Hello world, variables and operators

Question 1

what is the name given to a variable

Answer 1

the identifier

Question 2

words that cannot be used as variable names

Answer 2

reserved keywords

Question 3

byte

Answer 3

8 bit signed twos complement integer
default value 0
used to save space in large arrays

Question 4

short

Answer 4

16 bit sign twos complement integer
min (-2^15)
max (2 ^16)
can also be used to save memory as is two times smaller than an int

Question 5

int

Answer 5

32 bits
min value (-2^31)
max value(2^32)
default data type unless there is a concern about memory

Question 6

long

Answer 6

64 bit twos complement integer
default value is 0

Question 7

float

Answer 7

32 bit floating point default is 0.0f
mainly used to save memory in large arrays of floating point numbers

Question 8

double

Answer 8

64 bit IEEE 754 floating point
generally default data type used for floating point number values

Question 9

boolean

Answer 9

1 bit of information
default value is false

Question 10

char

Answer 10

16 bit is a single 16 bit unicode character
min value is \u0000 or 0
max value is \uffff or 65,535
used to store any character

Question 11

How are reference variables created

Answer 11

using defined constructors of classes

Question 12

why are reference variables used

Answer 12

to access objects

Question 13

Default value of a reference variable

Answer 13

null (as opposed to primitive data types where the default value is 0 which means you can store data with no value).

Question 14

steps of using a variable

Answer 14

declaration - when the variable is declared
initialisation - when the variable is initially given a value
assignment - when a variable is given assignment at some point after being createed

Question 15

what is a condition

Answer 15

an expression which can evaluate to true or false

Question 16

example of a selection statement

Answer 16

a selection statement allows a program to make a decision based on a conditions value
If and switch statements

Question 17

equality operators

Answer 17

== and !=

Question 18

relational operators`

Answer 18

> and < and >= and <=

Question 19

Under the bonnet

Answer 19

java is platform independent - this means it will run on almost any operating system
portability - java must run similarly on any supported hardware/operating system platform

Question 20

java portability

Answer 20

bytecode - portability achieved by compiling java into an intermediate representation called java bytecode instead of directly to platform specific machine code
bytecode instructions are analogous to machine code, but are interpreted by a JVM (java virtual machine)

Question 21

compiling and running java

Answer 21

step 1 - developers write the source code which is saved with the file extension .java
step 2 - Javac is used to to turn (compile) the source code into byte code
step 3 - After javac has finished compiling the bytecode, it creates a new file with the extension .class
step 4 - one the class file has been created, it can be passed onto and run on the JVM

Question 22

compiler vs interpreter

Answer 22

compiler - translates entire source code into machine or intermediate code, then an executor generates the ouput
interpreter - a program which reads one statement at a time and translates it to the machine code or virtual machine code and executes it straight away.

Question 23

JVM platform independent

Answer 23

there is a jvm for each platform