Sunday, May 10, 2015

Sorting a collection by more than one field

Let's say we have an Employee Object with the fields as given below.

class Employee implements Comparable {

    private int empid;
    private String deptname;
    private String name;
    private float salary;

    public int getEmpid() {
        return empid;
    }

    public String getDeptname() {
        return deptname;
    }

    public String getName() {
        return name;
    }

    public float getSalary() {
        return salary;
    }

    public Employee(int empid, String deptname, String name, float salary) {
        this.empid = empid;
        this.deptname = deptname;
        this.name = name;
        this.salary = salary;
    }

    @Override
    public String toString() {
        return "Employee{" + "empid=" + empid + ", deptname=" + deptname + ", name=" + name + ", salary=" + salary + '}' + "\n";
    }

    @Override
    public int hashCode() {
        int hash = 3;
        hash = 23 * hash + this.empid;
        return hash;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Employee other = (Employee) obj;
        if (this.empid != other.empid) {
            return false;
        }
        return true;
    }

    @Override
    public int compareTo(Object o) {
        if (o instanceof Employee) {
            Employee other = (Employee) o;
            return this.name.compareTo(other.name);
        } else {
            throw new RuntimeException("Comparison object is not matching");
        }
    }
}

Requirement: We want to sort Employee objects first by deptname and then followed by employee name within each department.
public class MultipleSort {

    public static void main(String[] args) {
        List emplist = new ArrayList();
        emplist.add(new Employee(353744, "MCA", "Athiruban", 47000));
        emplist.add(new Employee(311344, "CSE", "Nandhini", 41000));
        emplist.add(new Employee(351144, "EEE", "Kishore", 44400));
        emplist.add(new Employee(353321, "MCA", "Karthik", 37000));
        emplist.add(new Employee(353711, "CSE", "Gopal", 37000));
        emplist.add(new Employee(353743, "IT", "AthiNivas", 67000));

        defaultSortandDisplay(emplist);
        sortByDeptAndThenByName(emplist);
    }

    private static void defaultSortandDisplay(List emplist) {
        Collections.sort(emplist);
        System.out.println("DefaultSortandDisplay->\n" + emplist);
    }

    private static void sortByDeptAndThenByName(List emplist) {
        Map> mapSortedByDept = new TreeMap>();
        List finallist = new ArrayList();
        for (Employee e : emplist) {
            if (mapSortedByDept.containsKey(e.getDeptname()) == true) {
                mapSortedByDept.get(e.getDeptname()).add(e);
            } else {
                List templist = new ArrayList();
                templist.add(e);
                mapSortedByDept.put(e.getDeptname(), templist);
            }
        }
        System.out.println("Ordered by Department->\n" + mapSortedByDept);

        Collection> collectionSortedByDept = mapSortedByDept.values();
        for (List indvlist : collectionSortedByDept) {
            Collections.sort(indvlist);
            finallist.addAll(indvlist);
        }
        System.out.println("Ordered by name within Department->\n" + mapSortedByDept);
        System.out.println("Final list->\n" + finallist);
    }
}

Thursday, May 7, 2015

Observer Pattern example



package designpatterns;

/*
 * Observer Pattern example
 */

class WeatherData {

    float temperature;
    float humidity;
    
    public float getTemperature(){
        return temperature;
    }
    
    public float getHumidity() {
        return humidity;
    }
    
    public void setTemperature(float t) {
        temperature = t;
    }
    
    public void setHumidity(float h) {
        humidity = h;
    }
}

/*
 * The below interface adds functionality to add, remove subscribers/clients 
 *  to the internal list so that they can be notified whenever the state of
 *  the weather changes.
 */
 
interface WeatherNotifier {
    public void notify();
    public void addListener(Object client);
    public void removeListener(Object client);
}

/*
 * WeatherData after implementing the above interface
 */

class WeatherData implements WeatherNotifier {
    private List subscribers;
    {
        subscribers = new ArrayList();
    }
    
    public void notify() {
        // notify/wake up all the subscribers to update their view with latest state information
        Iterator iterator = subscribers.iterator();
        while (iterator.hasNext()) {
            iterator.next().update();
        }
    }

    @Override    
    public void addListener(Object client) {
        subscribers.add(client);
    }
    
    @Override
    public void removeListener(Object client) {
        subscribers.remove(client);
    }

   public void setTemperature(float t) {
        temperature = t;
        dataChanged();
    }
    
    public void setHumidity(float h) {
        humidity = h;
        dataChanged();
    }
    
    private void dataChanged() {
        notify();
    }

    public float getTemperature(){
        return temperature;
    }
    
    public float getHumidity() {
        return humidity;
    }

}

interface WeatherReader {
    public void update();
}

class WeatherClient implements WeatherReader{
    WeatherData source;
    
    public void update() {
        System.out.println("Temperature is " + source.getTemperature());
        System.out.println("Humidity is    " + source.getHumidity());       
    }
}

Wednesday, May 6, 2015

Adapter Pattern example

Adapter Pattern

It helps to assign/convert one interface (called as source) to another distinct interface (called as target).

Example Description:

The below code helps us to use legacy Enumeration class available in Vector class in place of modern Iterator class.

package designpattern;

import java.util.Enumeration;
import java.util.Iterator;
import java.util.Vector;

/**
 *
 * @author DELL
 */
public class AdapterPatternExample {

    public static void main(String[] args) {
        demo();
    }

    public static void demo() {
        Vector namelist = new Vector();
        namelist.add("Athiruban");
        namelist.add("Athinivas");
        Enumeration legacyIterator = namelist.elements();
        Iterator iterator;

        LegacyIteratorAdapter adapter = new LegacyIteratorAdapter(legacyIterator);
        //iterator = legacyIterator;    /* this code will fail */
        iterator = adapter;
        for (; iterator.hasNext();) {
            System.out.println(iterator.next().toString());
        }
    }
}

class LegacyIteratorAdapter implements Iterator {
    /*
     * The purpose of the class to adapt the legacy Enumeration interface to
     * modern Iterator interface.
     */

    Enumeration legacyIterator;

    public LegacyIteratorAdapter(Enumeration e) {
        legacyIterator = e;
    }

    @Override
    public boolean hasNext() {
        return legacyIterator.hasMoreElements();
    }

    @Override
    public Object next() {
        return legacyIterator.nextElement();
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }
}

Saturday, May 2, 2015

Coding refresher on Github

It gives an immense pleasure to share my technical experience with fellow people. I started pushing all my local code fragments/programs to github as a way of helping young/new programmers on how to code in Java.

Github link to my program list that helped me to attain "Oracle Certified Java Professional".

https://github.com/athiruban/CodingRefresher

Many new interesting topics on DataStructure and Algorithms will be added to the above repo.

Please visit and share your comments on how we can improve this further.