Simple Objects Phil Tayco Slide version 1 0

Simple Objects Phil Tayco Slide version 1. 0 Created Sep 10, 2017

Primitive Data Types • All variables are defined with a data type and variable name int age = 21; double gpa = 3. 9; age 21 gpa 3. 9 • Memory for age and gpa is instantly allocated and contents are initialized • Process is automatically done because the language already knows these simple data types • All variables in memory have a location, but knowing exactly where they are is not important for primitives • Advantages arise when knowing how these

Memory References • Memory locations can be referred to by other variables (in C/C++, this is called a “pointer”) • A memory reference is essentially a variable that contains a memory address as its value • The value refers to a memory location where information is stored • When memory references are defined, the location it is pointing to must be reserved (a. k. a. memory must be “allocated”) • Modern programming languages have “no pointers”, but in fact they are handling a lot of the pointer process automatically for you • C and C++ gives you more memory management control, which can be both a blessing and a curse!

Memory References • Memory references are useful when handling complex data • In Java, memory reference management is automatically done by default when you use data types that are not primitive – Exception: String data types are technically treated as complex data types, but can be used the same way as primitives in many ways • Java is often referred to as a language with “no pointers”, but in fact it does use memory reference management…it just does a lot of the hard work for you • Nevertheless, it is important to know the process that occurs behind the scenes as it will become useful to understand later with advanced concepts

Memory References • When we declare a variable with a complex data type, the memory location it is referring to is yet to be defined Student me; me ?

Memory References • To allocate the memory, we request it with the “new” keyword Student me = new Student(); me 0 x 1234 abcd (Student data) • When the compiler sees “new Student()” it uses what it knows a Student looks like and looks in memory for a location large enough to store that data • 0 x 1234 abcd is an example of a numeric location where the Student data could reside • “me” is the variable that now refers to the location of the Student data. In Java, this is transparent to us, but we should know that this structure exists

Complex Data Types • But how does Java know what “a Student looks like”? • Integers and doubles are simple data types because they store one value. (age stores 21 and gpa stores 3. 9) • Conceptually, a Student can store much more types of information. Think of every Student that you may want to model in a program. Every Student has: – – – First name Last name Age Gpa ID • Each one of these elements of a Student is called a “property” (also called an “attribute”)

Complex Data Types • These properties make up the template for defining a Student • In the example, we declared a variable “me” whose data type is “Student” (this is the same as when we declared a variable “age” whose data type is an integer) • We assigned age a value of 21 after we declared it. Similarly, we will want to assign values to me’s properties • This means Student’s properties are like variables grouped together within (or “encapsulated”) the overall Student variable (“me”) – me has its own first name, last name, ID, etc. • As such, each property is defined like a variable

Complex Data Types public class Student { public String first. Name; public String last. Name; public int age; public int id; public double gpa; } • These properties are defined in a separate Java file called a “class” • The file is separate because a class is designed to be a “stand-alone” data type. Put another way, the class is intended to be used by any number of other programs

Complex Data Types • In another program (such as in the “main” function), we can assign values to these properties for the “me” variable • In memory, technically the location where the Student data was allocated is found through the memory address – a process called “dereferencing” Student me = new Student(); me. id = 1234; me. age = 21; me. first. Name = “Phil”; me 0 x 1234 abcd first. Name “Phil” last. Name id 1234 age 21 gpa

Complex Data Types • You can define as many different variables defined with primitive data types as memory allows (int x, int y, int z, etc. ) • Similarly, you can define as many different variables with classes (Student me, Student you, Student[] roster, etc. ) • Variables with primitive types are commonly referred to as just variables. A variable whose data type is a class is called an “object” • Objects are seen as “instances” of a class. Creating objects is also called “instantiation” • Object-oriented programming is the study of designing solutions primarily through the manipulation of such objects (variables with complex data types)

Exercise 1 • Design a simple class with 4 properties. Remember to think of a property as an attribute of your class. Ask yourself “what properties does every instance of this class have? ” • Create a program with 2 files – One file must implement your class design – The other file is the main program that creates 2 objects of the class • In your main program, demonstrate using the objects by assigning and displaying different values of each object’s properties • You can be as simple or as clever as you like with your program as long as you show using class design and object creation and management