G52SWM
Lecture 02
OO and Java Refresher (2/2)
Peer-Olaf Siebers
Please note that the slides published AFTER the lectures and workshops are the official slides and are the ones that should be used for revision.
Week 2 Organisation
• Lecture 2:
– Going through more advanced Java topics
– Java Collections framework
– Implementation of object oriented principles
• Lab 2:
– Working further on the ZooApp example
– Looking at packages
• Workshop 2:
– CW1 Release
– IDEs + Java 9/10/11 additions
– Maintaining the ZooApp (basic maintenance)
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java collections framework
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Java Collections Framework
• What do we understand by “Collections” in Java?
– A collection is an object that represents a group of objects
– The Collections API is a unified framework for representing and manipulating collections,
independent of their implementation
• What does the abbreviation API stand for?
– Application Programming Interface
• What is the difference between a library and an API?
– A library contains re-usable chunks of code. These re-usable chunks of code are linked to your
program through APIs.
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Java Collections Framework
• Java Collections Framework principle ideas:
– We have container objects that contain objects
– All containers are either “collections” or “maps”
– All containers provide a common set of method signatures, in addition to their unique set of
signatures
• The framework contains data structures
– e.g. arrays; lists; maps
• The framework contains algorithmic operations
– e.g. searching; sorting
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Java Collections Framework
• Collection
– Something that holds a dynamic collection of objects
• Map
– Defines mapping between keys and objects (two collections)
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• Iterable
– Collections are able to return an iterator object that can
scan over the contents of a collection one object at a
time
Java Collections Framework
• Core collection framework interfaces
– Iterable: Represents an iterator object
– Collection: Represents a group of objects (elements)
– Map: Maps keys to values; no duplicate keys
– Queue: Represents FIFO queues or LIFO stacks
– Deque: Represents a double ended queue
– Set: A collection that cannot contain duplicate elements
– List: An ordered sequence of elements that allows duplicate elements
• Interface location
– Most interfaces can be found in the java.util.* package
– The “Iterable” interface can be found in the java.lang.* package
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Java Collections Framework
• Classes that implement the collection interfaces typically have names in the form
of
• Legacy classes (do not use)
– Vector (now ArrayList); HashTable (now HashMap); Stack (now ArrayDeque)
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“? extends E” means “some type
that either is E or a subtype of E”
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Java Collections Framework
• Non typesafe collections (do not use)
– Collection constructors are not able to specify the type of objects the collection is intended
to contain
– Need to cast objects when using them; a “ClassCastExeption” will be thrown if we attempt to
cast to the wrong type
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Java Collections Framework
• Typesafe collections with “Generics”
– Classes support generics by allowing a type variable to be included in their declaration; type
are declared for the reference and constructor
• You cannot type a collection using a primitive type
– Values of primitive types need to be put into objects of a suitable wrapper class
before they can be added to a collection
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ArrayList Class
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TreeSet Class
• TreeSet provides an implementation of the Set interface that uses a tree for
storage. Objects are stored in sorted, ascending order.
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HashMap Class
• HashMap is a Hash table based implementation of the Map interface. This
implementation provides all of the optional map operations, and permits null
values and the null key.
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Java Collections Examples
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http://www.java2novice.com/java-collections-and-util/
implementation of object oriented
principles
Aggregation and Composition; Inheritance; Polymorphism; Abstract Methods and
Classes; Interfaces
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Case Study: Zoo Management
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Aggregation and Composition
• What is the difference between the Aggregations and Compositions?
– Aggregation
• The object exists outside the other, is created outside, so it is passed as an argument (for example)
to the constructor
– Composition
• The object only exists, or only makes sense inside the other, as a part of the other
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Aggregation
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Composition
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Inheritance
• What is inheritance and why do we use it?
– Inheritance: Forming new classes based on existing ones
• A way to share/reuse code between two or more classes
– Superclass: Parent class being extended
– Subclass: Child class that inherits behavior from superclass
• Gets a copy of every field and method from superclass
– “is-a” relationship: Each object of the subclass also “is a(n)” object of the superclass and can
be treated as one
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Inheritance
• Example:
public class Zookeeper extends Employee {
…
}
• By extending Employee, each Zookeeper object now:
– Receives a copy of each method from Employee automatically
– Can be treated as an Employee by client code
– Zookeeper can replace (“override”) behavior from Employee
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Inheritance
• A subclass can call its parent’s method/constructor:
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Inheritance
• Every class is either
– a direct subclass of Object (no extends)
– a subclass of a descendant of Object (extends)
• Class Reptile extends Animal
• Class Amphibia extends Animal
• Class Animal extends Object
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Inheritance
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Inheritance
• Object creation process: Reptile r = new Reptile();
1. Create reference “r”
2. Start creating Reptile by entering Reptile constructor and making call to parent
3. Start creating Animal by entering Animal constructor and making call to parent
4. Create Object portion
5. Create Animal portion
6. Create Reptile portion
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Inheritance
• Which of these works?
– Reptile r = new Reptile();
– Animal a = new Reptile();
– Object o = new Reptile();
– Reptile r = new Animal();
– Animal a = new Object()
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Inheritance
• Casting primitives
double d;
float f;
d = f; // legal…no loss of information
f = d; // illegal…potential loss of information
• Casting references
Object o;
Reptile r;
o = r; // legal…a reptile is an object
r = o; // illegal…not all objects are reptiles
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Polymorphism
• What is the difference between polymorphism, method overloading, and
method overriding?
– Polymorphism
• Polymorphism is an object oriented concept
• Method overloading and method overriding are two forms of polymorphism
– Method overloading
• Methods with same the name co-exists in the same class but they must have different method
signature
• Resolved during compile time (static binding)
– Method overriding
• Method with the same name is declared in super and sub class
• Resolved during runtime (dynamic binding)
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Polymorphism
• Dynamic Binding
– At run time (dynamic) when a method is invoked on a reference the ACTUAL
OBJECT is examined and the “lowest” or closest version of the method is
actually run.
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Abstract Methods and Classes
• Any subclass of class
Animal has two choices:
– Define a eat method (i.e. { })
– Be abstract
• Note:
– Abstract classes may not be
used to instantiate or make
objects (new)
– References to abstract
classes are legal
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Abstract Methods and Classes
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Abstract Methods and Classes
• Abstract subclass
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Interfaces
• What is the difference between an abstract class and an interface?
– Java abstract class
• Can have instance methods that implement a default behaviour
• May contain non-final variables
– Java interfaces
• Methods are implicitly abstract and cannot have implementations
• Variables declared are by default final
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Interfaces
• Some explanations from the internet
– An interface is a contract: the guy writing the interface says, “hey, I accept things looking that
way”, and the guy using the interface says “Ok, the class I write looks that way”.
– An interface is an empty shell, there are only the signatures of the methods, which implies
that the methods do not have a body. The interface can’t do anything. It’s just a pattern.
– Abstract classes look a lot like interfaces, but they have something more: you can define a
behavior for them. It’s more about a guy saying, “these classes should look like that, and they
have that in common, so fill in the blanks!”.
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Reference: http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
http://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class
Interfaces
• Interfaces are less restrictive when it comes to inheritance
– While classes can only ever extend one other class (single inheritance), with interfaces we
can choose to implement as many interfaces as we like
– Implementing an interface means writing implementation code for each of the methods in
the interface
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Interfaces
• Some rules:
– Use the keyword “interface” instead of “class” to declare an interface
– Implement an interface with the “implements” keyword
– Because interfaces have no state and are only about method actions, using an action name
(ending in “able”) is often appropriate
– A class that implements an interface must provide implementations for all the methods in
the interface
– Similar to classes, you can build up inheritance hierarchies of interfaces by using the
“extends” keyword
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Interfaces
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Interfaces
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Useful Website
https://www.tutorialspoint.com/java/
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https://www.tutorialspoint.com/java/
https://www.tutorialspoint.com/java/
https://www.tutorialspoint.com/java/
And finally …
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Acknowledgement
• Slides based on material from
– Bill Leahy’s lecture slides
• http://www.cc.gatech.edu/~bleahy/xjava/cs1311xjava05_poly.ppt
– Maria Litvin’s & Gary Litvin’s book slides
• http://skylit.com/javamethods/ppt/Ch10.ppt
– Marty Stepp’s lecture slides
• http://www.cs.washington.edu/331/
• and others …
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http://www.cc.gatech.edu/~bleahy/xjava/cs1311xjava05_poly.ppt
http://www.cc.gatech.edu/~bleahy/xjava/cs1311xjava05_poly.ppt
http://skylit.com/javamethods/ppt/Ch10.ppt
http://skylit.com/javamethods/ppt/Ch10.ppt
http://skylit.com/javamethods/ppt/Ch10.ppt
http://www.cs.washington.edu/331/
http://www.cs.washington.edu/331/