Objektorienteeritud programmeerimine (Narva Kolledž) 2018/19 kevad

Session 6

Some Java interfaces

1. java.lang.Comparable

Interface Comparable has method compareTo to compare two objects. The interface is used in sorting algorithms. Using different implementations, we can define on what bases objects can be compared and sorted. It is a very important functionality when we want to compare instances of our own classes. For example, if we want to compare area of triangles (not triangle edges), we have to upgrade Triangle class as follows:

//modify Triangle class with the following lines, the rest leave unchanged
public class Triangle implements Polygon, Comparable<Triangle> {

  //implement abstract method compareTo of interface Comparable
  public int compareTo(Triangle compareWith) {
    if (this.area() < compareWith.area())
      return -1; // negative value shows that this object is smaller than the argument object (compareWith)
    if (this.area() > compareWith.area())
      return 1; // positive value indicate that this object is larger than the argument object
    return 0; // zero means that both objects have the same area
  }
}

2. java.util.List

Interface List describes functionality that we already know, e.g. add, get and size. List is an interface because it is implemented in different classes, e.g.:

• ArrayList implements a fast search of an element by its index;
• LinkedList implementation allows a fast insertion and removal of elements by indexes.

3. java.lang.Iterable

Iterable interface describes method iterator. Iterations are useful for discovering content of objects. For example, ArrayList implements Iterable interface. In the following syntax for (T element : elements), Java does not control if elements are placed in an array or a list; Java controls if the data type is Iterable. That means that everyone can create a class whose content can be looked through using for loop.

Session 6