Big Java, Late Objects

Question 1

What is a computer?

Answer 1

Machine that stores data, interacts with devices + executes programs. At heart of comp is CPU.

Question 2

What is the CPU?

Answer 2

Central Processing Unit, composed of millions of transistors. Performs program control + data processing + executes programs, carries out arithmetic operations + fetches + places processed data into storage, of which there are 2 kinds. May not be obvious where this data resides on a networked comp.

Question 3

What are the 2 kinds of storage?

Answer 3

Primary + secondary storage.

Question 4

What is primary storage?

Answer 4

Made from memory chips, electronic circuits that can store data so long as they have electric power.

Question 5

What is secondary storage?

Answer 5

Provides slower, less expensive storage which persists without power. Data + programs located here + loaded into memory when program starts. Program then updates program in memory + writes modified data back to sec storage. e.g. hard disk

Question 6

What is a hard disk?

Answer 6

Secondary storage. Consists of rotating platters coated with magnetic material + read/write heads which can detect + change magnetic flux on platters.

Question 7

What is magnetic flux?

Answer 7

Quantity/strength of magnetic lines produced by magnet.

Question 8

How does comp interact with user?

Answer 8

Peripherals.

Question 9

How do high level programming languages work?

Answer 9

Created to make writing program instructions less tedious. Specify actions your program should carry out + compiler translates these into language of CPU.

Question 10

What are applets?

Answer 10

Java code that can be located anywhere on internet. Java was first created for programming consumer devices but successfully used to write these in 1995.

Question 11

Why is Java so good?

Answer 11

Has rich library so you can write portable programs that can bypass proprietary operating systems. Micro + enterprise edition of this to allow for a wide range of uses. It is safe + portable as it as designed for the internet. Safety features allow it to be run in browsers without fear of attack. Help you learn lang faster too as when you make an unsafe error, it tells you what’s wrong.

Question 12

What makes Java portable?

Answer 12

The compiler doesn’t translate it directly into CPU instructions but instructions for the Java virtual machine, which simulates a real CPU.

Question 13

What are the cons of Java?

Answer 13

It’s not simple to write basic programs + you need a certain amount of technical machinery to write even the simplest program.

Question 14

What is an editor?

Answer 14

Program for entering + modifying text, such as a Java program.

Question 15

What is a compiler?

Answer 15

Java compiler translates source code (java) into class files which contains the info for the virtual machine. The VM then executes the program.

Question 16

What should be pointed out in relation to backups?

Answer 16

Back up often, rotate backups (where you backup), pay attention to the backup direction (don’t overwrite newer files with older files), check your backups + relax, then restore.

Question 17

What are classes?

Answer 17

These are the fundamental building blocks of Java programs. Each java program contains a class with a main method. When app starts, instructions in main method are executed. Each class contains declarations of methods which contain sequence of instructions.

Question 18

What does it mean if a program crashes?

What if it is hung?

Answer 18

It has quit spontaneously.

It failed to respond to your input.

Question 19

What is pseudocode?

Answer 19

An informal description of a sequence of steps for solving a problem. Use indentation to indicate which statements should be selected/repeated.

Question 20

What are the steps to creating a program?

Answer 20

Understand problem, develop + describe algorithm, test algorithm with simple inputs, translate into java + compile + test program.

Question 21

What are the steps to describing an algorithm with pseudocode?

Answer 21

Determine inputs + outputs, break down problem into smaller tasks, describe each subtask in pseudocode + test pseudocode by working out example problem.

Question 22

What does initialise mean?

Answer 22

To specify the value stored in the variable.

Question 23

What is a number literal?

Answer 23

When a value such as 6 or 0.335 occurs in a Java program.

Question 24

What is an assignment statement?

Answer 24

Used to name variable. Stores new value in variable, replacing previously stored value e.g. cans = 8; Doesn’t include data type.

Question 25

What is a variable declaration?

Answer 25

Used to name variable but includes data type + doesn’t necessarily include value.

Question 26

What is a constant?

Answer 26

A variable defined using reserved word ‘final’. Its value can never change. Written with capital letters + underscores e.g. final double BOTTLE_VOLUME = 2;

Question 27

Why are roundoff errors a fact of life?

Answer 27

There isn’t always an exact representation of the decimal number you want in floating point numbers.

Question 28

What is a magic number?

Answer 28

A numeric constant that appears in code without explanation e.g. totalVolume = bottles * 2. Name variable e.g. totalVolume = bottles * BOTTLE_VOLUME; Also allows us to change value rather than having to go back and find and change every 2.

Question 29

What do you do if you want to compute with really large nums?

Answer 29

Use big number objects e.g. BigInteger + BigDecimal classes in java.math package. No limits on size + precision. However, computations using them is much slower + have to use diff operators e.g. add, subtract, multiple, not +, -, *. e.g. BigInteger oneHundred = new BigInteger (“100”); \n System.out.print(oneHundred.multiply(fiftyMillion)); BigDecimal carries out floating point calculations without roundoff errors.

Question 30

What is an expression?

Answer 30

Combination of variables, literals, operators +/or method calls.

Question 31

How can we make this expression shorter:

total = total + cans;

Answer 31

Combine arithmetic + assignment, total += cans

Question 32

What is a package?

Answer 32

Collection of classes with related purpose. Only classes in java.lang packae are automatically available in programs, must import all other classes.

Question 33

What is the API documentation?

Answer 33

Application Programming Interface documentation. Lists all classes + methods of Java library.

Question 34

What is an application programmer?

Answer 34

A programmer who uses the Java classes to put together a Java program/application.

Question 35

What is a system programmer?

Answer 35

A programmer who designs + implements the library classes.

Question 36

What happens when one of the arguments of the operator is a string?

Answer 36

The other argument is converted into a string.

Question 37

What is true of objects?

Answer 37

Strings + Scanners are objects. Each object belongs to a class e.g. strings are objects of string class. Class declares methods you can use with its objects. Method invoked with dot notation (object, method name, method, parameters in parentheses) Can’t invoke methods on nums.

Question 38

What are static methods?

Answer 38

Classes can declare these methods that aren’t invoked on objects. Used when manipulating nums. e.g. Math.sqrt(2).

Question 39

What is an instance method?

Answer 39

A method that is invoked on an object. Used when processing strings or inputs/outputs.

Question 40

What is an if statement for?

Answer 40

Used to implement decision. When condition fulfilled, one statement executed, otherwise, other set of statements executed. Can carry out diff actions depending on nature of data being processed. No semicolon after if condition. If we have duplicate code, move to outside of if statement.

Question 41

What is a relational operator?

Answer 41

> , >=,

Question 42

How do you test whether strings are equal?

Answer 42

string1.equals(string2) == would check if they’re stored in same location.

Question 43

How do we test if floating point numbers are the same?

Answer 43

Don’t test if they’re the same but if they’re close enough to each other. (x-y)

Question 44

What is a switch statement?

Answer 44

Similar to if statement but compares value against multiple alternatives. Each branch must end with break (or else executes rest of program) + tests same variable. Can only use with int, String, enum + values must be constant, can’t test greater than.

Question 45

What is a nested statement?

Answer 45

When decision statement contained inside branch of other decision statement.

Question 46

What is hand tracing?

Answer 46

Working out problem by hand by using columns. Work through + change values as values in program change.

Question 47

What is a dangling else?

Answer 47

If you don’t use braces, statement will attach itself to wrong if/else statement. Occurs in nested decision statements.

Question 48

What is an enum?

Answer 48

Has finite set of values E.g. public enum FilingStatus {SINGLE,MARRIED,MARRIED_FILING_SEPARATELY} To declare: FilingStatus status = FilingStatus.SINGLE;
If wrong value typed in, error flagged. Use the == value to compare enum values. Place enum declaration inside class that implements program.

Question 49

What are flowcharts for?

Answer 49

Made up of elements for tasks, input/output and decisions. Show structure of decisions + tasks that are required to solve problem. Each branch of decision can contain tasks + further decisions. Never point arrow inside another branch (spaghetti code)

Question 50

What are test cases?

Answer 50

Each branch of program should be covered by test case. Test boundary conditions + invalid inputs. If you have flowchart, can check off each branch. Design them before coding.

Question 51

How do you get a printout of the program flow?

Answer 51

Insert trace messages into program. E.g. if (status==SINGLE){System.out.println(“Status is SINGLE”);} However, must remove these when done + put back in if another error so use Logger class. Use global logger object (Logger.getGlobal()) Then (Logger.getGlobal().info(“Status is SINGLE”);) By default, message is printed, but if you use (Logger.getGlobal().setLevel(Level.OFF);) at start of main method, log printing suppressed. Set level to Level.INFO to turn back on.

Question 52

Describe boolean.

Answer 52

Boolean variable = true/false. AKA flag variable. Use && and || (boolean operators) && = both true. Or = both/either one true. Boolean operators have lower precedence than relational operators. To invert condition, use ! not operator. && and II operators computed using short circuit evaluation. Once true, no other conditions evaluated.

Question 53

What is De Morgan’s law?

Answer 53

Used to simplify Boolean expressions. E.g. !(A&&B) = !A || !B. !(A || B) = !A && !B. And and or operators reversed by moving not inward.

Question 54

How can you validate input?

Answer 54

If wrong value input, runtime exception occurs + program terminated. To avoid, call hasNextInt/hasNextDouble method which checks whether next input is int. If true, can safely call nextInt. Otherwise, print error message + exit program.

Question 55

What is a while loop?

Answer 55

Repeats step while variable certain value. Executes instructions repeatedly while condition true. Statements being executed are body of statement. Variables declared inside them are local and not seen anywhere else in program. Condition controlled/indefinite. Pre-test loop.

Question 56

What are infinite loop?

Answer 56

Loops which run forever, stopped only by killing program or restarting comp. Continuous output or program just hangs.

Question 57

What are off by one errors?

Answer 57

Common with loops, use simple test cases to avoid them.

Question 58

When is a for loop used?

Answer 58

When value runs from starting point to ending point with constant increment/decrement. Can use while loop with counter instead. Count controlled/definite. Pre-test loop.

Question 59

What are symmetric values?

Answer 59

If same operator is on both bounds e.g. 1 <=i<=10

Question 60

What is a do loop?

Answer 60

Loop body only executed once. Body executed first then condition tested. Post-test loop.

Question 61

What is a sentinel value?

Answer 61

Denotes end of data set but not part of data. Can use boolean variable to control loop. Set variable before entering loop, then set to opposite to leave loop. May have 2 checks which ends loop, then ensures sentinel isn’t processed as input value.

Question 62

What is a loop and a half?

Answer 62

When check is in middle of program so must go halfway through program until you know if you need to terminate program. Can use break otherwise {break;}

Question 63

What is input/output redirection?

Answer 63

Can use input redirection to read input from file + output redirection to capture program output in file e.g. java.SentinelDemooutput.txt

Question 64

What is a storyboard?

Answer 64

Annotated sketches for each step in action sequence. Helps to understand inputs + outputs. Must ask what info provided, what inputs/outputs + in what order?

Question 65

How could you compute average?
How could you count values that fulfil condition?
How could you find largest value?
How could you compare adjacent inputs?

Answer 65

Keep total + count of all values, initalise total + count with 0. Check all values + increment counter for each match, if goal to find match, exit loop when found. Update largest value so far when see larger one. Store preceding value in variable e.g. in previous = input

Question 66

What is a nested loop?

Answer 66

When body of loop contains another loop. Typical use is printing table with rows + columns.

Question 67

How are simulation programs used?

Answer 67

Use comp to simulate real life activity. Can introduce randomness with Math.random() which gives float num >=0 and <1/ Not truly random as drawn from sequence of nums that don’t repeat for long time. Called pseudorandom nums. To choose range, e.g. Math.random()*6

Question 68

What is the monte carlo method?

Answer 68

Find approx solutions to problems that can’t be precisely solved. Use Math.random() to do it.

Question 69

What is the draw method?

Answer 69

Receives object of type Graphics which has methods for drawing shapes + remembers colours used for drawing operations.

Question 70

What is a method?

Answer 70

Named sequence of instructions. Can call method to execute instructions. Args supplied when method called, return value is result that method computes. Must provide name, variable for each arg + result type when declaring method. declaration is called header. Should be reusable where possible. Keep short, should fit on 1 screen.

Question 71

What is a parameter?

Answer 71

Parameter values hold arguments supplied in method call + yield method result. Compile time error if some branches return values and others don’t. Use return type void to indicate method doesn’t return value.

Question 72

What is stepwise refinement?

Answer 72

Decompose tasks into smaller ones.

Question 73

What is a stub?

Answer 73

When writing larger programs, not always possible to implement + test all methods at once so can use temp stub method to replace one which hasn’t yet been implemented. Returns simple value to test method.

Question 74

What is a recursive method?

Answer 74

Calls itself. Solves problem by using solution of same problem with simpler inputs. For it to terminate, must have special cases for simplest inputs.

Question 75

What is an array?

Answer 75

Collects sequence of values of same type. Num of elements is length of array. Start at index 0. Each index in array contains element. Bounds error if try to access something out of index range. Can’t add values to array you’ve already declared length of. If you copy array ref, changing value in 1 also changes value in other. Can create partially filled array but must keep companion variable to store how many elements in use. Use Arrays.sort(values,0,currentSize) to sort. e.g. double {}values=new double[10] OR double[]values={1,2,3,4,6,8}

Question 76

What is an enhanced for loop?

Answer 76

Can use to visit all elements of array. Element variable assigned value[0] while in basic for loop, index variable assigned these. Use enhanced if don’t need index values in loop body. When separating elements, don’t place separator before first element. Can also use for ArrayList.

Question 77

What is a linear/sequential search?

Answer 77

Inspects elements in sequence until match found. Before inserting element, move elements to end of array starting with last one. Can copy array using x.copyOf. Can sort it using Arrays.sort(values). If array sorted, use binary search algorithm. This means check if value in first half, then check in second half. Arrays can occur as method arguments and return values.

Question 78

How can you store a 2D array e.g. Matrix?

Answer 78

Supply num of rows + columns e.g. new int[7][3]. Can do same with multidimensional array. Array list stores sequence of values whose size can change + ArrayList class supplies methods for common tasks like inserting + removing elements. ArrayListnames=new ArrayList(); To use it, must do import java.util.ArrayList. ArrayList is generic class.

Question 79

How do you collect numbers in ArrayList?

Answer 79

Must use wrapper classes. Cap letter for type + use full name. e.g. ArrayList. Conversion between primitive types + corresponding wrapper class automatic, called auto-boxing. Can use similar code for them but yse get. To find length of ArrayList use a.size()

Question 80

How do we read files in Java?

Answer 80

Use Scanner to read files. Constuct File object with name of input file (File inputFile=new File(“input.txt”), then use file object to construct scanner object (Scanner in = new Scanner (inputFile). To write output to file, construct PrintWriter object with file name (PrintWriter out=new PrintWriter(“output.txt”). Must declare what we want program to do if file doesn’t exist so may put throws FileNotFoundException at main method declaration. When spec file name as string literal, use 2 backslashes due to escape character.

Question 81

How do we read contents of webpage?

Answer 81

String address = https://www.bbc.co.uk;URL pageLocation=new URL(address);Scanner in = new Scanner(pageLocation.openStream();

Question 82

How do we use file dialog boxes?

Answer 82

JFileChooser class implements file dialog box for Swinh user interface toolkit. Construct object, then call showOpenDialog or showSaveDialog method. Can use InputStream to read binary data. To write binary data to file, use FileOutputStream. Next method rads string that is delimited by white space. Can use UseDelimiter method to discard anything that’s not a letter. Can use Character class to analyse + classify characters. e.g. Character.isDigit(ch); To read entire line of string, use String line = in.nextLine(); If string contains digits of num, use Integer.parseInt to obtain num value.

Question 83

How can we signal an exceptional condition?

Answer 83

Use throw statement to throw exception object. When we do this, processing continues in exception handler. If withdraw too much money, throw IllegalArgumentException. Place statements that can cause exception inside try block + handler inside catch clause. Call exception.getMessage() to retrieve message associated with exception. Internal errors reported by descendants of type Error e.g. outOfMemoryError. Descendants of RuntimeException like OutOfBoundsException indicate code errors, unchecked exceptions.

Question 84

What are checked exceptions?

Answer 84

All other errors, indicate something gone wrong for external reason beyond our control. Once try block entered, statements in finally clause guaranteed to be executed, whether exception throws or not. Better to throw exception than get incorrect answer. Don’t use catch + finally in same try statement as hard to understand + often wrong. For each exception, must decide which part of program can competently handle it.