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HOPE
Structured Problem Solving
2009-2010
Week 7: Java basics
Stewart Blakeway
[email protected]
0151 291 3113
www.hope.ac.uk
Faculty of Sciences and Social Sciences
Java: The Basics
HOPE
Pages 69 - 87
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2
What we have done already
• Seen what an algorithm is
– a set of instructions that, if carried out, will lead
to a successful conclusion
HOPE
• Learned how to represent algorithms in
– Structured English
– Flow charts
• Used variables to remember
• Applied the top down, stepwise refinement
approach to creating algorithms
• Looked at problems more oriented towards
being solved on a computer – stacks,
queues, functions, procedures
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3
What we shall do today
HOPE
•
•
•
•
What a computer program is
Low level programming
High level programming
The Java programming language
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What we shall do today
HOPE
•
•
•
•
What a computer program is.
Low level programming
High level programming
The Java programming language
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5
A computer program
•
•
•
•
HOPE
.. is a set of instructions
Stored on disc
Patterns of 0s and 1s
Copied into main memory
when required
• Executed by CPU fetching
and executing the
instructions from main
memory
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6
ThreeBit
HOPE
• Machine code (non-mnemonics mode)
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What we shall do today
HOPE
•
•
•
•
What a computer program is.
Low level programming
High level programming
The Java programming language
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8
ThreeBit Instruction format
HOPE
001 01101
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ThreeBit Instruction format
HOPE
001 01101
Op code
What the instruction
is to do
(add, multiply, move)
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ThreeBit Instruction format
HOPE
001 01101
Op code
What the instruction
is to do
(add, multiply, move)
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Operand
What the instruction
is to do it to
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Programming in machine code
HOPE
• Almost impossible to spot errors.
• Easy to mistype a 1 for a 0 and vice versa
without noticing
• Boring to do
• Meaningless to read
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Programming in machine code
HOPE
• First generation language
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1950s solution: Assembly language
HOPE
• Assembly language
– uses mnemonics for op codes
– like ThreeBit
•
•
•
•
•
•
•
•
LDI
LDD
STD
ADD
SUB
JMP
JEZ
STP
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HOPE
1950s solution: Assembly language
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1950s solution: Assembly language
• LDI 20
– Load the CPU accumulator register with the value
20
HOPE
• STD 30
– Copy the CPU accumulator register contents to
memory location 30
• ADD 10
– Add the contents of memory location 10 to the
CPU accumulator register
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16
1950s solution : Assembly language
• Other processors use different mnemonics
• mov a, 0
• moves 0 into a register labelled a
HOPE
• Special program translates assembly
language into machine code
• Assembler
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1950s solution : Assembly language
• One line of assembly code for one line of
machine code
HOPE
• Assembly language is a low level language –
it is very close to being in written in terms the
machine understands
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1950s solution : Assembly language
HOPE
• Easier to write but still difficult to get right
• Many hundreds of instructions to do simple
things like input and output
• Can only run on one type of machine
• Not easy to tell what an assembler program
is intended to do by reading it
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19
What we shall do today
HOPE
•
•
•
•
What a computer program is.
Low level programming
High level programming
The Java programming language
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20
1960s solution: High level
languages
• Pioneers decided to:
HOPE
– enable scientist, engineer and business programmers to
write programs in a language that looked familiar to them
– avoid needing to know the internal structure of the
computer
– high level programming language
• Invented compiler programs that would translate
high level programs into machine code
automatically
– the compiler can generate machine code from the high
level language
• E.g. GOTO becomes JMP op code
– the compiler knows the internal structure of the computer
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1960s solution: High level
languages
• FORTRAN, COBOL were the earliest
– FORTRAN aimed at scientist and engineers
– COBOL aimed at business people
HOPE
• Look like Structured English
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HOPE
FORTRAN Example
READ *, A, B, C
X1 = (-B + SQRT(B*B - 4*A*C)) / (2*A)
X2 = (-B - SQRT(B*B - 4*A*C)) / (2*A)
Science and Engineering Oriented Language
FORmula TRANslation
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COBOL Example
IF HOURS_WORKED OF PAYROLL_RECORD IS GREATER THAN 40
PERFORM PAY_CALCULATION_WITH_OVERTIME
ELSE
PERFORM_PAY_CALCULATION_NO_OVERTIME
HOPE
COMPUTE GROSS_PAY = REGULAR_PAY + OVERTIME_PAY
Business oriented language
COBOL == COmmon Business Oriented Language
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1960s solution: High level
languages
HOPE
• Key features of a high level language
– Written in a language the user can understand
– Program looks like an algorithm so easier to write
– Does not depend on knowledge of internal
workings of the computer
– Can be run in different types of machines
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1960s solution: High level
languages
• Compiler
– Special program translates high level language
into machine code
HOPE
• Lots of different languages
– C, C++, Java, Python, PHP, Basic, Visual Basic,
C#, Pascal, Fortran, COBOL, Lisp, Prolog
– which one to use ?
– discussion on page 71
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HOPE
Compiling a program
• This program is written in C
– another high level language
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On this module ...
HOPE
• ... we shall use the high level programming
language called Java
• But what we do with Java can be done
with many other languages e.g. Pascal, C,
C++
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Making a Program
HOPE
• The steps to be followed in creating a
successful program include:
– writing down a design for the program in
Structured English
– translating the design into the chosen high level
language: Java in your case
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Programming house style
HOPE
• Using a house style looks more professional
• Follow the Liverpool Hope house style
– See Appendix B, page 106
– variable names and method names in lower case
– Three spaces for each indentation
Three spaces
in indentation
while (a>b)
{
System.in.read(first);
}
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HOPE
Structure of a Java program in
Java Trainer
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HOPE
Structure of a Java program in
Java Trainer
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HOPE
Translating a design into Java
The design …
A := 8
B := 11
C := A + B
display ‘Answer is ‘, C
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Translating a design into Java
• We need to create two sections
HOPE
– declarations
– statements
Design
A := 8
B := 11
C := A + B
display ‘Answer is ‘, C
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Translating a design into Java
• Create a data table
HOPE
Design
A := 8
B := 11
C := A + B
display ‘Answer is ‘, C
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Translating a design into Java
HOPE
• Create the
declarations
int a;
int b;
int c;
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HOPE
Translating a design into Java
If we want to store numbers
with a decimal fractional part
e.g. money values, then we
must declare the variable as a
real number having double
precision:
double x;
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Translating a design into Java
HOPE
Design
A := 8
B := 11
C := A + B
display ‘Answer is ‘, C
• Create the statements
a = 8;
b = 11;
c = a + b;
System.out.println(“Answer is:
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” + c);
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Read and write/display in Java
Design
Java
System.in.read(x);
display X
System.out.println(x);
display ‘X is’, X
System.out.println(“x is
HOPE
read(X)
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”+x);
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print and println
System.out.print(“One ”);
System.out.print(“Two ”);
Produces:
One Two Three
HOPE
System.out.print(“Three ”);
System.out.println(“One ”);
Produces:
System.out.println(“Two ”);
One
System.out.println(“Three ”);
Two
Three
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Printing text and variable values
double a;
HOPE
int b;
a = 4.7;
Output:
a is 4.7 b is 8
b = 8;
System.out.println(“a is ” + a + “ b is “ + b);
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Printing text and variable values
double a;
HOPE
int b;
a = 4.7;
Output:
a is 4.7 b is 8
b = 8;
System.out.println(“a is ” + a + “ b is “ + b);
Text between quotes
printed exactly as written
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42
Printing text and variable values
double a;
HOPE
int b;
a = 4.7;
Output:
a is 4.7 b is 8
b = 8;
System.out.println(“a is ” + a + “ b is “ + b);
Text outside quotes treated
as variable names – print
their values
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43
Pascal and Java
Pascal Program
Java Trainer equivalent
HOPE
PROGRAM AddVat;
VAR Price, VAT, Total: Integer;
BEGIN
Price := 20;
VAT := 3;
Total := Price + VAT;
WriteLn(VAT, Total);
END.
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int Price;
int VAT;
int Total;
Price = 20;
VAT = 3;
Total = Price + VAT;
System.out.println(VAT, Total);
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Pascal and Java: declarations
Pascal Program
Java Trainer equivalent
HOPE
PROGRAM AddVat;
VAR Price, VAT, Total: Integer;
BEGIN
Price := 20;
VAT := 3;
Total := Price + VAT;
WriteLn(VAT, Total);
END.
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int Price;
int VAT;
int Total;
Price = 20;
VAT = 3;
Total = Price + VAT;
System.out.println(VAT, Total);
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Pascal and Java: statements
Pascal Program
Java Trainer equivalent
HOPE
PROGRAM AddVat;
VAR Price, VAT, Total: Integer;
BEGIN
Price := 20;
VAT := 3;
Total := Price + VAT;
WriteLn(VAT, Total);
END.
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int Price;
int VAT;
int Total;
Price = 20;
VAT = 3;
Total = Price + VAT;
System.out.println(VAT, Total);
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Let’s try a program
int age;
int yearOfBirth;
System.out.print ("How old are you?");
System.in.read(age);
yearOfBirth = 2010 - age;
HOPE
System.out.println ("Ah, you must of been born in " + yearOfBirth);
int age;
System.out.print ("How old are you?");
System.in.read(age);
System.out.println ("Ah, you must of been born in " + (2010-age) );
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Another
String name;
String doing;
System.out.println ("Who are you?");
System.in.read (name);
System.out.println ("What are you doing");
HOPE
System.in.read (doing);
System.out.println ("Hello " + name + " why are you " + doing + " when
you could be programming?");
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Arrgh! Syntax!
int a;
int B;
int c;
HOPE
a = 9;
b = 12;
3 Syntax Errors
Can you spot them?
C = 0;
c = a;
a = b;
b = c
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3 Syntax Errors
Arrgh! Syntax!
Can you spot them?
double wages;
double tax;
HOPE
System.out.println ("How much did you get paid
this week?");
System.In.read(wages);
tax = wage * 0.20;
System.out.println ("You owe £" + tax " in
taxes");
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Any Questions?
HOPE
• We have covered
–
–
–
–
What a computer program is.
Low level programming
High level programming
The Java programming language
www.hope.ac.uk
Faculty of Sciences and Social Sciences