COMP SCI EXAM 1

Object

a software bundle of related state and behavior often used to model real life behavior.

have states and behaviors. Example: A dog has states - color, name, breed as well as behavior such as wagging their tail, barking, eating. It is an instance of a class

Class

a template/blueprint that describes the behavior/state that the object of its type supports.

Method

basically a behavior. It is a chunk of code that can be called upon or invoked to perform a set of actions. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed

Instance variable

Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables. HAs separate copies created for every object.

UML

Notation used to depict objects and to illustrate OO concepts

interface

a contract between objects on how to communicate with each other

Encapsulation

objects encapsulate expertise, attributes and actions that it needs to carry out its role

Information hiding

objects can be designed to hide certain info and implementation details from other objects

Generality

objects are designed as general as possible so it can be applied to multiple similar problems

extensibility

designed to potentially be extended to perform a certain task

Literal value

A value expressed as itself, they are constant. EX: int number = 20;

Case sensitivity

Java is sensitive, which means that Hello and hello are different

Class names

1) First letter should be upper case

2) If several words are present, each inner word’s first letter should be uppercase

EX: public class MyFirstJava

Method names

1) Starts with lower case letter

2)if several words, then each inner word’s first letter needs to be uppercase

EX: public void myMethodName(parameters) {

}

Program file name

1) NEEDS TO MATCH class name , it won’t compile otherwise

2) add .java to the end

3) If public isnt included in the class name then the file can have a different name

main method

public static void main (String[] args) {

}

Identifiers

Names used for classes, variables, and methods

Rules of identifiers

1. All should begin with a letter (A-Z), currency characters ($), or an underscore(_)

2. after the first character, there can be any combination of characters

3. A key word cannot be used

4. CASE SENSITIVE

5. EX legal: age, $salary, _value

6. EX illegal: 123abc, -salary

modifiers

modifies classes and methods

Acess modifiers

default, public, protected, private

Non access modifiers

final, abstract, strictfp

Final

keyword used to define a constant value or method/classes. Its a non-access modifier applicable only to a variable, a method or a class. Once declared the variable can’t be changed when using this keyword.

Extends

Keyword used to inherit the properties of a class

package

Set of classes and interfaces grouped together

static

access modifier and keyword used to create variables that will exist independently of any instances created for the class. The method can be called without using an individual object. Belongs to the class rather than an instance. It is only applicable for methods and variables . Only ONE object which is shared by every object of the class.

Inheritence

Allows you to reuse fields and methods of an exisitng class w/o having to rewrite the code. Classes can be derived from classes. Basically if you need to create a new class and you already have a class that has some of the code you require, then it is possible to derive your new class from the already existing code

Superclass

existing class

Subclass

class made from superclass

Types of variables

local, class (Static), instance(non-static)

Variables

named storage units that the program can manipulate. Each has a specific type, which determines the size and layout of the variables memory

Declare: int a,b,c;

Initialize: a = 2, b = 4, c =5

Local

declared in methods. CANNOT have access modifiers

Instance variables

Declared in a class but outside of a method. Created when an object is created with the use of the keyword new and are destroyed when the object is destroyed

They hold values that may be referenced by one or more methods

They can have access modifiers and be declared in the class level before or after use

They have default values

They can be accessed directly by calling the variable name inside the class

Within static methods(if they are given accessibility) they should be called using the full name

Default values:

numbers - 0 , booleans - false, objects - null

Class/Static variables

labeled with the static keyword in a class but outside of a method

There is only ONE copy of each variable per class, regardless of how many objects are created from it.

Created when the program starts and destroyed when the program stops

Data types

primitive and non primitive

primitive data types

byte

short

char

int

long

float

double

boolean

byte

8 bit

short

16 bit

char

16 bit

int

32 bit

long

64 bit

float 32 bit

double

64 bit

Boolean

true/false

Non primitive

Complex data types that store collections of values in various formats rather than just a stingle value

String

Array

Classes

Array

Structures that store multiple values of the same type in a single variable

declaration

int [] arrayName; → ont can be replaced with other data types

arrayName = new int[Array length];

setting and getting an element of an array

int[] numbers = new int [5];

numbers[0] = 10; //setting first element

int firstElement = numbers[0]; //getting first element

Getting length of an array

int length = arrayName.length;

Numeric promotion

converting one type to another type

Type promotion

the smaller type is converted to the larger type before the operand is performed

The smallest value taken is an int. Byte/short will become an int

implicit type conversion

allowed in cases where no information will be lost (narrower to wider) → small to larger, done automatically by Java

Cast operator

Must be used when converting a wider type into a narrower type. This is explicit casting. doing this manually/explicitly, making a larger type become a smaller type

preincrement

++k The operation is done before the operans value is used

EX: k = k + 1; j = k; OUTPUT k = 1, j = 1

postincrement

k++ the operation is done after the operand value is used

EX: j = k; k + 1; OUTPUT k = 1, j = 0;

predecrement (same as preincrement)

-- k

k = k -1; j = k; OUTPUT k = -1, j = -1

Postdecrement (same as postincrement)

k- -

j = k; k - 1; OUTPUT k = -1, j = 0;

Public

An access modifier used for attributes, methods, and constructors, making them accessible from any class

private

access modifier used for attributes, methods, and constructors, making them only accessible within the declared class

Formatting numbers

%n→ names a new line, %07 makes seven spaces where any extra space is a 0. The comma , is used like a decimal

Method overloading

technique used in OOP where multiple methods can have the same name but they need different parameters

Unary vs Binary operations

Unary operation is between one operand like increment/decrement

Binary operation is between two operands, +, - , *, /, and, or

Representation

the objects/characteristics

action

methods/behvaior

class constant

A field that is declared with the static and final keywords, They cannot be changed

class variable

a variable defined in a class of which a single copy exists, regardless of how many instances of the class exist

instance variable

each class has its own copy

Helper method

USed to perform particular repetitive tasks common across multiple classes. A function that is used to assist the main funcion

class method

methods declared within a class, perform specific operations

operator overloading

allows operators to be redefined and used in a way where they have a different meaning based on their operands

method overloading

having two or more methods in a class with the same name but different parameters/arguments

method call

block of code that only runs when it is called

Type conversion/type casting

convert one data to another, this has to be done MANUALLY

Type promotion

Dont naturally, smaller → bigger type

modularity

dividing a program into smaller parts

float is the same as

long

byte is the same as

short

Class Hierarchy

tree like

has one root class, called Object , which is superclass of any class

• Each class below the Object class is a subclass (child) of its parent class and can have its own subclasses.

• Subclasses inherit the characteristics (fields and methods) of their parent classes, allowing them to reuse code and create specialized classes based on the more general ones.

Three types of comments in java

Single-Line Comments for brief notes, Multi-Line Comments for detailed explanations or disabling code, and Documentation Comments for generating JavaDoc.

// single

/* multiline */

/** javadoc

*@Author

*@Param

*@return

*/

Consider the following code

public class MainClass {

public static void main(String[] args) {

System.out.println(args[2]+","+args[0]); S

ystem.out.println(args[2] + ", " + args[0] + " "+args[1]);

System.out.println(args[0]); } //end of main method

}

Name of the file - > MainClass

How to compile and run in CD

1. javac filename,java

2. java filename

output if it takes the arguments Alan M.Turin

1. Turin, Alan

2. Turin, Alan M

3. Alan

What does the code do

Reads the index of the argument and prints them whith different delimeters like , and space shown by ““

Math package

java.lang.Math

all methods in the math class are static so you can call them directly like

Math.max(24, 9 , 3, 22);

Using static import in the header class will allow you to invoke the methods and use the fields without writing the class name

While loops

Used when you dont know how many times youre going to loop through.

Will only enter the body if the condition is met

for loops

used when you do know how many loops you want to do.

for(initialization; continuation statement; update)

do - While

will always enter the body of the loop and then will check the condition

infinite loop

will run continuously

for(;;){

}

while(true){

}

break

terminates any loop

continue

skips the current iteration and continues with the next one

Ternary operator

Works like an if else statement but is more compact

variable = expression ? statement1 : statement2

str = (num > 0,0) ? “Positive” : “Negative”;

Create Array

data_type[] variableName = new data_type[Length];

or

data_type[] variableName = {Comma seperated literal values}

Elements of an array

anArray [i-1]

Elements of anArray are indexed from

0 to (# of elements - 1)

Limitations of arrays

only one type

Fixed size

Off- by -One error

When you are off by one index.

It will create an ArrayOutofBoundsException

value semantics

behavoir where values are copied when assigned, passed as parameters or returned

FOR primitive types

reference semantics

Behvaior where variables actually store the address of an object in memory

FOR objects

Aliasing

having more than one reference to the same object

Traversing an array

int[] data = {2,4,6,8,10};

for(int i = 0; i < data.length; i++) {

System.out.println(data[i]);

}

Array manipulation

java.util.Arrays

copyFrom, indexFrom, indexTo-exclusive

Two arrays are equivalent in values if they have the same

length and same sequence of values

Test if two arrays are equal

.equals

Which of the following would return a random number from 1 to 5 inclusive? - Choose all that apply.

A. (int) (Math.random() 5 )

B. myRandom.nextInt(5) + 1

C. (int) (Math.random() 6 )

D. (int) (Math.random() 5 ) + 1

E. (int)Math.random()5+1

B & D

String

String myStr = “NEW STRING”;

String variableName = new String(“My new String”);

The string class is immutable

String methods return new String objects