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COMP200 JAVA REVIEW
OOP using Java, from slides by Shayan Javed
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Introduction to Java
High-level language.
Paradigm: Object-Oriented.
WORA (“write-once, run anywhere”)
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Introduction to Java
Some languages are compiled (C/C++)
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Introduction to Java
Some languages are compiled (C/C++)
Some are interpreted (Python, Perl, Ruby, etc.)
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Introduction to Java
Some languages are compiled (C/C++)
Some are interpreted (Python, Perl, Ruby, etc.)
What about Java?
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Introduction to Java
Source code (.java)
Bytecode (.class)
Interpret and run
Compilation and Interpretation
.java = compiled to .class
.class = interpeted by Java Virtual Machine (JVM)
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Java Syntax
Similar to C/C++. Variables: byte, short, int, long, float, double, char, boolean
Field Modifiers: final (“constant”) , static (applies to classes)
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Java Operators
Numerical: +, -, *, /, % Boolean: >, <, <=, >=, ==, !=, !, instanceof Others: ++, --, Bitwise operators: & (AND),
^ (XOR), | (OR), <<, >> (shift)
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if statements
if (boolean-expression) {
…
}
else if (…) {…
}
else {
…
}
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Conditional expression
boolean-expression ? expression1 : expression2
Example:
int x = 3;
int y = (x > 0) ? 1 : 5;
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switch statement
switch (byte/short/char/int/String/etc.) { case x:
break; case y: …..
break; case …:
break; default: …..
}
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Loops
while (boolean-expression) {// do something
}
do {// something
} while (boolean-expression);
Difference?
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Loops
for (expression1; boolean-expression; expression2) {
// do something
}
Example: int i;
for (i = 0; i <= 10; i++)
i++; // Value of i?
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keyword break
break = used to “break out” of a loop. Rest of the code is not executed.
int sum = 0; int number = 0;while (number < 20) {
number++;sum += number;if (sum > 100)
break;}
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keyword continue
continue = used in loops.
Break out of current statement, but continue with the rest of the loop.
int sum = 0;int number = 0;while (number < 20) {
number++;if (number == 10)
continue;sum += number;
}
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Methods
Program modularity.
Avoid redundant code! Use whenever possible
Methods can be “called”
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Methods
modifier returnValueType name (list of parameters) {
...
}
public static int max (int num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
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Methods - modifiers
public = can be called by any class private = can only be called by the class Also protected (will look at it later)
static = don’t require an “instance” of the class to call the method.
ClassName.method(...)The Math class – Math.sin(), Math.acos(), etc.
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Methods
returnValueType = Can be primitive, class, etc. Even void (nothing to return)
list of parameters = a list of primitives, classes, etc. (or nothing)
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Recursion
Methods calling themselves
Write base case first! Otherwise might be stuck forever.
Classic example: Fibonacci numbers
Integer sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21...
F(n) = F(n-1) + F(n-2) F(0) = 0, F(1) = 1
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Recursion
public int fibonacci (int n) {
if (n == 0 || n == 1) // base case(s)
return n;
else
return fibonacci (n-1) + fibonacci (n-2);
}
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Recursion
Later on we will look at recursion for other algorithms (searching/sorting)
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Method overloading
Can have multiple methods with the same name. Showed “max” method with ints Write one with double:
public static double max (double num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
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Commenting
Single-line: // This is a single-line comment
Multi-line: /* This is going to be on multiple lines */
Comment your code properly! Very helpful – to you and others.
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Object-Oriented Programming
Paradigm which uses “objects” and “classes”.
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Object-Oriented Programming
Paradigm which uses “objects” and “classes”.
Used to represent real-life objects or concepts that can be distinctly identified.
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Object-Oriented Programming
Paradigm which uses “objects” and “classes”.
Used to represent real-life objects or concepts that can be distinctly identified.
Objects have properties, methods.
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Object-Oriented Programming
Paradigm which uses “objects” and “classes”.
Used to represent real-life objects or concepts that can be distinctly identified.
Objects have properties, methods.
Interaction between objects.
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Object-Oriented Programming
Most modern languages support OOP
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Object-Oriented Programming
Most modern languages support OOP
Alternatives: ! Procedural/Imperative ( C ) ! Functional (Lisp/PROLOG)
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Classes in Java
A template for objects of the same type.
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Classes in Java
A template for objects of the same type.
You create “objects” (or “instances”) of a class.
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Objects in Java
Unique identity, state and behavior.
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Objects in Java
Unique identity, state and behavior.
state (properties/attributes): Data fields and their current values.
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Objects in Java
Unique identity, state and behavior.
state (properties/attributes): Data fields and their current values.
behavior: The methods for that class
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Examplepublic Circle {
// Propertiesprivate double radius;
// Constructorspublic Circle() {
radius = 0.0;}
public Circle(double radius) {this.radius = radius;
}
// Methodspublic double getArea() {
return radius * radius * Math.PI;}
}
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Properties
// Propertiesprivate double radius;
private = only accessible by that class directly.
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Properties
// Propertiesprivate double radius;
private = only accessible by that class directly.
Not a good idea to have public properties (for security reasons).
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Properties
// Propertiesprivate double radius;
private = only accessible by that class directly.
Not a good idea to have public properties (for security reasons). What if another class needs to access/modify the property?
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Properties
Add get/set methods:
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Properties
Add get/set methods:
public double getRadius() {
return radius;
}
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Properties
Add get/set methods:
public double getRadius() {return radius;
}
public void setRadius(double radius) {this.radius = radius;
}
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this keyword
Refers to the property of this specific class
Used to distinguish between similar-named variables
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Constructors
// Constructors// default constructorpublic Circle() {}
public Circle(double radius) {this.radius = radius;
}
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Constructors
Special kind of method
Same name as the class
No return type (even void)
Used to initialize objects (using the new keyword)
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Constructors
Initialization example:
Circle circle1 = new Circle();
Circle circle2 = new Circle(4.5);
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Constructors
Should always provide a default constructor. ! Does not take in any properties
Good idea to have multiple constructors and default values
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Reference Variables
Objects accessed via reference variables.
Example from before:
Circle circle2 = new Circle(4.5);
circle2 = Reference variable used to access the object.
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Reference Variables
Can declare without initializing
Circle circle2; // What’s the value?
Initialize later:
circle2 = new Circle(4.5);
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Accessing properties/methods
referenceName.property // only if public!
referenceName.method(...)
Example:
double radius = circle1.radius;
double area = circle2.getArea();
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Revisiting static
Variables in classes can be static
Associated with the class, rather than a specific object.
Every object shares that variable
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Revisiting static
Example: public class Student {
private String name;public int ID;
private static int numberOfStudents = 0;
public Student(String name) {this.name = name;
this.ID = ++numberOfStudents;}
}
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Revisiting static
Example:
Student student1 = new Student(“John”);
Student student2 = new Student(“Smith”);
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Revisiting static
Example:
Student student1 = new Student(“John”);
Student student2 = new Student(“Smith”);
System.out.println(student1.ID); // Output?
System.out.println(student2.ID); // Output?
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Revisiting static
Example:
Student student1 = new Student(“John”);
Student student2 = new Student(“Smith”);
System.out.println(student1.ID); // Prints 1
System.out.println(student2.ID); // Prints 2
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Summary
Creating classes
specifying properties, methods
reference variables, initialization
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Next lecture
Arrays (single and multi-dimensional)
Strings
Inheritance
