Built in Data Types

• Java originally had many predefined classes for some built in collective data types:
  • Vector Stack Hashtable Bitset ….
• You could choose the one you need.
  • But changing from one to another required significant updates.
• Idea: Organize the data structures by functionality.

<aside> 💡 Create a hierarchy of abstract interfaces and concrete implementations → Collection

</aside>

The Collection Interface

• The collection interface abstracts properties of grouped data:
  • Like Arrays, lists, sets,….
  • But NOT key-value structures like dictionaries.

public interface Collection<E>{
	boolean add(E element);
	Iterator<E> iterator();
	...
}

• add() → Add to collection

• iterator() → Get an object that implements Iterator interface.

  public interface Iterator<E>{
  	E next();
  	boolean hasNext();
  	void remove();
  	...
  }
  

• With this iterator: you can loop through the elements.

  Collection<String> cstr = new .... ;
  Iterator<String> iter = cstr.iterator();
  while(iter.hasNext()){
  	String element = iter.next();
  	// Do something with element.
  }
  

<aside> 💡 Java later added for each loop → Implicitly creates an iterator and runs through it.

</aside>

Collection<String> cstr = new .... ;
for(String element : cstr){
	// do something with element
}

• Can use generic functions to operate on collections due to its hierarchy.

  public static <E> boolean contains(Collection<E> c, Object obj){
  	for(E element : c){
  		if(element.equals(obj)){
  			return true;
  		}
  	}
  	return false;
  }
  

  • Note: How does element.equals(obj) works??? → Answer

Removing Elements

• Recall the interface Iterator:

  public interface Iterator<E>{
  	E next();
  	boolean hasNext();
  	void remove();
  	...
  }
  

• Note the remove() method → which element does it remove ( first element, last element, any other element….????)