Mid Term Study Concepts

Question 1

JDK (Java Developer Kit) is:

Answer 1

-What we use to program Java
-Comes with the compiler(Javac.exe) and the JRE
-The compiler converts source code into byte code
-Source code is the code programmers type into VS code

Question 2

JRE (Java Runtime Environment) contains:

Answer 2

-Ships with every operating system out there
-Contains the JVM and Java API

Question 3

JVM (Java Virtual Machine or java.exe) is:

Answer 3

-Converts byte code into machine code
-Exists on many operating systems, allowing java to be cross platform

Question 4

Java API is:

Answer 4

-A library of java code that we can use

Question 5

Primitive variables are:

Answer 5

Variables that store values and start with lower case(int, boolean, double, float)

Question 6

Reference variables are:

Answer 6

-Reference variables store a reference to an object in the heap

-Anytime a object or array is referenced in a method, it is referenced using a reference variable

-String is a reference variable(Reference variables typically start with capital)

Question 7

Local variable is:

Answer 7

-any variable declared in a method

-stored in the stack frame

Question 8

Class variables also known as ____ variables, declared with the _____ prefix

Answer 8

Class variables also known as static variables, declared with the static prefix
example: public static int age = 5;

Question 9

These are static/instance variables from the Class BankAccount:

private static double interest = 4.25;
private double balance = 0;

Why is interest static and balance not, why are they both private?

Answer 9

-Static variables should be values that apply to all objects of a class without being changed, while instance variables can expect to be different from object to object.

-The principle of encapsulation to protect the data means the static and instance variables should be private(inaccessible unless using a getter/accessor).

Question 10

Arrays are:

Answer 10

An object, so their data is stored in the heap while a reference exists on its stack frame.

Question 11

Below is an array of objects, explain where in the memory everything is stored:
(Assume student1 and 2 are previously declared)

Student [ ] arr = {student1, student2}

Answer 11

Student [] arr is a reference, with data in the heap. In the heap each of Student [] arr’s indexes store references to the student objects which are also in the heap.

Question 12

JOptionPane Syntax:

Answer 12

String name = JOptionPane.showInputDialog(“What is your name?”);

System.out.println(“Hello “+name);

Question 13

How many classes can be made from objects? How about vice versa?

Answer 13

Can’t make classes from objects but the opposite is an infinite amount

Question 14

What are the four types of access modifiers?

Answer 14

public, private, protected, default

-all these are case sensitive

Question 15

Static is ___ ______

Answer 15

Static is not OOP.

-all objects would share the same values for their static variables. Instance variables allows for objects to be distinct.

Question 16

What are the principles of OOP?

Answer 16

Encapsulation: Protect instance variables and only allow them to be accessed or modified through the use of getter or setter methods

Abstraction: managing complexity by hiding unnecessary details from the user. It allows programmers to focus on the essential features of an object, rather than its internal workings.

Question 17

What are Special classes and Regular classes?

Answer 17

Special Classes:Classes with a main method. Are passed into JVM to run the program

Regular Classes: Don’t need a main method

Question 18

Where do classes typically exist?

Answer 18

Stored as a .java file on the hard drive

Question 19

Difference between a class and a object?

Answer 19

A Class is like a blueprint, tells the JVM how to create an object.

An Object is an instance of a class.

Question 20

An object is a self sustained collection of what two things?

Answer 20

Instance variables and methods

Question 21

What problem did OOP solve?

Answer 21

OOP solved the GUI problem(Allow for better creation of GUIs) and allowed you to reuse code.

Question 22

Create an Object Constructors for the class called Object:

Answer 22

Constructors for the class Object would be the class’s method that would have the same name as the class in this example they would be Object() or Object(String name) or Object(int age), etc…

Question 23

What are overloaded methods/constructors?

Answer 23

Overloaded method is when a method has the same name as another method but just takes different arguments or returns a different data type

Question 24

Scope of these variables:

Local, Instance, Static(class)

Answer 24

local - method level scope
instance - class level scope
static - class level scope

Question 25

How can you access an instance variable and a static variable?

Answer 25

Instance variables would require having an object and using a accessor to return the value

Static variables don’t require you to create a class to access them. Simply use the class name(Example: Classname is Object)
int staticvalue = Object.staticvariable

Question 26

When a program starts what happens?

Answer 26

-The Class Loader will load the .class file into the method section of memory
-The JVM will begin running code in the main method first

Question 27

What part of the memory deals with bytecode?

Answer 27

method area

Question 28

Why do we not have to JOptionPane obj = new JOptionPane or the same for parseInt, to use the showInputDialog or parseInt class methods?

Integer.parseInt()

Answer 28

Because they are static methods

Question 29

Why should we use this.variable syntax?

Answer 29

If two variable names collide, the JVM will always choose the variable with the smallest scope (usually the local variable).

The this. keyword allows you to force the JVM to use the instance variable instead

They picked the word this because we are referring to instance variables and methods in this object

Question 30

The Stack holds:

Answer 30

Contains local variables, when a method is entered a new stack frame is created, and when a method is exited the current stack frame is destroyed.

Question 31

The Heap stores:

Answer 31

Objects are stored in the heap, including arrays as they are considered objects

-When an array is first declared in the method, it is created in the heap, where a reference is created of that array in the current stack frame.

Question 32

The Method stores:

Answer 32

Where .class files are loaded when the JVM starts the program. Holds all the information about classes.

Question 33

What happens when the program runs into an Exception?

Answer 33

The JVM will look for a try/catch to handle the exception. If it cannot find one in the current stack frame it will go to the next highest stack frame looking for a method with a try/catch. If there are no approriate tries/catches, it terminates the program.

Question 34

What is a Stack?

Answer 34

A data structure that operates on the principle of LIFO “Last in, First out”.

Has methods: push(), pop(), peek(), size()

Question 35

What is an enum data type?

public enum AnimalType {
ELEPHANT, LION, TIGER
}

Animal a1 = new Animal(AnimalType.LION);

Answer 35

Enums (short for enumerations) in Java are a special data type that enables for variables to be a set of predefined constants. Enums are used to define a collection of constant values in a more readable and organized manner

Question 36

What is a final data type?

Answer 36

It can be used on instance variables and also the class itself. Makes the instance variables and the class immutable.