1. Expressions and Assignment Statements
This slide set was compiled from the Absolute Java textbook slides (Walter Savitch) and the professor’s own class materials.
CSS161: Fundamentals of Computing

2. CSS161: Fundamentals of Computing
Identifiers
The name of a variable or other item (class,
method, object, etc.)
Must be a~z, A~Z, 0~9, and _.
Must not start with 0~9.
Can be of any length.
Is case-sensitive: Rate, rate, and RATE are the names of three different variables.
Excludes keywords and reserved words.
public class void static
Excludes system-predefined class names
System String
CSS161: Fundamentals of Computing

3. CSS161: Fundamentals of Computing
Naming Conventions
The names of variables, methods, and objects
Start with a lowercase letter
topSpeed bankRate1 timeOfArrival
The names of classes
Start with an uppercase letter
FirstProgram MyClass String
Word boundaries
Delimited with an uppercase letter
CSS161: Fundamentals of Computing

4. CSS161: Fundamentals of Computing
Practice
X 34 count main
firstData %growth data-1 numberOfBeans
ABC123z7 first.data while PROG.CLASS
variable? x_1 data2 variable!
variable for x1 5A
mainProgram do doIt int
Integer integer String string
Choose bad identifiers
Choose recommended variable names
CSS161: Fundamentals of Computing

5. Variable Declarations
Every variable must be declared before it is used.
Tell the compiler what kind of data you will be storing in the variable
Declare variables either just before they are used or at the start of a block
Syntax
Type Variable_1, Variable_2, . . .;
Examples
int count, numberOfDragons, numberOfTrolls;
char answer;
double speed, distance;
CSS161: Fundamentals of Computing

6. CSS161: Fundamentals of Computing
Primitive Types
CSS161: Fundamentals of Computing

7. Assignment Statements with Primitive Types
Syntax
Variable = Expression;
Evaluates the expression of the right-hand side and store the value into the variable of the left-hand side.
Example
float distance, speed, time;
speed = 15;
time = 20;
distance = speed * time;
distance
time 15 20
300 15 20 *
300
CSS161: Fundamentals of Computing

8. Assignment Statements with Primitive Types (Cont’d)
Note that a variable can occur on both sides of the assignment operator
count = count + 2;
The assignment operator is automatically executed from right-to-left, so assignment statements can be chained
number2 = number1 = 3; 10 12 10
+ 2 3 3
CSS161: Fundamentals of Computing

9. Variable Initialization
Syntax
Type Variable_1 [= Expression_1]{, Variable_2 [= Expression_2]};
[..] can be omitted
{..} can be repeated 0 or more times.
Each variable is declared as well as initialized or left uninitialized.
Example
int a = 10, b; // initialize a but not b
System.out.println( a );
System.out.println( b );
Javac Test.java
Test.java:5: variable b might not have been initialized ^
1 error
CSS161: Fundamentals of Computing

10. Shorthand Assignment Statements
The general form is
Variable Op = Expression
which is equivalent to
Variable = Variable Op (Expression)
Op can be +, -, *, /, or %
Example:
Equivalent To:
count += 2;
count = count + 2;
sum -= discount;
sum = sum – discount;
bonus *= 2;
bonus = bonus * 2;
time /= rushFactor;
time = time / rushFactor;
change %= 100;
change = change % 100;
amount *= count1 + count2;
amount = amount * (count1 + count2);
CSS161: Fundamentals of Computing

11. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook p20’s 12 ~ 16 with your neighboring classmate.
CSS161: Fundamentals of Computing

12. Assignment Compatibility
Illegal
int intVarible;
intVariable = 2.99;
possible loss of precesion
found : double
required : int ^
Legal/Acceptable
double doubleVariable;
doubleVariable = 2;
intVariable = ( int )2.99 // Type Casting
CSS161: Fundamentals of Computing

13. CSS161: Fundamentals of Computing
Type Casting
byteshortintlongfloatdouble
Char
An explicit type cast is required to assign a value of one type to a variable whose type appears to the left of it on the above list (e.g., double to int)
The type resulted from arithmetic operations with combined types is adjusted to the highest type.
int a = 1;
double b = 2.0;
doule c = a + b; // int + double -> double
CSS161: Fundamentals of Computing

14. CSS161: Fundamentals of Computing
Constants
Integer
0, 2, -1, 5, 1000
Decimal-fraction
0.0, 2.0, -1.5, 5.0,
Floating-point
3.65e5 (= 3.65 * 105 = 3.65 * = )
5.89e-4 (= 5.89 * 10-4 = 5.89 * = )
Character
‘A’, ‘0’, ‘Welcome to Java’
String
“A”, “0”, “Welcome to Java”
CSS161: Fundamentals of Computing

15. Arithmetic Operations
Division /
15.0 / 2 // equals 7.5
15 / 2 // equals 7
int a = 15, b = 2;
double c = a / b; // equals 7.0
double c = a / (double)b; // equa;s 7.5
Modulus operator %
15 % 2 = 1;
Increment and decrement operators ++, --
int n = 2;
int a = 3 * (n++);// a equals 6, n incremented to 3
int b = 3 * (++n);// n incremented to 4, b equals 12
CSS161: Fundamentals of Computing

16. Arithmetic Operations (Cont’d)
Precedence
Associativity rules
Binary operators: left-to-right order
a + b + c equals (a + b) + c
Unary operators: right-to-left order
+-+a equals +(-(+a))
Assignment operators: right-to-left order
a = b = c equals a = (b = c)
CSS161: Fundamentals of Computing

17. CSS161: Fundamentals of Computing
Practice
Evaluate z, and m1 ~ m5
public class Test {
public static void main( String[] args ) {
int x = 1, y = 2, z = 3;
z += y += x += 5;
System.out.println( "z += y += x += 5; z = " + z );
int m1 = 0; x= 1; y = 2; z = 3;
m1 = x +(++y);
System.out.println( "m1 = x +(++y); m1 = " + m1 );
int m2 = 0; x = 1; y = 2; z = 3;
m2 = x +++y;
System.out.println( "m2 = x +++y; m2 = " + m2 );
int m3 = 0; x = 1; y = 2; z = 3;
m3 = x ---y;
System.out.println( "m3 = x ---y; m = " + m3 );
int m4 = 0; x = 1; y = 2; z = 3;
m4 = x z;
System.out.println( "m4 = x z; m4 = " + m4 );
int m5 = 0; x = 1; y = 2;
m5 = x *-y;
System.out.println( “m5 = x *-y; m5 = " + m5 ); }
CSS161: Fundamentals of Computing

18. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook p28’s 17 ~ 21 with your neighboring classmate.
Work on Textbook p32’s 22 ~ 23 with your neighboring classmate.
CSS161: Fundamentals of Computing

19. CSS161: Fundamentals of Computing
Static Methods
A set of statements to repeat the same action
Being called from the main( )
static void main( String[] args ) {
static void main( String[] args ) {
printMessage( );
System.out.println( “D” );
System.out.println( “A” );
System.out.println( “B” );
System.out.println( “C” );
System.out.println( “D” ); }
static void printMessage( ) {
System.out.println( “A” );
System.out.println( “B” );
System.out.println( “C” ); } }
CSS161: Fundamentals of Computing

20. Static Methods (Cont’d)
What will be printed out?
public class Test2 {
public static void main( String[] args ) {
repeatHello( );
repeatBye( ); }
static void repeatHello( ) {
System.out.println( "hello" );
static void repeatBye( ) {
System.out.println( "bye" );
CSS161: Fundamentals of Computing