Java / OO

Question 1

Name the access modifiers

Answer 1

public, private, protected

Question 2

What’s another name for a mutator method?

Answer 2

setter

Question 3

What’s another name for an accessor method?

Answer 3

getter

Question 4

Whats happens in the first stage of Java compilation?

Answer 4

Compiled into bytecode. Low-level but not hardware-specific. Catches syntax errors.

Question 5

What happens when you run a Java program?

Answer 5

Bytecode compiled further by a JVM (Java Virtual Machine). Executed.

Question 6

What’s the name for the Java style of compilation?

Answer 6

‘Just in time’ compilation.

Question 7

Is the Java style of compilation faster or slower than the normal type and why?

Answer 7

Can be faster because can be compiled in smaller chunks depending on which bit of the code you are running, and therefore optimised.

Question 8

What’s the file extension on a source code file?

Answer 8

.java

Question 9

What’s the file extension on a compiled Java file?

Answer 9

.class

Question 10

What’s the command to compile a java file or files?

Answer 10

javac *.java

Question 11

How do you run a Java program with a main method?

Answer 11

java ClassName

Question 12

How do you run a Java program with a package name and a main method?

Answer 12

java my.package.ClassName

Question 13

How do you control what “a string” + myObject outputs?

Answer 13

Implement the toString() method on myObject

Question 14

Explain what it means in practice that strings are immutable

Answer 14

Methods that appear to modify a string actually create and return a new string - so you have to reassign the result to a new variable.

Question 15

What’s a wrapper class (types)?

Answer 15

e.g. Integer, Character - provides useful methods to perform on primitives, and is an ‘object’, which is useful e.g. for using as the generic in ArrayList

Question 16

What’s a wrapper class (types)?

Answer 16

e.g. Integer, Character - provides useful methods to perform on primitives, and is an ‘object’, which is useful e.g. for using as the generic in ArrayList.

Question 17

List out the types of integer and their sizes

Answer 17

byte; int; short; long

8-bit; 16-bit; 32-bit; 64-bit

Question 18

List out the types of floating point number and their sizes

Answer 18

float; double

32-bit; 64-bit

Question 19

What is a char stored as?

Answer 19

16-bit unicode character

Question 20

Explain checked vs unchecked exceptions

Answer 20

checked at compile time

have to be handled or declare that they’re thrown

Question 21

guarding

Answer 21

if clause

Question 22

defensive programming

Answer 22

against unexpected exceptions from outside

Question 23

how to respond to an error

Answer 23

throw exception, print message, return false

Question 24

sources of input to a program

Answer 24

file, network, command line, database

Question 25

streams

Answer 25

System.in, System.out, System.err

Question 26

Type of System.in

Answer 26

PrintStream

Question 27

Classes for handling streams

Answer 27

BufferedReader, BufferedWriter, Scanner

Question 28

Non-blocking IO

Answer 28

e.g. async code

Question 29

Polymorphism

Answer 29

calling a method, deciding at runtime which object to call it on