Designing utility methods in ruby

This demo is to introduce ways of defining static methods in ruby. Generally utility classes will not store any state information and thread safe.

# Demo for defining utility methods in ruby
# In Java it is known as static methods

module Universe
  module Earth
    def sayHello
      puts "Hello from earth!"
    end

    def sayHi
      puts "Hi from earth!"
    end

    def sayBye
      puts "Bye from earth!"
    end
  end
end

module Universe
  module Moon
    class << self
      include Universe::Earth
      private
        def foobar
          puts "Hello from foobar!"
        end
    end
  end
end

Universe::Moon.sayHello

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);
    }
}

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());       
    }
}