Test 1 Flashcards
Java Structure
public class myProgram {
variable_type variable_name;
public static void main(String[] args){
// variable declaration
// IO statements
// if- statements
// loops
// method calls
} //end of main
}// end of classPackages
▪ A package creates a namespace for a set of classes.
▪ With packages, it is possible to have different classes/methods with the same name
in different packages.
▪ A package can contain a subpackage that should be accessed by
name_of_the_package.name_of_the_subpackage
▪ Java.lang, java.io
Variable
A variable is a name that refers to a value
It’s a placeholder to store a value
To store, assignment operator is used.
To declare a variable, its data type should be given.
Data Types
Specifies the size of storage in which you store data, a set of acceptable values and operations. 33 Name Size (bit) Value Range Defined operators byte 8 (-128) – (127) + , -, * , / short 16 (-32,768) – (32,767) + , -, * , / int 32 (-2 31 ) - (231 -1) + , -, * , / long 64 (-2 63 ) – (263 -1) + , -, * , / float 32 + , -, * , / double 64 + , -, * , / boolean 1 True/ false (0 or 1) &&, ||, ! char 16 unicode 0-65535 , >=, ==, !=
Memory Diagram/Model– Primitive Variable
• Memory is like a set of lockers. • A locker has a number • Lockers accommodate items with different sizes. • Memory is different from lockers • All memory spaces are the same size int height = 165; double weight = 58.5; char initial = 'D'; boolean found = true;
A Chain of Function Call
Function A (….){
A_stmt1;
B(…);
A_stmt2;
}
Function B (….){
B_stmt1
;
C(…);
B_stmt2;
}
Function C (….){
C_stmt1;
D(…);
C_stmt2;
}
Function D (….){
D_stmt;
}
Variables Types
Java cares about the type. Java is a strongly typed language.
▪ Variables must have a type to indicate the size of the container (i.e. memory space)
that it requires.
▪ A primitive data type is the one, whose size is fixed.
▪ Non-primitive data types are flexible in size.
▪ Can you give an example when data types with flexible size come handy?
Non-primitives
There are a lot of non-primitive built-in data types that you are familiar
with
▪ String, Math, Scanner, ArrayList, Vector, StringBuilder, StringBuffer
▪ Non-primitive data types are also called reference types.
▪ Why do you think is the rational behind this naming?
4
Fixed
Memory
Spaces
=
Program
Stack
growable
Memory
Spaces =
Garbage
Collectible
Heap
100
3000
public static void main (String[] args) {
double grade = 98.5;
String name = "Marzieh";Non-primitive data types
▪ Reference types have zero or more states.
▪ Their states are presented by their instance
variables (AKA fields, attributes)
▪ There are zero or more behaviours
associated with them.
▪ Behaviours are presented by functions.
▪ In object-oriented programming, functions are
called methods.
▪ Example: Self-driving cars
Classes
A bundle of instance variables and methods is called a class. • A class is a data type.
Objects
A variable defined using a class (non-primitive/ reference type) is called an
object.
• An object is an instance of a class.
• To make an object ‘new’ operator is used followed by the name of the class.
• To get access to the objects’ property (i.e. fields & methods), dot ‘.’ operator is
used.
• With one class (reference type), you can define as many objects as you require.
Just like primitive variables, when you define an object you tell java to reserve a
space in memory for the object
The keyword to order the reservation is ‘new’.
• In which part of the memory, the space is reserved?
▪ Creating an object is a 3 steps process:
▪ Declaration: Student john
▪ Space reservation: new Student();
▪ Address Assignment: Student john = new Student();
11
Stack
GCH
int noOfStudents = 1;
Student john = new Student();
Arrays are non-primitive data type
Just like other reference types, arrays need to
request for a memory space.
▪ Example: initialize the courseTaken array in class
student.
A class is a blueprints for objects
A class represent a data type by which you can define a variable. ▪ The variable that you define using a class, is called an object. ▪ This variable holds a reference to the actual object. ▪ You define one class (i.e. Student) by which you can define as many objects as you need (i.e. all the students who enrolled in EECS2030) ▪ Up to now, you used built-in classes (e.g. String, Math, Scanner, etc) to define an object, but from now on you can create your own classes. ▪ It’s good to know: For simplicity in the memory diagram presented in this course both GCH and Stack grow down when a variable is added. In reality, however, stack grows down, and heap grows up.
Constructors
A special method with no return value. ▪ It is not a void. ▪ It is the same name as the class. ▪ It is used to initialize the instance variables. ▪ Does it make sense now as to why you use the name of the class after ‘new’? ▪ new Student(); ▪ When I didn’t have any constructor for Student, I didn’t get any error for new Student(); why? ▪ This type of the constructor, in which you initialize the attributes to the default values, is called default constructor.
To combine the object creation and instance variable initialization, a constructor can accept parameters.
this
‘this’ is a keyword that refers to the instance variables of the current object.
public Student (char initial, int studentId, int [] courseTaken, int enrolmentYear) {
this.initial = initial;
this.studentId = studentId;
this.enrolmentYear = enrolmentYear;
this.courseTaken = new int [courseTaken.length];
For (int i= 0; i < courseTaken.length; i++)
this.courseTaken[i] = courseTaken[i];
}
Overloaded Constructors
▪ To give the user of the class the flexibility on creating an object, multiple version of the constructors can be defined. ▪ These constructors are different in the number or types of the arguments that they accept. ▪ They are called overloaded constructors.
Constructor Chaining & Overloaded constructors
To avoid duplicate code, constructors are chained.
▪ To chain the constructors, this()is used.
▪ this() can have zero or more parameters, depending on which constructor is
called.
Copy constructors
It is possible to clone an object.
▪ This type of constructors are called copy constructors.
▪ The input parameter is an object of the same type.
Garbage Collection Mechanism
Objects are stored in the heap. ▪ If an object is no longer needed, the memory space that is allocated to them should be released. ▪ This is a process that is performed by Garbage Collection in Java.
Encapsulation
The first object-oriented principle.
▪ Data encapsulation refers to hiding the internal states
of an object
▪ an implementer controls the clients’ access to the instance
variables.
▪ The first step is to make the instance variables
inaccessible to the clients.
▪ This is done by choosing right access modifiers.
Access Modifiers
To define an access level for
a variable, method or
constructors access
modifiers are used.
Access modifiers
Access Modifier 1 within class 2 within package 3 outside package by subclass only 4 outside package Private Y N N N Default Y Y N N Protected Y Y Y N Public Y Y Y Y
Accessors and Mutators (AKA getters & Setters)
When data is encapsulated, the only way by which you can get access to the data is
via the object methods.
▪ Mutators (setters): updates the value of the instance variable.
▪ You can check the validity of the value that you assign to the instance variable.
