ECE122 Feb

Introduction to Class and Object Java is an object-oriented language. Java’s view of the world is a collection of objects and their interactions. Class is a template, or blueprint to build objects. Objects are instantiated (“created”) from the class specification.

What is a class? A class is a template that defines the form of an object. It specifies both the data and the code that will operate on that data. A class is a set of plans that specify how to build an object. A class is a logical abstraction. It is a blueprint that resides in the the Java class file. It has data: variables with certain data types. Some data are associated with each individual object, called instance variables. Other data are associated with each class, called class variables, which are identified by keyword “static”. It has methods: the actions that act on those data. The methods also have two types. Instance methods are associated with each object, and class methods (identified as “static”) are associated with the whole class.

General Forms of class definition class classname { //declare variables type var1; type var2; …. //declare methods type method(parameters) { //body of method, consists of statements } type method2(parameters) { //body of method, consists of statements } …. }

A class example public class Vehicle { //declare variables int passengers; //number of passengers int fuelcap; //fuel capacity in gallons int mpg; //fuel consumption in miles per gallon //omitted method declaration }

What is an object? An object is an instantiation of a class. According to blueprints specified in the class, an object has data (variables) and actions (methods) that act on those data. One or more object of the same class can be created in computer memory according to their blueprint, class.

How is an object created? Objects are created through operator “new”. Vehicle minivan; //declare reference to object minivan = new Vehicle(); //construct a Vehicle object in memory Step 1: declare a variable called minivan of the class type Vehicle. This is just a variable that can refer to an object. At the point, minivan contains value “null”, meaning it is not referring to any object yet. Step2: Allocate memory in computer and construct an object of type “Vehicle” in that memory segment. This is done by using operator “new”. Then, assign to “minivan” a reference to that object.

How do I access an object’s data? Object contains data that you can access with operator “.” in the form of object.variablefor instance variable e.g. minivan.passengers = 7; minivan.fuelcap = 16; minivan.mpg = 21; Or class.variablefor class variable e.g. boolean b = Vehicle.hasEngine; Use variable directly without operator “.” if you reference object data within a method of same object.

Primitive variable and assignment Assignment of primitive type is a straight- forward copy of value from one variable to another. E.g. int i = 1; int j = i;//the value contained in I is copied to j

Reference Variable and Assignment When you assign one reference variable to another reference variable, the second variable begins to reference the object the first reference variable is referencing to. v = minivan; //both v & minivan are objects of Vehicle class After this assignment, object v will reference the memory that object minivan references to. What happens to the memory block the v referenced to before the assignment? It depends. If there are other object variables reference it. That memory block stays untouched. If no more object variables reference it, it is no longer usable, and become a “garbage”. It will wait for the garbage collector to free the memory and recycle it back to system resource.

Illustration of Reference Variable Assignment v minivan v v=minivan; Garbage collected Memory block

Methods Methods are subroutines that act on object’s data. A method can call other methods of its own class, it can also call methods of other classes. Object interacts with each other through method calls. You can model the real world through object interactions. Instance method is created and associated with each object created. Class method is created when the class is loaded and class method is shared among all the objects of the same class.

General Form of a Method ReturnType MethodName(parameter-list) { //body of method } MethodName is the name of the method as a string with no space. Capitalize first letter of each word as a convention. ReturnType is any primitive type, or object reference. If nothing is returned from the method to the method-caller, a “void” is declared. Parameter-list is a comma separated list of type and variable pair. Each pair pass a value of certain data type to the method. The body of a method consists of statements.

Add methods to Vehicle Class Add an instance method to Vehicle class: void range() { System.out.println("Range is " + fuelcap*mpg + " miles."); } Add a class method to Vehicle class: static int calcRange(int fuelCap, int mpg) { int theRange = fuelCap * mpg; return theRange; }

Call a method Call an instance method objectReference.methodName(parameterlist); minivan.range(); Call a class method ClassName.methodName(parameterlist); Vehicle.calcRange(18,33); If the caller of the method is another method of the same object, just use the methodName, without operator dot.

Java methods pass by value A java method can use parameters. A method caller can passes arguments. The data type of argument normally is the same as that of the parameter. Otherwise, an explicit/implicit cast will happen. Java pass everything (primitive type & reference type) by VALUE. For primitive type, this means copy of value. For reference type, this means you cannot change the object the original argument variable reference to, but you can change the data contains within the original object.

Assignments Practice Vehicle.java Read “Java2” P Read “Head First Java” p26 – 29, 32-37,52- 57, 69-77