Java 8 Lambda: Comparator

Question

I want to sort a list with Lambda:

List messagesByDeviceType = new ArrayList();      
messagesByDeviceType.sort((Message o1, Messag         


        

Answer 1

Comparator

We use comparator interface to sort homogeneous and heterogeneous elements for default, customized sorting order.

int compare(T o1, T o2);

it takes two arguments for ordering. Returns a

    negative integer(-1) « if first argument is less than the other
    zero             (0) « if both are equal
    positive integer (1) « if first greater than the second.

Anonymous Classes how to sort a list of objects in prior versions of Java 8 using inner Classes.

An anonymous class cannot access local variables in its enclosing scope that are not declared as final or effectively final.

Comparator timeCompare = new Comparator() {
    @Override public int compare(Employee e1, Employee e2) {
        return e1.getCreationTime().compareTo( e2.getCreationTime() );
    }
};

Java 8 Lambda Expressions uing compare method

A lambda expression is like a method: it provides a list of formal parameters and a body - an expression or block - expressed in terms of those parameters.

LambdaExpression: LambdaParameters -> LambdaBody

Any local variable, formal parameter, or exception parameter used but not declared in a lambda expression must either be declared final or be effectively final, or a compile-time error occurs where the use is attempted.

Comparator functional_semantics = (e1, e2) -> {
   return e1.getCreationTime().compareTo( e2.getCreationTime() );
};

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Basic Sort with Lambda Support

Comparator timeCompareLambda = (o1, o2) -> (int) ( o1.getCreationTime() - o2.getCreationTime());
Collections.sort(java8, timeCompareLambda );

Using Extracted Key and Comparing method: A comparator that compares by an extracted key. Pass references using :: keyword.

static  Comparator comparingLong(ToLongFunction keyExtractor)

ToLongFunction keyExtracor = Employee::getCreationTime;
Comparator byTime = Comparator.comparingLong( Employee::getCreationTime );

Sample Test Code:

public class Lambda_Long_Comparator {
    public static void main(String[] args) {

        List java7 = getEmployees();

        // Sort with Inner Class
        Comparator timeCompare = new Comparator() {
            @Override public int compare(Employee e1, Employee e2) {
                return e1.getCreationTime().compareTo( e2.getCreationTime() );
            }
        };

        // Collections.sort(list); // Defaults to Comparable « @compareTo(o1)
        Collections.sort(java7, timeCompare); // Comparator « @compare (o1,o2)
        System.out.println("Java < 8 \n"+ java7);

        List java8 = getEmployees();
        Collections.sort(java8, Comparator
                .comparing( Employee::getCreationTime )
                .thenComparing( Employee::getName ));
        //java8.forEach((emp)-> System.out.println(emp));
        System.out.println("Java 8 \n"+java8);
    }

    static List getEmployees() {
        Date date = Calendar.getInstance().getTime();
        List list = new ArrayList();
        list.add( new Employee(4, "Yash", date.getTime()+7));
        list.add( new Employee(2, "Raju", date.getTime()+1));
        list.add( new Employee(4, "Yas", date.getTime()));
        list.add( new Employee(7, "Sam", date.getTime()-4));
        list.add( new Employee(8, "John", date.getTime()));
        return list;
    }
}
class Employee implements Comparable {
    Integer id;
    String name;
    Long creationTime;

    public Employee(Integer id, String name, Long creationTime) {
        this.id = id;
        this.name = name;
        this.creationTime = creationTime;
    }

    @Override public int compareTo(Employee e) {
        return this.id.compareTo(e.id);
    }

    @Override public String toString() {
        return "\n["+this.id+","+this.name+","+this.creationTime+"]";
    }

    // Other getter and setter methods
}

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