, Grace : 38, Arpit : 25] After Sorting: [Adam : 22, Arpit : 28, Arpit : 25, Grace : 38, John : 35, John : 30]] |
Sort Employee list by name using lambda expression without type definitions:
Here we will not pass type definition to arguments in lambda expression.It will be interpreted in context
Just change line no. 16 of Employee name from :
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Collections.sort(employeeList,(Employee e1,Employee e2)->e1.getEmployeeName().compareTo(e2.getEmployeeName())); |
To
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Collections.sort(employeeList,(e1,e2)->e1.getEmployeeName().compareTo(e2.getEmployeeName())); |
and Run EmployeeMain.java. You will get same output as above
Reverse sort:
JDK 8 has introduced a new help method for reverse sorting a list.
Change the main method in employee main to below method:
Change the main method in employee main to below method:
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public static void main(String[] args) { List employeeList=getEmployees(); System.out.println("Before Sorting: "); System.out.println(employeeList); // using lambda expression Collections.sort(employeeList,(Employee e1,Employee e2)->e1.getEmployeeName().compareTo(e2.getEmployeeName())); System.out.println("After Sorting: "); System.out.println(employeeList); } |
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