CLASS AND OBJECTS Valerie Chu, Ph.D. LeMoyne-Owen College 1 Existing Data Types Declare a variable: 2 LeMoyne-Owen College Create your own data type 3 LeMoyne-Owen College Class & Objects Class blueprint e.g., Course Object house e.g., JAVAI, JAVAII, Data Structures,etc. Characteristic of an object Instructor, number of seats,etc. Behavior Change instructor, increase number of seats,etc. LeMoyne-Owen College 4 5 Class & Objects (cont.) Characteristic Instance Variable A storage not take any action, no (). private only methods within the same class can view or change it. Data is protected. encapsulation. data type to know how large of storage to be reserved. variable name no space (one word only) e.g., numOfStudents. LeMoyne-Owen College 6 Class & Objects (cont.) Behavior Method or Constructor Take an action () is needed. public client (main method) can call it. Interface between client and methods. return data type what type information the method return to caller? void - no return information. parameter information needed from caller. - Can be only used within the block of the method, it located. - data type is needed. LeMoyne-Owen College 7 Constructor A special method Take an action : Constructors job: Initialize objects; i.e. initialize instance variables. Be called when a new object of the class is declared. Has the same name as the class name. No return data type. Number of parameters usually, not always, is same as number of instance variables. public LeMoyne-Owen College 8 Declare a class 9 LeMoyne-Owen College 10 LeMoyne-Owen College 11 LeMoyne-Owen College 12 LeMoyne-Owen College Default constructor If a programmer did not set any constructor, the Java compiler has a default constructor with no parameters. Set zero for primitive number types. Set false for boolean values Set null for references (e.g. String type) 13 LeMoyne-Owen College Example 14 LeMoyne-Owen College What if a constructor is set? 15 LeMoyne-Owen College 16 LeMoyne-Owen College What if a constructor is set? (cont.) 17 LeMoyne-Owen College What if a constructor is set?(Cont.) A constructor is created in a class. Default constructor is turned off. The no-argument constructor is required if a client creates an object with no initial values. Constructor Overload 18 LeMoyne-Owen College 19 Constructor Overload LeMoyne-Owen College 20 LeMoyne-Owen College 21 LeMoyne-Owen College 22 LeMoyne-Owen College 23 LeMoyne-Owen College Compile and run javac *.java java CourseTest 24 LeMoyne-Owen College Compare two objects 25 LeMoyne-Owen College