1. MID-TERM TEST REVIEW FOR CECS 220
PRESENTED BY REACH

2. Java Language: BASIC RULES
Java is case sensitive
Identifier names should be descriptive and readable
Keywords are reserved words that cannot be used as identifier names
Java generally ignores whitespace (except when it is used to separate words)
Proper formatting of your codes is important for readability
Variable is named memory location used to hold a value of a particular type
Accessing a variable leaves it intact but an assignment statement overwrites it
Constants hold a particular value for the duration of their existence

3. Example 1: VARIABLES
Is the following a valid name for a variable in Java?
_special_character
A.) Yes
B.) No
$vari_A1
A, A

4. SCANNER CLASS This class is part of the standard Java class library
Provides a convenient way for reading input values of various types {
String message;
Scanner scan = new Scanner(System.in);
System.out.println(“Enter a line of text: “);
message = scan.nextLine();
System.out.println(“Your text: “ + message); }

5. Scanner Class Scanner scan = new Scanner(System.in);
For integer: scan.nextInt()
For double: scan.nextDouble()
For string: scan.nextLine()
For byte: scan.nextByte()
For float: scan.nextFloat()
For Boolean: scan.nextBoolean()

6. JAVA CLASSES AND OBJECTS: RANDOM CLASS
import java.util.Random; {
Random random = new Random();
int num1, num3;
float num2, num4;
num1 = random.nextInt();
System.out.println(num1);
num2 = random.nextFloat();
System.out.println(num2);
num3 = random.nextInt(10);
System.out.println(num3);
num4 = random.nextFloat() * 5;
System.out.println(num4); }
Creates pseudorandom numbers
It is part of the java.util class
Can be accessed by importing the java.util class
Random random = new Random();
For random integer: random.nextInt();
For random integer within a range: random.nextInt(int num);
For float: random.nextFloat();

7. JAVA CLASSES AND OBJECTS: MATH CLASS{
//program calculates the roots of a quadratic equation
int a,b,c;
double discriminant, root1, root2;
//discriminant = (sqrt(b^2 - 4 * a * c))
Scanner scan = new Scanner(System.in);
System.out.println("Enter value of a: ");
a = scan.nextInt();
System.out.println("Enter value of b: ");
b = scan.nextInt();
System.out.println("Enter value of c: ");
c = scan.nextInt();
discriminant = Math.pow(b, 2) - (4 * a * c);
root1 = ((-1 * b) + Math.sqrt(discriminant))/(2 * a);
root2 = ((-1 * b) - Math.sqrt(discriminant))/(2 * a);
System.out.println("Root 1 = " + root1);
System.out.println("Root 2 = " + root2); }
Provides a large number of basic mathematical functions
It is part of the java.lang package
All methods in this class are STATIC
Some methods include:
Math.sqrt();
Math.pow();
Math.ceil();
Math.exp();
Math.random();
Math.floor();
Math.cos();
Math.sin();
Math.tan();
Among many others…

8. NUMBERFORMAT CLASS
The NumberFormat class and DecimalFormat class are used to format information in order to display them properly
They are both include in the Java standard class library and can be found in the java.text package
NumberFormat object does not need to be instantiated using the new keyword
Some of its methods include:
String Format(double number) : for decimal point formatting
NumberFormat.getCurrencyInstance() : for currency formatting
NumberFormat.getPercentInstance() : for percentage formatting

9. NumberFormat Class: Example
final double TAX_RATE = 0.06; // 6% sales tax
int quantity;
double subtotal, tax, totalCost, unitPrice;
Scanner scan = new Scanner (System.in);
NumberFormat fmt1 = NumberFormat.getCurrencyInstance();
NumberFormat fmt2 = NumberFormat.getPercentInstance();
System.out.print ("Enter the quantity: ");
quantity = scan.nextInt();
System.out.print ("Enter the unit price: ");
unitPrice = scan.nextDouble();
subtotal = quantity * unitPrice;
tax = subtotal * TAX_RATE;
totalCost = subtotal + tax;
// Print output with appropriate formatting
System.out.println ("Subtotal: " + fmt1.format(subtotal));
System.out.println ("Tax: " + fmt1.format(tax) + " at "
+ fmt2.format(TAX_RATE));
System.out.println ("Total: " + fmt1.format(totalCost));

10. ENUMERATED TYPES
This is a special kind of class and the variables of an enumerated types are object variables
Enumerated types are type-safe, ensuring invalid values will not be used
There is no limit to the number of values that you can list for an enumerated type
Once the type is defined, a variable can be declared of that type
enum Season = {Summer, Winter, Fall, Spring}
Season time;
time = Season.Summer;
It has two main methods : .ordinal() and .name()

11. WRAPPER CLASSES A wrapper class represents a particular primitive type
A wrapper class allows a primitive value to be managed as an object
Integer ageObj = new Integer(40);
The ageObj object effectively represents the integer 40 as an object.
It can therefore be used wherever an object is required within the program rather than a primitive type
They also provide various methods related to the management of the associated primitive type

12. Wrapper Classes: Cont’d
For each primitive type, there exists a corresponding wrapper class in Java
Autoboxing: this is the automatic conversion between a primitive value and a corresponding wrapper object
Integer obj1;
int num1 = 69;
Obj1 = num1; //automatically creates an Integer object
Integer obj2 = new Integer(62);
int num2;
num2 = obj2; //automatically extracts the int value
Primitive Type
Wrapper Class
byte
Byte
short
Short
int
Integer
long
Long
double
Double
char
Character
boolean
Boolean
void
Void

13. Object Oriented Programming CONCEPTS
A class is the blueprint of an object
A class represents the concept of an object and any object created from that class is a realization of that concept
The heart of OOP is defining classes that represent objects with well-defined state and behavior
Student
public String firstName
public String lastName
public int Age
private bool onProbation
Class Attributes or Properties
Class Methods or Behavior
public Student(fName, lName, Age)
public void setProbation(bool probation)
public bool getProbation()
public String toString()

14. Attributes, Methods and Constructors
Variable names:
Public string firstName; //declaration
Public string firstName = “John”; //initialization
private boolean probation;
public string toString(){
return firstName + “ ” + lastName;
} //returns a value
public void displayName(){
System.out.println(firstName);
} //prints directly to screen but returns no value
public String setName(String fName){
firstName = fName;
}//sets firstName to fName given as a parameter
private Boolean getProbation(){
return probation = true;
} //private method that returns value of probation attribute
Public Student(String Fname, String Lname, int Age){
firstName = Fname;
lastName = Lname;
age = Age;
probation = false;
}//the constructor sets initial values for attributes in a new object

15. Encapsulation
An object should be encapsulated to guard its data from inappropriate access
We should make it difficult, if not impossible, for code outside of a class to “reach in” and change the value of a variable that is declared in that class. This is referred to as ENCAPSULATION
Other code should only be able to interact with the class attributes using methods defined by the programmer
This is usually achieved using access modifiers

16. Visibility Modifiers
Public: can be accessed by other classes. Can be used for both attributes and methods
Private: can only be accessed within the class. Can be used for both attributes and methods.
Private methods can only be used within the class while private attributes can only be used by methods of the class
Protected: Can only be accessed by descendants of the parent class. Can be used for both attributes and methods
Constants are usually given public visibility

17. Conditionals and Loops
These allow us to control the flow of execution through a method
An if statement allows a program to choose whether to execute a particular statement
A loop allows a program to execute a statement multiple times (with the same or different parameters)
Loops: while, do and for
Conditionals: if, if-else, switch and nested-if

18. Conditionals If-statement: if(height < 10.0)
setAdjustment(0);
If-else statement: if(height < 10.0)
adjustment = 0;
else
setAdjustment(10);

19. Comparing Objects
The Unicode relationships among characters make it easy to sort characters and strings of characters
Using the == or relational operators to compare String objects is highly discouraged; use the .equals(String toCompare) method instead
if(name1.equals(name2))
System.out.println(“The names are the same”);
else
System.out.println(“The names are different”);
The compareTo() method can be used to determine the relative order of strings
int result = name1.compareTo(name2);
If(result < 0)
System.out.println(name1 + “ comes before ”+ name2);
else if(result == 0)
System.out.println(“They are equal”);
system.out.println(name2 + “ comes before ”+name1);

20. Loops
While statement is a loop that evaluates a boolean condition just as an if statement does and executes a statement (called the body of the loop) if the condition is true
while(total > max){
total = total / 2;
system.out.println(“Current total: ”+total); }
int i = 1;
While(i < 5){
System.out.println(i);
i++;

21. Break and Continue statements
When a break statement is executed, the flow of execution transfers immediately to the statement after the one governing the current flow
The continue statement is similar to break but the loop condition is evaluated again and the loop body is executed again if it is still true

22. ITERATORS
An iterator is an object that helps in processing a group of related items
The key is that an iterator provides a consistent and simple mechanism for systematically processing a group of items
It is closely related to the idea of loops
Technically, all iterators use the Iterator interface in java
All iterators implement the .hasNext(), .next() methods in various ways
An example of an iterator is the scanner class

23. Example of Using Scanner Class as an Iterator for File Reading
String url;
Scanner fileScan, urlScan;
fileScan = new Scanner (new File("urls.inp")); // Read and process each line of the file
while (fileScan.hasNext()) {
url = fileScan.nextLine();
System.out.println ("URL: " + url);
urlScan = new Scanner (url);
urlScan.useDelimiter("/"); // Print each part of the url while (urlScan.hasNext()) System.out.println (" " + urlScan.next()); System.out.println(); }

24. ARRAYLIST
This is a very useful class for managing a set of objects in java
An ArrayList object stores a list of objects and lets you access them using an integer index
When an ArrayList object is created, the programmer specifies the type of element that will be stored in the list
ArrayList<String> myNames = new ArrayList<String>();
myNames.add(“Jane”);
myNames.add(“Harry”);
myNames.add(“Sara”);

25. ArrayList Cont’d Its methods include: .add(E obj)
.remove(int index, E obj)
.clear()
.contains(object obj)
.size()
isEmpty()
.indexOf(object obj)

26. STATIC VARIABLES
A static variable is shared among ALL instances of a class
Static variables are sometimes referred to as CLASS VARIABLES
There exists only 1 copy of the static variable for all objects of the class
Changing the value in one object changes its value in all objects of the class
private static int count = 0;
Constants, which are declared using the final modifier, are often declared using static modifier

27. STATIC METHODS These are also referred to as class methods
These methods are usually invoked through the class name i.e we don’t have to instantiate an object of the class in order to invoke the method e.g Math.random()
A method is made static by using the static modifier in the method declaration e.g public static void main(String[] args)
Static methods cannot reference instance variables (which exist only in an instance of a class)
Static methods can only reference static variables because they exist independent of specific objects
This is why the main method can only access only static or local variables

28. INTERFACES
An abstract method is a method that does not have an implementation
An interface is a collection of abstract methods and can therefore not be instantiated
public interface Complexity {
public void setComplexity(int complexity);
public void getComplexity(); }
Methods in interfaces are all public by default
A class that implements an interface uses the keyword implements followed by the interface name in the class header
It serves as a “contract” between the interface and class that the class MUST implement all methods declared in the interface
A class can have other methods other than those in the interface
A class can implement multiple interfaces

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