Java Basics

Question 1

The way Java works

Answer 1

1. Source
2. Compiler
3. Output (Bytecode)
4. Virtual Machines (JVM)

Question 2

What goes in source file?

Answer 2

Class definition

Question 3

What goes in a class?

Answer 3

Methods

Question 4

What goes in a method?

Answer 4

A set of statements, and for now you can think of a method kind of like a function or procedure

Question 5

Why does everything have to be in class?

Answer 5

Java is object-oriented, thus each object represents a set of blueprint

Question 6

Do I have to put a main in every class I write?

Answer 6

Nope. You might use a dozens of classes, but only one main method to evoke the program

Question 7

What is the difference between a class and an object?

Answer 7

A class is blueprint for an object.

A object is like one entry in your address book

Question 8

What if I need global variables and method? How do I do that if everything has to go in a class?

Answer 8

There isn’t a concept of ‘global’ variables and methods in a Java OO program.

Question 9

What is a Java program? What do you actually deliver?

Answer 9

A Java program is a pile of classes. In a Java application, one of the classes must have a main method, used to start-up the program

Question 10

How is this object oriented if you can still make global functions and global data?

Answer 10

Everything in Java goes in a class. So the constant for pi and the method for random(), although both public and static, are defined within the Math class

Question 11

Object Oriented Programming

Answer 11

1. Flexibility

2. Extensibility

Question 12

Instance Variable

Answer 12

The object’s state (the data), and can have unique values for each object of the type

Question 13

Class

Answer 13

A blueprint that tells the virtual machine how to make an object of that particular type

Question 14

Main

Answer 14

1. To test your real class

2. To launch/start your Java application

Question 15

Garbage-Collectible Heap

Answer 15

1. When the object is created, Java allocates memory space on the heap according to how much that particular object needs
2. When the object never in use anymore, then that object becomes eligible for garbage collection

Question 16

Primitive type

Answer 16

Hold fundamental values including integers, booleans, and floating point numbers

Question 17

Object reference

Answer 17

1. Hold bits that represent a way to access an object.

2. Hold an address of the object

Question 18

Does all object references are the same size, regardless of the size of the actual objects to which they refer?

Answer 18

Yes. All references for a give JVM will be the same size regardless of the objects they reference, but teach JVM might have a different way of representing references on one JVM maybe smaller or larger than references on another JVM

Question 19

Can I do arithmetic on a reference variable, increment it, you know - C stuff?

Answer 19

No. Java is different from C

Question 20

Array

Answer 20

Arrays are always objects, whether they’re declared to hold primitives or object references

Question 21

How do Java pass its parameters?

Answer 21

Java is pass-by-value, meaning everything is a copy

Question 22

Can a method declare multiple return values? Or is there some way to return more than one value?

Answer 22

Sort of, A method can return an array

Question 23

Do I have to return the exact type I declared?

Answer 23

You can return anything that can be implicitly promoted to that type. So, you can pass a byte where an int is expected. The caller won’t care, because the byte fits just fine into the int the caller will use for assigning the result.
You must use an explicit cast when the declared type is smaller than what you’re trying to return

Question 24

Getter

Answer 24

A getter’s sole purpose in life is to send back, as a return value, the value of whatever it is that particular Getter is supposed to be Getting

Question 25

Instance variables

Answer 25

1. Always get a default value

2. If no assignment for instance variable, then it still has a value

Question 26

Local variable

Answer 26

Do not get a default value. The compiler complains if you try to use a local variable before the variable is initialized

Question 27

Comparing Variable

Answer 27

1. Use == to compare two primitives

2. Use the equals() method to see if two different objects are equal