Java 8 Lambda : Comparator example

In this example, we will show you how to use Java 8 Lambda expression to write a Comparator to sort a List.

1. Classic Comparator example.

	Comparator<Developer> byName = new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getName().compareTo(o2.getName());
		}
	};

2. Lambda expression equivalent.

	Comparator<Developer> byName = 
		(Developer o1, Developer o2)->o1.getName().compareTo(o2.getName());

Developer.java

package com.favtuts.java8;

import java.math.BigDecimal;

public class Developer {

    @Override
    public String toString() {
        return "Developer [" +
            " name='" + getName() + "'" +
            ", salary='" + getSalary() + "'" +
            ", age='" + getAge() + "'" +           
            "]";
    }

    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;
    }

    public Developer(String name, int age, BigDecimal salary) {
        this.name = name;
        this.age = age;
        this.salary = salary;
    }
    
    private String name;
    private int age;
    private BigDecimal salary;
}

1. Sort without Lambda

Example to compare the Developer objects using their age. Normally, you use Collections.sort and pass an anonymous Comparator class like this :

TestSorting.java

package com.favtuts.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class TestSorting {

    public static void main(String[] args) {
        
        List<Developer> listDevs = getDevelopers();

        System.out.println("Before Sort");
        for (Developer developer : listDevs) {
            System.out.println(developer);
        }

        // sort by age
        Collections.sort(listDevs, new Comparator<Developer>() {
            @Override
            public int compare(Developer o1, Developer o2) {
                return o1.getAge() - o2.getAge();
            }
        });

        System.out.println("After Sort");
		for (Developer developer : listDevs) {
			System.out.println(developer);
		}
    }

    private static List<Developer> getDevelopers() {
        List<Developer> result = new ArrayList<Developer>();

		result.add(new Developer("favtuts", new BigDecimal("70000"), 33));
		result.add(new Developer("alvin", new BigDecimal("80000"), 20));
		result.add(new Developer("jason", new BigDecimal("100000"), 10));
		result.add(new Developer("iris", new BigDecimal("170000"), 55));
		
		return result;
    }
}

Output

Before Sort
Developer [ name='favtuts', salary='70000', age='33']
Developer [ name='alvin', salary='80000', age='20']
Developer [ name='jason', salary='100000', age='10']
Developer [ name='iris', salary='170000', age='55']
After Sort
Developer [ name='jason', salary='100000', age='10']
Developer [ name='alvin', salary='80000', age='20']
Developer [ name='favtuts', salary='70000', age='33']
Developer [ name='iris', salary='170000', age='55']

When the sorting requirement is changed, you just pass in another new anonymous Comparator class :

	//sort by age
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getAge() - o2.getAge();
		}
	});
	
	//sort by name	
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getName().compareTo(o2.getName());
		}
	});
				
	//sort by salary
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getSalary().compareTo(o2.getSalary());
		}
	});	

It works, but, do you think it is a bit weird to create a class just because you want to change a single line of code?

2. Sort with Lambda

In Java 8, the List interface is supports the sort method directly, no need to use Collections.sort anymore.

        //List.sort() since Java 8
	listDevs.sort(new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o2.getAge() - o1.getAge();
		}
	});	

Lambda expression example :

TestSorting.java

package com.favtuts.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

public class TestSorting {

	public static void main(String[] args) {

		List<Developer> listDevs = getDevelopers();
		
		System.out.println("Before Sort");
		for (Developer developer : listDevs) {
			System.out.println(developer);
		}
		
		System.out.println("After Sort");
		
		//lambda here!
		listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());
	
		//java 8 only, lambda also, to print the List
		listDevs.forEach((developer)->System.out.println(developer));
	}

	private static List<Developer> getDevelopers() {

		List<Developer> result = new ArrayList<Developer>();

		result.add(new Developer("favtuts", new BigDecimal("70000"), 33));
		result.add(new Developer("alvin", new BigDecimal("80000"), 20));
		result.add(new Developer("jason", new BigDecimal("100000"), 10));
		result.add(new Developer("iris", new BigDecimal("170000"), 55));
		
		return result;

	}
	
}

Output

Before Sort
Developer [ name='favtuts', salary='70000', age='33']
Developer [ name='alvin', salary='80000', age='20']
Developer [ name='jason', salary='100000', age='10']
Developer [ name='iris', salary='170000', age='55']
After Sort
Developer [ name='jason', salary='100000', age='10']
Developer [ name='alvin', salary='80000', age='20']
Developer [ name='favtuts', salary='70000', age='33']
Developer [ name='iris', salary='170000', age='55']

3. More Lambda Examples

3.1 Sort By age

	//sort by age
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getAge() - o2.getAge();
		}
	});
	
	//lambda
	listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());
	
	//lambda, valid, parameter type is optional
	listDevs.sort((o1, o2)->o1.getAge()-o2.getAge());

3.2 Sort by name

	//sort by name
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getName().compareTo(o2.getName());
		}
	});
		
	//lambda
	listDevs.sort((Developer o1, Developer o2)->o1.getName().compareTo(o2.getName()));		
	
	//lambda
	listDevs.sort((o1, o2)->o1.getName().compareTo(o2.getName()));	

3.3 Sort by salary

	//sort by salary
	Collections.sort(listDevs, new Comparator<Developer>() {
		@Override
		public int compare(Developer o1, Developer o2) {
			return o1.getSalary().compareTo(o2.getSalary());
		}
	});				

	//lambda
	listDevs.sort((Developer o1, Developer o2)->o1.getSalary().compareTo(o2.getSalary()));
	
	//lambda
	listDevs.sort((o1, o2)->o1.getSalary().compareTo(o2.getSalary()));

3.4 Reversed sorting.

3.4.1 Lambda expression to sort a List using their salary.

	Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
	listDevs.sort(salaryComparator);

Output

Developer [ name='favtuts', salary='70000', age='33']
Developer [ name='alvin', salary='80000', age='20']
Developer [ name='jason', salary='100000', age='10']
Developer [ name='iris', salary='170000', age='55']

3.4.2 Lambda expression to sort a List using their salary, reversed order.

	Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
	listDevs.sort(salaryComparator.reversed());

Output

Developer [ name='iris', salary='170000', age='55']
Developer [ name='jason', salary='100000', age='10']
Developer [ name='alvin', salary='80000', age='20']
Developer [ name='favtuts', salary='70000', age='33']

Download Source Code

$ git clone https://github.com/favtuts/java-core-tutorials-examples

$ cd java-basic/java8

References

1. Start Using Java Lambda Expressions
2. Oracle : Lambda Expressions
3. Oracle : Comparator