Introduction to Java Chapter 7 Classes and ObjectOriented

Introduction to Java Chapter 7 Classes and Object-Oriented Programming Chapter 7 - Classes & Object-oriented Programming 1

Introduction to Java Classes • A class is the basic unit of the Java language. It is the “blueprint” for the objects created from that class. • Each class contains some data definitions (called fields), together with methods to manipulate that data. – When the object is instantiated from the class, an instance variable is created for each field in the class. • The methods serve as an interface to isolate the data in the class from the outside world. Chapter 7 - Classes & Object-oriented Programming 2

Introduction to Java Class Hierarchy • All classes form a part of a class hierarchy. – Classes below a given class are subclasses of the class. – Classes above a given class are superclasses of the class. The class immediately above a given class is known as its immediate superclass. • A class inherits both instance variables and methods from it’s immediate superclass. It can additional variables and methods, and it can override (change) the inherited methods. Chapter 7 - Classes & Object-oriented Programming 3

Introduction to Java Structure of a Class • The major components (members) of a class are: – Fields - define the instance variables to be created when an object is instantiated from the class. – Constructors - special methods the define how to initialize variables when an object is instantiated. – Methods - implement the behaviors of a class. – Finalizer - a special method to perform cleanup before an object is destroyed. Chapter 7 - Classes & Object-oriented Programming 4

Introduction to Java The Member Access Operator (. ) • The members of a class (instance variables and methods) are accessed using the member access operator, or dot operator (. ) – To access a member, the user names a reference to a object, followed by the dot operator, and followed by the member name (with no spaces) • Examples: Obj. a Obj. method. A Access instance variable a Access method. A Chapter 7 - Classes & Object-oriented Programming 5

Introduction to Java Example: Timer Class definition Instance variable definition Constructor Method reset. Timer Note: This method does not have a finalizer. Method elapsed. Time Chapter 7 - Classes & Object-oriented Programming 6

Introduction to Java Scope • Instance variables have class scope, meaning that they are visible everywhere within a class, including within methods – Example: variable saved. Time in class Timer. • Local variables within a method have block scope, meaning that they are only visible within the block in which they are defined. – Example: variable e. Time in method elapsed. Time. Chapter 7 - Classes & Object-oriented Programming 7

Introduction to Java Hidden Instance Variables • It is possible for a local variable to have the same name as an instance variable, hiding it within a block. • In this case, the hidden instance variable can be accessed using the this reference. Instance variables Local variables Reference hidden instance variables using this Chapter 7 - Classes & Object-oriented Programming 8

Introduction to Java Common Types of Methods Found in a Class • Certain type of methods are common to many classes – “set methods” are used to set the values of instance variables – “get methods” are used to read the values of instance variables – predicate methods return a true/false result based on some test. – to. String method creates a string representation of the contents of the object Chapter 7 - Classes & Object-oriented Programming 9

Introduction to Java Packages • A package is a group of classes and methods that share some related purpose. • All standard Java classes are grouped in packages, such as java. lang, java. io, etc. • To use the contents of any package except java. lang, it must first be imported into a program with an import statement. • The import statements must be the first noncomment lines in a program! Chapter 7 - Classes & Object-oriented Programming 10

Introduction to Java Creating Your Own Packages • You can create packages for your own methods: – First, add a package statement to each class specifying the package that it belongs to. Place the statement before the class definition: package chapman. testpackage; – Then, compile using the -d option to specify the root directory of the package structure: javac -d d: packages My. Class. java; – These commands will place file My. Class. class in directory d: packageschapmantestpackage. Chapter 7 - Classes & Object-oriented Programming 11

Introduction to Java Example Creating My. Class package statement Compilation statement The class file will now be in directory d: packageschapmantestpackage Chapter 7 - Classes & Object-oriented Programming 12

Introduction to Java Using Your Own Packages • To use the classes in your packages, you must add an import statement to each class wanting to access the packages. • In addition, you must add the root directory of the package structure to the CLASSPATH environment variable. • Example: If class chapman. testpackage. My. Class is in directory d: packagechapmantestpackage, then the root directory of the package structure is d: package, and it must appear in a CLASSPATH statement. Chapter 7 - Classes & Object-oriented Programming 13

Introduction to Java Example Using My. Class Set class path in environment import statement Chapter 7 - Classes & Object-oriented Programming 14

Introduction to Java Member Access Modifiers • Member access modifiers control where a class member can be accessed from: – public: Members can be accessed from any class – private: Members can only be accessed from within the class that they are defined in – <none>: With no modifier, members can be accessed from any class in the same package as the class that they are defined in. – protected: Access from the same package or from any subclass of the class that they are defined in Chapter 7 - Classes & Object-oriented Programming 15

Introduction to Java Finalizers • A finalizer is a special method named finalize, which performs any necessary clean-up (releasing resources, etc. ) before an object is destroyed. • It is automatically called just before an object is destroyed. • Most classes do not need a finalizer. Chapter 7 - Classes & Object-oriented Programming 16

Introduction to Java Garbage Collection • When an object is no longer needed, it is automatically destroyed by a low-priority thread called the garbage collector. • Destroying old objects returns their resources to the system for re-use. • Any object that no longer has a reference pointing to it is a candidate for garbage collection. • The garbage collector calls a class’s finalizer before destroying it. Chapter 7 - Classes & Object-oriented Programming 17

Introduction to Java Static Variables • A static variable is a special type of variable that is shared by all objects instantiated from a class. • Static variables are useful for keeping track of global information such as the number of objects instantiated from a class, etc. • They are declared with the static keyword: private static int created; // Static! • Static variables are also useful for declaring a single shared copy of a constant: static final double C = 2. 99792458 e 8; Chapter 7 - Classes & Object-oriented Programming 18

Introduction to Java Class A definition with two instance variables and one static variable Example x an y are unique in each object, while s is common to both Two objects a 1 and a 2 instantiated from class A Chapter 7 - Classes & Object-oriented Programming 19

Introduction to Java Static Methods • Static methods are methods that can be executed without first instantiating an object of the class containing the method. • Static methods can access the static variables in a class, but they cannot access instance variables. • Static methods are normally used for utility calculations that are independent of the data in a class, such as sin(), cos(), tan(), etc. Chapter 7 - Classes & Object-oriented Programming 20

Introduction to Java Example: Extended Math Class static variable static methods Chapter 7 - Classes & Object-oriented Programming 21