1. Overview
In this tutorial, We'll learn how to use the Comparator interface with lambda expression in Java 8 onwards.
Lambda expression is to replace the anonymous implementations and to reduce the boilerplate code.
This makes the code cleaner and focus more on the business logic rather than semantics.
Look at the few examples before and after java 8 using Lambda expressions.
Java community provided a few sets of rules and guidelines in creating the lambda statements.
Lambda syntax and invalid one are discussed in detail in the previous discussions "java 8 lambda expressions".
First, we will see the examples without lambda and then the next section will see the equivalent lambdas.
For sorting to work with lambda comparator, we have to use the Collections.sort() method.
2. Usage of Java Comparator Without Lambda
Comparator in Java is an interface that is used to sort the collections based on the custom objects fields.
The comparator must be used with the combination of List or Set or Queue implementations.
Here the comparator creation is called an Anonymous comparator.
2.1 Sort Engineers by int id
Simple example 1
package com.javaprogramto.java8.comparator.lambda;
import java.util.Collections;
import java.util.Comparator;
import java.util.LinkedList;
import java.util.List;
public class WIthoutLambdasExample {
public static void main(String[] args) {
List<Engineer> engineers1 = getEngineersList();
// printing engineer objects sorted by id
System.out.println("Before sorting");
engineers1.forEach(System.out::println);
// comparator before java 8 and without lambds
Comparator<Engineer> idComparator = new Comparator<Engineer>() {
@Override
public int compare(Engineer e1, Engineer e2) {
return Integer.valueOf(e1.getId()).compareTo(e2.getId());
}
};
// sorting with collections.sort()
Collections.sort(engineers1, idComparator);
// printing engineer objects sorted by id
System.out.println("\nAfter sorting");
engineers1.forEach(System.out::println);
}
private static List<Engineer> getEngineersList() {
// creating the List of engineers
List<Engineer> engineers = new LinkedList<>();
// adding engineer objects
engineers.add(new Engineer(100, "Jeo", 100000, false));
engineers.add(new Engineer(103, "Sunny", 500000, true));
engineers.add(new Engineer(104, "Zeon", 300000, false));
engineers.add(new Engineer(105, "Neon", 400000, true));
engineers.add(new Engineer(102, "Ammon", 200000, false));
return engineers;
}
}
class Engineer {
private int id;
private String name;
private long salary;
private boolean fullTime;
public Engineer(int id, String name, long salary, boolean fullTime) {
super();
this.id = id;
this.name = name;
this.salary = salary;
this.fullTime = fullTime;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public long getSalary() {
return salary;
}
public void setSalary(long salary) {
this.salary = salary;
}
public boolean isFullTime() {
return fullTime;
}
public void setFullTime(boolean fullTime) {
this.fullTime = fullTime;
}
@Override
public String toString() {
return "Engineer [id=" + id + ", name=" + name + ", salary=" + salary + ", fullTime=" + fullTime + "]";
}
}
Output
Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true]
2.2 Sort Engineers by string name
Example 2
// sort engineers by name
List<Engineer> engineers2 = getEngineersList();
// printing engineer objects sorted by id
System.out.println("Before sorting");
engineers2.forEach(System.out::println);
// comparator before java 8 and without lambds
Comparator<Engineer> nameComparator = new Comparator<Engineer>() {
@Override
public int compare(Engineer e1, Engineer e2) {
return e1.getName().compareTo(e2.getName());
}
};
// sorting with collections.sort()
Collections.sort(engineers2, nameComparator);
// printing engineer objects sorted by id
System.out.println("\nAfter sorting");
engineers2.forEach(System.out::println);
Output
Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false]
2.3 Sort Engineers by Long Salary
Example 3
// sort engineers by salary
List<Engineer> engineers3 = getEngineersList();
// printing engineer objects sorted by id
System.out.println("\nEngieer Sort by Salary");
System.out.println("\nBefore sorting");
engineers3.forEach(System.out::println);
// comparator before java 8 and without lambds
Comparator<Engineer> salaryComparator = new Comparator<Engineer>() {
@Override
public int compare(Engineer e1, Engineer e2) {
return Long.valueOf(e1.getSalary()).compareTo(e2.getSalary());
}
};
// sorting with collections.sort()
Collections.sort(engineers3, salaryComparator);
// printing engineer objects sorted by id
System.out.println("\nAfter sorting");
engineers3.forEach(System.out::println);
Output
Engieer Sort by Salary Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=103, name=Sunny, salary=500000, fullTime=true]
2.4 Sort Engineers by Boolean FullTime
Example 4
// sort engineers by full time
List<Engineer> engineers4 = getEngineersList();
// printing engineer objects sorted by id
System.out.println("\nEngieer Sort by fulltime");
System.out.println("\nBefore sorting");
engineers4.forEach(System.out::println);
// comparator before java 8 and without lambds
Comparator<Engineer> fulltimeComparator = new Comparator<Engineer>() {
@Override
public int compare(Engineer e1, Engineer e2) {
return Boolean.valueOf(e1.isFullTime()).compareTo(Boolean.valueOf(e2.isFullTime()));
}
};
// sorting with collections.sort()
Collections.sort(engineers4, fulltimeComparator);
// printing engineer objects sorted by id
System.out.println("\nAfter sorting");
engineers4.forEach(System.out::println);
Output
Engieer Sort by fulltime Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=105, name=Neon, salary=400000, fullTime=true]
3. Usage Lambda With Java Comparator
By using lambda, we can reduce code and make focusing much on core logic.
Lambda is indicated with the arrow symbol (->). Look at the below syntax
Here the data types are optional and no need to use the return statement inside the lambda body if there is only one statement.
When working with lambda or java 8 then you do not need to go with the Collections.sort() method.
because java 8 list is added with the sort() method directly. So, we can call list.sort() with the comparator instance or lambda.
3.1 Compartor sort by id with Lambda
below is the complete example with Lambda and list.sort() to sort the list of engineers by id in ascending order.
Example 5
public class WithLambdasExample {
public static void main(String[] args) {
List<Engineer> engineers1 = getEngineersList();
// printing engineer objects sorted by id
System.out.println("Before sorting");
engineers1.forEach(System.out::println);
// comparator with lambdas
Comparator<Engineer> idComparator = (e1, e2) -> Integer.valueOf(e1.getId()).compareTo(e2.getId());
// sorting with list.sort()
engineers1.sort(idComparator);
// printing engineer objects sorted by id
System.out.println("\nAfter sorting");
engineers1.forEach(System.out::println);
}
private static List<Engineer> getEngineersList() {
// creating the List of engineers
List<Engineer> engineers = new LinkedList<>();
// adding engineer objects
engineers.add(new Engineer(100, "Jeo", 100000, false));
engineers.add(new Engineer(103, "Sunny", 500000, true));
engineers.add(new Engineer(104, "Zeon", 300000, false));
engineers.add(new Engineer(105, "Neon", 400000, true));
engineers.add(new Engineer(102, "Ammon", 200000, false));
return engineers;
}
}
Output
Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true]
3.2 Compartor sort by name with Lambda
Example 6
// comparator in java 8 with lambdas Comparator<Engineer> nameComparator = (e1, e2) -> e1.getName().compareTo(e2.getName()); // sorting with list.sort() engineers2.sort(nameComparator);
Output
Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false]
3.3 Compartor sort by salary with Lambda
Example 7
// comparator in java 8 with lambds Comparator<Engineer> salaryComparator = (e1, e2) -> Long.valueOf(e1.getSalary()).compareTo(e2.getSalary()); // sorting with list.sort() engineers3.sort(salaryComparator);
Output
Engieer Sort by Salary Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=103, name=Sunny, salary=500000, fullTime=true]
3.4 Comparator sort by full time with Lambda
Example 8
// comparator in java 8 with lambdas Comparator<Engineer> fulltimeComparator = (e1, e2) -> Boolean.valueOf(e1.isFullTime()).compareTo(Boolean.valueOf(e2.isFullTime())); // sorting with list.sort() engineers4.sort(fulltimeComparator);
Output
Engieer Sort by fulltime Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=105, name=Neon, salary=400000, fullTime=true]
4. Java 8 Comparator with Lambda sorting by multiple fields
The below example is to sort based on the full time and id.
First, create two separate comparators with lambda by full time and id.
Next, call thenComparint() method on the full-time comparator as below.
Example 9
Comparator<Engineer> fullTimeAndIdComparator = fulltimeComparator.thenComparing(idComparator);
This creates a new comparator with by full time and id fields together.
Look at the full code for multiple fields.
Full Example
public class WithLambdasExample2 {
public static void main(String[] args) {
List<Engineer> engineers1 = getEngineersList();
// printing engineer objects sorted by id
System.out.println("Before sorting");
engineers1.forEach(System.out::println);
// comparator with lambdas
Comparator<Engineer> fulltimeComparator = (e1, e2) -> Boolean.valueOf(e1.isFullTime())
.compareTo(Boolean.valueOf(e2.isFullTime()));
Comparator<Engineer> idComparator = (e1, e2) -> Integer.valueOf(e1.getId()).compareTo(e2.getId());
Comparator<Engineer> fullTimeAndIdComparator = fulltimeComparator.thenComparing(idComparator);
// sorting with collections.sort()
engineers1.sort(fullTimeAndIdComparator);
// printing engineer objects sorted by id
System.out.println("\nAfter sorting");
engineers1.forEach(System.out::println);
}
}
Output
Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=105, name=Neon, salary=400000, fullTime=true]
5. Java 8 Comparator with Lambda Sorting in reverse order or descending order
To get sorted the objects with comparator by reverse order and objects in the descending order.
For the descending order, after creating the Comparator with lambda then you need to call reversed() method of Comparator as below.
Example 10
Comparator<Engineer> fullTimeAndIdComparator = fulltimeComparator.thenComparing(idComparator).reversed();
// sorting with collections.sort()
engineers1.sort(fullTimeAndIdComparator);
// printing engineer objects sorted by id
System.out.println("\nAfter sorting");
engineers1.forEach(System.out::println);
Output
Before sorting Engineer [id=100, name=Jeo, salary=100000, fullTime=false] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] After sorting Engineer [id=105, name=Neon, salary=400000, fullTime=true] Engineer [id=103, name=Sunny, salary=500000, fullTime=true] Engineer [id=104, name=Zeon, salary=300000, fullTime=false] Engineer [id=102, name=Ammon, salary=200000, fullTime=false] Engineer [id=100, name=Jeo, salary=100000, fullTime=false]
Compare this output with the above section output for the reversed order.
6. Conclusion
In this article, We've seen how to use lambda with comparator in java 8.
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