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Variables and memory

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Title: Variables and memory

1
Variables and memory

Every variable has
a name, a type, a size, and a value
Concept name corresponds to a memory location
If primitive type actual value stored there
long needs more space than int, and so on
If object type just reference to object stored
there (just need space for memory address)
Actual object is somewhere else
But reference can be null means no actual object

2
Variables and constants

Java is strongly-typed
Must declare type for memory locations used
e.g., declare 2 doubles, and one String
reference
double a, b
String s
Declaring allocates space, but value is
undefined
Must assign value, or compiler wont let you use
it
final variables are constants
May only assign value once usually when
declared
e.g., final double TAX_RATE 0.0775

3
Identifiers

Names of classes, variables, methods
3 simple rules
Must consist of a sequence of letters, digits, _,
or
No other characters allowed including no
spaces
Must not begin with a digit
No Java reserved words allowd
Unwritten rule Use meaningful names
Conventions
NameOfClass begin with uppercase
other or otherName, unless name of constant, like
PI

4
java.lang.Math static methods

Maths public methods are all static
So invoke by class name and the dot .
operator
double r Math.toRadians(57.)
System.out.println(Sine of 57 degrees is
Math.sin(r))
Some methods in chapter 4, Table 2 (p. 150)
Math.max(x,y) and Math.min(x,y)
Math.random() (and more versatile
java.util.Random class)
e.g., int dice (int)(Math.random()6) 1
Math is in the package called java.lang (the one
you neednt import)

5
Standard Output, and Strings

System.out an object of type PrintStream
println(string) prints string and newline
print(string) prints string, no newline
String delimited by quotes a string
Remember special characters start with \
e.g., \n is a newline character
So println(Hi) is same as print(Hi\n)
concatenates e.g., a 5 b becomes
a5b
Note first 5 is converted to a String.

6
Some String methods

Accessing sub-strings (Note positions start
at 0, not 1)
substring(int) returns end of string
substring(int, int) returns string from first
position to just before last position
charAt(int) returns single char
length() the number of characters
toUpperCase(), toLowerCase(), trim(),
valueOf() converts any type to a String
But converting from a String is more difficult

7
Standard input, and more Strings

Normally have to read keyboard or other input as
a String (also requires error handling and a
reader object)
And must parse string to interpret numbers or
other types
e.g., String s1 426, s2 93.7
Then s1 can be parsed to find an int or a double,
and s2 can be parsed to find a double
int n Integer.parseInt(s1)
double d Double.parseDouble(s2)

8
java.util.Scanner

Important Java 5 enhancement
Greatly simplifies processing standard input
No need to handle IOExceptions
No need to deal with parsing input strings
First construct a Scanner object pass it
System.in
Scanner in new Scanner(System.in)
Then get next string, int or double (others too)
int x in.nextInt()
double y in.nextDouble()
String s in.next()
String wholeLine in.nextLine()

9
Other ways to get data from user

JOptionPane simplest type of GUI
Quick way to get an input String from the user
Must parse string to convert to numbers/other
e.g., old texts InputTest.java
Before Java 5 harder to read standard input
Basically, associate a Reader object with
System.in
Must handle or throw IOExceptions
Data actually are integers representing char
Reader object converts whole line to a String
then parse
e.g., old texts ConsoleInputTest.java

10
Formatted printing with printf

Java 5 printf(String format, Object... args)
Method of PrintStream class so System.out has
System.out.printf(x d, x) // x is an
integer
d means print integer as decimal. Can be octal
or hex too
printf(octal onhex xn, x, x)
Note variable length argument list also new
Java 5 feature
f or e or g for floating point, and s for
strings
Also control field width, precision, and other
formatting
printf(-9s7.2fn, Value, v)
See Tables 3 and 4, p. 168
Complete details in java.util.Formatter
Format dates, times, Works for String objects
too
String s String.format(pt d, d", x, y)

11
Some operators

is the assignment operator
Basic arithmetic operators , -, , /,
is modulus operator (remainder)
Compound arithmetic/assignment operators
e.g., a 5 // same as a a 5
Also -, , /, and
Increment and decrement operators
is same as 1 and -- is same as - 1
e.g. counter // increments counter by 1

12
Pre vs. post or --

Post-increment is not exactly the same as
pre-increment (same goes for decrement)
i.e., x is not exactly the same as x, but the
final value of x is the same in both cases
Post uses value then changes it pre is reverse
e.g., say x 7, then
System.out.println(x) // would print 7
System.out.println(x) // would print 8
In either case, x equals 8 after the print.

13
Type conversions

Automatically applies to promotions only
e.g., int n 5 double d n // okay
n is promoted to double before assignment
happens
e.g., int n 5 double d n/2.0 // okay
n promoted to double before division result is
double
Must cast to force other conversions
e.g., double d 5. int n d // error
double d 5. int n (int)d // okay
But not all casts are legal (basically must make
sense)
String s dog int n (int)s // error

14
Some object reference issues

null a reference to no object at all
Cannot send message to no object, of course
e.g., BankAccount mySavings null
mySavings.withdraw(100) // error at runtime
this an objects reference to itself
Often used just for clarity
e.g., in a BankAccount method, balance 0 is
same as this.balance 0
Also used to call one constructor from another
one
e.g., public BankAccount() this(0)
Copying a reference does not copy the object.

15
Copying values

Copying values (of primitive data types) actually
does copy the value
int balance1 1000
int balance2 balance1
// now there are separate copies of the value
1000

16
vs. copying references

Copying references to objects does just that
BankAccount account1 new BankAccount(1000)
BankAccount account2 account1
// now there are separate copies of the
reference
// but there is still just one bank account
object

17
Java has 7 control structures

1st is trivial sequence structure
3 choices of selection structures (decisions)
if
if/else
switch
3 choices of iteration structures (loops)
while
for (Note Java 5 also has two versions of it)
do/while

18
Sequence (it really is a structure)
19
if Selection Structure
20
Implementing if

Either
if (boolean expression)
one statement
Or
if (boolean expression)
multiple statements
separated by
boolean expressions evaluate to true or false

Indented here too
Note indentation
21
Simple boolean expressions

Relational operators , , , !
e.g., int x1, y2, z3
x y // false
x z - y // true
Note lower precedence than arithmetic
x z y // false
Note not same as x z y // makes x be 5
z ! x y // false (if x still is 1)

22
Comparing objects, like Strings

Do NOT use to test equality
That just compares references! For example,
String s1 dog
String s2 DOG.toLowerCase()
s1 s2 // false! different objects
Use equals method instead (if defined by class)
s1.equals(s2) // true same contents
But not all classes define equals method. Be
careful.
Some objects (like Strings) are Comparable, so
s3.compareTo(s4) // returns -1, 0, or 1

23
if/else Selection Structure
24
Implementing if/else

With if and else
if (grade 60)
message Pass
else
message Fail
Or with selection operator
message grade 60 ? Pass Fail
// same result as if/else above
This version does not allow blocks or nesting
but it returns a value, so more useful in many
cases

25
Nesting indenting