Java : Primitive Data Types, Operators and Control

1
Java Primitive Data Types, Operators and
Control
September 17, 2007
2
Things to Ignore (for now)

• In Chapter 2 of Raposa, be aware of but dont
  worry about
• Bitwise operators
• Shift operators

3
What is Java?

• Java
• Compiled language
• Executes within a virtual machine
• This is what makes it highly portable
• Is C like in terms of syntax and simple control
  constructs
• Is purely object-oriented
• Is a strongly typed language

4
The basics of the Java

• Rules for identifiers
• The first character must be an underscore, upper-
  or lower-case letter, or a dollar sign
• Java is case sensitive
• Subsequent characters may be any upper- or
  lower-case letter, digit, underscore or a dollar
  sign
• Cannot be a keyword, true, false or null
• Recommendation!
• Do not use dollar signs
• Dollar signs are typically intended for use by
  code generation tools

5
Reserved Words

• Java has, like, 50 keywords
• Get to know them
• Interesting trivia
• const and goto are keywords
• They have no implementation in Java
• They do cause compilation errors

6
Built-in Types

• There are eight built-in types
• Java calls the primitive types

-27 to 27-1
-215 to 215-1 -231 to 231-1 -263 to 263-1
(/-)1.410-45 to 3.41038 (/-)4.910-324
to 1.810308
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Literals
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Quick!

• What would System.out.println(x) produce?
• int x 45
• int x 045
• int x 0x45

• 45
• 37
• 69

9
Constants

• Java calls constants final
• Java has blank finals
• Have no initial value
• Are fixed upon first assignment

Syntax for declaration constants final int x
7 //x is fixed to the value 7 x 2
//compilation error final int y //blank
final y 3 //y is now fixed at 3 y
7 //compiler error
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Declaring Variables, Assigning Values and
Comparisons

• Java is typed
• Variables must include a type
• May include an initial value
• Assignment
• Comparison

Syntax for declaration, assignment and
comparison int x ? declare that x is a
variable x 5 ? assign x the value 5 x 7 ?
compare value of x to 7
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Operators

• Java has numerous built-in operators
• Operators mostly apply only to the primitive
  types
• There are exceptions which we will point out
• Most operators apply to all primitive data types
• There are exceptions which we will point out

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Assignment Operator

• The assignment operator

• Used to assign the value on the right to the
  value on the left
• For primitive data types, this works as expected
• For all other Java objects, we need further
  discussion

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Objects vs. References

• Identifiers of primitive data types represent
  objects

memory
int i
ltintegergt
When this line is executed, space is allocated to
store i. Where i is stored, an integer lives
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Objects vs. References

• Identifiers of classes represent references

memory
String s
ltaddressgt
When this line is executed, space is allocated to
store s. Where s is stored, lives the address of
where the string is located.
String
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Objects vs. References

• Assignment copies what is stored where an
  identifier lives

int i 5int j 7i j
String s new String(Fred)String t new
String(Ethel)s t
After the last line executes, both i and j have
the same value.
After the last line executes, both s and t store
the same address.
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Relational Operator

• Equality tests

!

• Used to test if the value on the left is equal to
  the value on the right
• For primitive data types, this works as expected
• For all other Java objects, this only test
  addresses, not referenced values!

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Example String
String s new String(Fred)String t new
String(Fred)System.out.println(s t)s
tSystem.out.println(s t)
Prints false. The address of s and t are not
the same.
Prints true. The address of s and t are the
same.
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Example String
String s FredString t FredSystem.out
.println(s t)
What gets printed?
Why?
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Relational Operators

• Other relational operators for primitive types

lt lt gt gt

• These do not apply to
• Boolean
• Any class-based objects

20
Mathematical Operators

• Standard math operators (apply to all primitives
  but boolean)

- / - (unary) (unary)

• Short-cut math operators (apply to all primitives
  but boolean)

-- - /
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Logical Operators

• Logical operators apply only to boolean values

( short-circuit and) (and)
(short-circuit or) (or) (exclusive
or)! (not)
22
Short Circuiting

• Care must be taken with compound logical
  statements
• Java uses short-circuiting
• Statements are evaluated only as far as they need
  to be to determine the value

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Example Short Circuit
g() is called only if f() returns false. h() is
called only if g() and f() return false.
boolean f()boolean g()boolean h()if (
f() g() h() ) . . .if ( f() g()
h() )
g() is called only if f() returns true. h() is
called only if g() and f() return true.
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Example - Bitwise
f(), g() and h() are all invoked and then the
expression is evaluated
boolean f()boolean g()boolean h()if (
f() g() h() ) . . .if ( f() g() h()
)
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Casting

• Objects can and may need to be converted from one
  type to another
• This is called casting

int i 5double d 4.2i di (int)
d
Generates a syntax error. Loss of precision will
occur
We need to explicitly convert since there is loss
of precision.
26
Promotion

• In certain circumstances, the Java will cast for
  us
• When there is no chance of loss of information
• This is called promotion

int i 5double d 4.2d i
Going from int to double is represents no loss of
precision. Java converts automatically.
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Execution Control

• Available constructs

if if-else if-else ifwhiledo-whilefor
switch
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if statement

• Provides a decision point

Syntax
Example


if (divisor 0) System.out.println
(Error Division by zero!)
if (boolean-exp) statements
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if-else statement

• Provides a decision with alternative

Syntax
Example


if (divisor ! 0) quotient value /
divisor else System.out.println
(Error Division by zero!)
if (boolean-exp) statements else
statements
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if-else if statement

• Provides a set of decisions with multiple
  alternatives

Syntax
Example


if (status DNE) System. out.println
(Error File does not exist!) else if
(status ERROR) System.out.println
(Error Cant open file!)
if (boolean-exp) statements else if
(boolean-exp) statements
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if-else if - else statement

• Provides a set of decisions with multiple
  alternatives and a default alternative

Syntax
Example


if (n lt 0) System.out.println (n is
negative) else if (n gt 0)
System.out.println (n is positive)
else System.out.println (n is
zero)
if (boolean-exp) statements else if
(boolean-exp) statements else
statements
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Be Careful!

• Compound if statements are evaluated in the order
  they are specified

Incorrect
Correct


if (n gt 0) System.out.println (n
has 1 digit) else if (n gt 10)
System.out.println (n has 2 digits)
else if (n gt 100) System.out.println
(n has 3 digits)
if (n gt 100) System.out.println (n
has 3 digits) else if (n gt 10)
System.out.println (n has 2 digits)
else if (n gt 0) System.out.println
(n has 1 digit)
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Returning from a statement

• If statements are often used in operations to
  decide a return value

// Operation splits the string into two strings
at the position // indicated. The following must
be true // // 0 lt pos lt length-2 //
public String split(String s, int pos)
String result (null, null) if ( (pos
lt 0) (pos gt s.length()-2))
System.out.println(Error Bad value for pos!)
return result result0
s.substring(0,pos) result1
s.substring(pos) return result
If pos was not specified correctly, the operation
would return without executing the splitting
statements
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Good Practice

• Have only 1 point of return

// Operation splits the string into two strings
at the position // indicated. The following must
be true // // 0 lt pos lt length-2 //
public String split(String s, int pos)
String result (null, null) if ( (pos
lt 0) (pos gt s.length()-2))
System.out.println(Error Bad value for pos!)
else result0
s.substring(0,pos) result1
s.substring(pos) return result
This makes the operation easier to follow. There
is only one way to leave the opertion. This is
especially useful when writing complex operations.
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while loops

• Enables a sequence of statements to be iterated
  while a given condition is true
• Condition is checked before entering loop

Syntax

while (boolean-exp) statements
Example

char response while ( !( (response
Y) (response N) ) ) response
getResponse()
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do-while loops

• Like while, enables a sequence of statements to
  be iterated while a given condition is true
• Condition is checked after entering loop

Syntax

do statements while (boolean-exp)
Example

char response do response
getResponse() while ( !( (response Y)
(response N) ) )
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while vs. do-while

• Statements are similar but
• while loops may never execute the statements
  inside the loop
• do-while loops execute the statements inside the
  loop at least once

do-while
while


do statements while (boolean-exp)
while (boolean-exp) statements
Condition is checked at the top of the loop. If
the condition initially is false, the loop
doesnt execute.
Condition is checked at the bottom of the loop.
The loop executes at least once before checking
condition.
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for loops

• Use a for loop when you need to do something a
  set number of times

Syntax

for (initialization boolean-exp step)
statements
Example

for (int n2 nlt100 n) if (isPrime(n))
System.out.println(n is prime!)

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for loops

• Think of the parameters as follows

Syntax

for (initialization boolean-exp step)
statements
initialization Where do I start?boolean-exp
When do I continue?step How do I
progress?
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for loops

• How it works

Syntax

for (initialization boolean-exp step)
statements

• Perform initialization
• If boolean-exp is true, execute statements in
  loop
• Perform the step instructions
• Goto 2

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for loops

• Parameters could be operations

Example

for (initialize() seeIfDone() getNext())
statements
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for loops

• There are more exotic forms
• Dont worry about for the time being

43
Loops and returning

• It is possible to return from within a loop
• It is not a good practice
• Defeats the purpose of the loop

Bad practice
Good practice


for (int i0 ilts.length() i) if
(letterToFind s.charAt(i)) return
true return false
result false for (int i0 ilts.length() i)
if (letterToFind s.charAt(i))
result true return result
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Breaking

• It is also possible to break out of a loop
  without returning
• Again, this is typically a bad practice

Bad practice
Good practice


result false for (int i0 ilts.length() i)
if (letterToFind s.charAt(i))
result true break return false
result false for (int i0 ilts.length() i)
if (letterToFind s.charAt(i))
result true return result
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Continuing

• We can also can also force the next iteration of
  a loop
• Similar to breaking, use of the continue should
  be avoided when possible

Bad practice
Better practice


for (int i3 ilt99 i) if (i3 ! 0)
continue System.out.println(i)
for (int i3 ilt99 i3)
System.out.println(i)
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switch statements

• Provides a selection based on an integral value

Syntax
Example


switch (charFromInput) case Y case
y start() break case N
case n stop() break
default System.out.println
(Invalid response!) break
switch (integral-selector) case
integral-value statements
break case integral-value case
integral-value case integral-value
statements break default
statements break
47
Two Java Quirks

• String
• Arrays
• We will understand these later