Java 8 Lambda排序 : Comparator example

1. Classic Comparator example.

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

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

    }
    
}
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Output

Before Sort
Developer [name=mkyong, 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=mkyong, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]
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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());
        }
    });                
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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();
        }
    });    
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Lambda expression example :

TestSorting.java
package com.mkyong.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("mkyong", 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;

    }
    
}
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Output

Before Sort
Developer [name=mkyong, 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=mkyong, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]
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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());
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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()));        
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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()));
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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);
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Output

Developer [name=mkyong, 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]
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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());
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Output

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

http://www.mkyong.com/java8/java-8-lambda-comparator-example/