Lilian Blot BUILDING CLASSES Java Programming Spring 2014 TPOP 1

Lilian Blot Overview Instance Attributes & constants Access Level Modifier Constructors Methods Method overloading & method’s signature First Attempt at Inheritance Spring 2014 TPOP 2

Lilian Blot Instance Attributes What does public/private means?  Access modifiers in Java  a 3 rd one exists: protected One public class per file Instance attributes are declared outside the constructor. Spring 2014 TPOP 3 public class Item { private String title, UID; private Member member; private boolean status; … } Java Code

Lilian Blot Access Level Modifier modifierClassPackageSubclassWorld public YYYY protected YYYN no modifierYYNN private YNNN Spring 2014 TPOP 4 Access Level Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:  At the top level—public, or package-private (no explicit modifier).  At the member level—public, private, protected, or package-private (no explicit modifier).

Lilian Blot Tips on Choosing an Access Level If other programmers use your class, you want to ensure that errors from misuse cannot happen.  Access levels can help you do this. Use the most restrictive access level that makes sense for a particular member.  Use private unless you have a good reason not to. Avoid public fields except for constants.  Public fields tend to link you to a particular implementation  limit your flexibility in changing your code. Spring 2014 TPOP 5

Lilian Blot Constants Keyword final to create a constant  must be declared and initialised in same statement  convention: use upper case, word separated by underscores Keyword static for declaring a class constant/variable  called using name of class, e.g. Item.ON_LOAN. Spring 2014 TPOP 6 public class Item { // Declaration of constants (keyword final) public static final boolean AVAILABLE = true; public static final boolean ON_LOAN = false; public static final int PRINT_ALL = 1; private String title, UID; private Member member; private boolean status; Java Code

Lilian Blot Constructor The constructor is declared using the name of the class, followed by zero, one or more parameters  no need to have a self parameter  this is the equivalent of self in Python Spring 2014 TPOP 7 public class Item { … private String title, UID; … // the def __init__(self,...) in python public Item(String title, String UID){ this.title = title; // is the equivalent of in Python this.UID = UID; this.member = null; this.status = AVAILABLE; } Java Code

Lilian Blot Constructor Spring 2014 TPOP 8 public class Item { private String title, UID; // the def __init__(self,...) in python public Item(String title, String UID){ this.title = title; // is the equivalent of in Python this.UID = UID; this.member = null; this.status = AVAILABLE; } Java Code class Item(object): # the def __init__(self,...) in python def __init__(self, title, UID){ self.title = title; self.UID = UID; self.member = None; self.status = AVAILABLE; Python Code

Lilian Blot toString( ) method For a non-void function/method we must declare the type of the returned value  we MUST use the return statement Spring 2014 TPOP 9 public class Item { … public Item(String title, String UID){ … } public String toString(){ return ("<Item: uid = " + this.UID + ", " + this.title + ", available= "+this.status+", borrower= "+this.member+'>'); } Java Code Type of the returned value non-void function/method MUST use return statement method’s name

Lilian Blot print( ) method For a void function/method we must declare the type of the returned value as void.  No return statement needed  the return; statement could be used to exit the method Spring 2014 TPOP 10 public class Item { … public Item(String title, String UID){ … } public void print(int option){ if(…){ … } … Java Code Use keyword void to declare a void function/method method’s name

Lilian Blot Calling methods Spring 2014 TPOP public class Item {... public static void main(String[] args){ Item anItem = new Item(“The Joy of programming”,”book1001A”); anItem.print(Item.PRINT_ALL); System.out.println(anItem.toString()); // Equivalent to: System.out.println(anItem); } Java Code Method’s name Constructor’s nameKeyword to create a new object Object’s name Implicit call to the toString() method 11

Lilian Blot Method Signature In the class Item, we have two methods:  public void print() {…}  public void print(int option){…} It is authorised in Java (not in python) Java use method signature to differentiate them  the signature is composed of the name of the method  the number, type of parameters, and their order  NOT the returned type Nor the modifier Nor the parameter’s name  e.g. print() and print(int). This is called method overloading.  constructors can be overloaded Spring 2014 TPOP 12

Lilian Blot First Attempt at Inheritance Can only inherit from one super class  we MUST call a constructor from the super class  use of keywords extends and super. There is more, but not for now! Spring 2014 TPOP 13 public class DVD extends Item{ … public DVD(String title, String UID, String director){ super(title, UID); // calling the constructor Item(String, String) from the // super class Item // MUST be the first line of code in the DVD's constructor this.director = director; } Java Code Keyword extends to indicate inheritance MUST call a constructor from super class Name of super class

Lilian Blot Summary Now you should be able to create your own classes  declare instance attributes  declare class constants  declare constructors and methods  method signature and overloading Understand Access Level Modifier in Java Use inheritance  call constructors from super class Spring 2014 TPOP 14

Lilian Blot Exercises Rewrite part of your Library program in Java Spring 2014 TPOP 15