Primitive Data Types• There are exactly eight primitive data types in

Java– four of them represent integers:

• byte (class Byte), short (class Short), int (class Integer), long (class Long)

– two of them represent floating point numbers• float (class Float), double (class Double)

– one of them represents characters:• char (class Character)

– and one of them represents boolean (logical) values:• boolean (class Boolean)

Numeric Primitive Data• “Objects” of different numeric data types

occupy different number of cells

Type

byteshortintlong

floatdouble

Storage

8 bits16 bits32 bits64 bits

32 bits64 bits

Min Value

-128-32,768-2,147,483,648< -9 x 1018

+/- 3.4 x 1038 with 7 significant digits+/- 1.7 x 10308 with 15 significant digits

Max Value

12732,7672,147,483,647> 9 x 1018

IEEE 754format

Recap: Arithmetic Operations

• The storage of a computer is limited; this leads to potential surprises:

numerical data types, especially float and double, have limited precision: a computer cannot store all of the numbers precisely in its claimed range– e.g., the numbers a double can represent: +/-

1.7 x 10308; but there are infinite number of numbers in between

– Historical example: The Patriot Missile Failure in 1991a computer cannot represent 0.1 precisely; for a 24-bit floating point number they used, it is off by 0.000000095. After 100 hours in operation, it is off by about 0.34 seconds, leading to an error of 600 meters for the detected Scud missile.

(http://www.ima.umn.edu/~arnold/disasters/patriot.html)

4

Variables: Revisited• We already know that a variable must be

declared, specifying the variable's name and the type of information that will be held in it

• As of now, think of a variable as a name for a location in memory cell (we will revisit the concept later)

int total;

int count, temp, result;

Multiple variables can be created in one declaration

data type variable name

Variables

• A variable can be given an initial value in the declaration

• When a variable is referenced in a program, its current value is used

int sum = 0;int base = 32, max = 149;String msg1 = new String( “Hello” );String msg2 = “Hello” ;

6

Change the Value of a Variable: Assignment Statement

• An assignment statement changes the value of a variable

• The assignment operator is the = signtotal = 55;

• Remember: you can only assign a value to a variable that is consistent with the variable's declared type

• The expression on the right is evaluated and the result is stored in the variable on the left

• The value that was in total is overwritten

Constants

• A “constant variable” is an identifier that is similar to a variable except that it holds one value for its entire existence

• Why constants:– give names to otherwise unclear literal values– facilitate changes to the code– prevent inadvertent errors

• In Java, we use the final modifier to declare a variable constant, and the convention is to use all capital words to name a constant

final double PI = 3.14159265;

• The compiler will issue an error if you try to assign value to a constant variable after its declaration

Arithmetic Expressions• An expression is a combination of operators and

operands• Arithmetic expressions (we will see logical expressions

later) are essentially special methods applied to numerical data objects: compute numeric results and make use of the arithmetic operators:

Addition +Subtraction -Multiplication *Division /Remainder %

Division and Remainder

• If both operands to the division operator (/) are integers, the result is an integer (the fractional part is discarded)

• The remainder operator (%) returns the remainder after dividing the second operand into the first

8 / 12 equals?

14 / 3 equals?

14 % 3 equals?

8 % 12 equals?

Recap: Arithmetic Operations

• In daily life, we generally do not distinguish different types of numbers, but a computer does; this leads to the third type of surprise

• the result of an operation depends on data types:

– e.g., 4 / 8 vs 4.0 / 8.0– e.g., “Year “ + 2005

Arithmetic Expressions• Arithmetic operators can be combined into

complex arithmetic expressions, e.g., result = total + count / max - offset;

• The evaluation order of the operators in an arithmetic expression is determined by a well-defined precedence which determines the order in which they are evaluated

• Precedence rules– multiplication (*), division (/), and remainder (%) are

evaluated prior to addition (+) and subtraction (-)– operators with the same precedence are evaluated

from left to right– parentheses can always be used to force the

evaluation order

Operator Precedence

• What is the order of evaluation in the following expressions?

a + b + c + d + e432

a + b * c - d / e3 241

a / (b + c) - d % e2 341

a / (b * (c + (d - e)))4 123

1

Assignment Revisited (again)• You can consider assignment as an operator, with a

lower precedence than the arithmetic operators

First the expression on the right handFirst the expression on the right handside of the = operator is evaluatedside of the = operator is evaluated

Then the result is stored in theThen the result is stored in thevariable on the left hand sidevariable on the left hand side

answer = sum / 4 + MAX * lowest;

14 3 2

Assignment Revisited

• The right and left hand sides of an assignment statement can contain the same variable

First, one is added to theFirst, one is added to theoriginal value of original value of countcount

Then the result is stored back into Then the result is stored back into countcount(overwriting the original value)(overwriting the original value)

count = count + 1;

Variables and Assignment

• What do these two statements do?

x = y;y = x;

Value stored in x y a b b b b b

Question: How do I swap the values of two variables?

How do I Swap the Values of Two Variable?

t1 = x;t2 = y;x = t1;y = t2;

Use two temporaries:

t = x;x = y;y = t;

Use one temporary:

Swapping Values of Two Variables

x = x + y;y = x - y;x = x - y;

Value stored in x y a b a+b b a+b a b a

• No temporaries!

Don’t write such code!!

More Assignment-related Operators

• Increment and decrement operators: ++, --

• Assignment operators: +=, -=, *=, /=

count = count + 1;

count += 1;

count ++;

count = count - 10;

count -= 10;

these three expressions have the same effect

these two expressions have the same effect

Data Conversions• What is data conversion?

– data conversion is the conversion of an object of one type to an object of a different type, e.g., an int to a double, a double to an int, an int to a string

• Why data conversions?– Java is a strongly typed language, i.e., every object has a type, and the

result of an operation depends on the type of the operands• remember: 4 / 8 4.0 / 8.0

– however, sometimes it is more efficient (and natural) to store data as one type, but during a computation, we may want to treat the data as a different type to get desired results

• the Ariane 5 example, the Patriot missile example• for example, we may want to store data as integers, but during a

computation, we want to treat integers as floating point values to get the desired results, e.g., we want miles / gallons to be treated as floating point /

int miles; int gallons; System.out.print( miles / gallons );

– sometimes we just write mixed-type expression

Mixed-type Expressions: Example

• An integer literal is by default of type int• that is, a literal number 4 in Java is of type int• to say that the number 4 is of type long, write 4l or 4L (4L is

preferred over 4l since lower case “l” is hard to distinguish from 1)

• A floating point literal is by default of type double• to day that the number 0.5 is of type float, write 0.5f or 0.5F

• Sometime we write 4.0 / 8, which is a mixed-type expression– Java defines only: int / int, double / double– to perform the mixed-type numerical operation, Java needs to

convert the operands to be of the same type

Mixed-type Expressions: Example 2

• We have already seen examples such as System.out.println( “Year: ” + 2005 );

Here “Year: ” is of type String, but 2005 is an integer

• We know that Java has defined

string1 + string2

but not a

string + integer

Different Type Conversions in Java

• Identity conversion (i.e., no conversion)• Conversions related to primitive data types

widening primitive conversions narrowing primitive conversions

• Conversions related to general classes– widening reference conversions– narrowing reference conversions– we will cover these two cases later in the course; they are

powerful tools to allow polymorphism

• Conversions related to Strings string conversions: i.e., convert a numerical object to a

string, e.g., the number 17 to the string “17”

Widening Primitive Conversions • Widening primitive conversions are those that do not

lose information about the overall magnitude of a numeric value

• Java defines 19 primitive conversions as widening primitive conversionsbyte short, int, long, float, doubleshort int, long, float, doublechar int, long, float, doubleint long, float, doublelong float, doublefloat double

• They are generally safe because they tend to go from a small data type to a larger one (such as a short to an int)– can problems happen in some of the cases?

Narrowing Primitive Conversions

• Java defines 23 primitive conversions as narrowing primitive conversionsbyte charshort byte, charchar byte, shortint byte, short, charlong byte, short, char, intfloat byte, short, char, int, longdouble byte, short, char, int, long, float

• Narrowing primitive conversions may lose overall magnitude of a numeric value, or precision

How Do Data Conversions Happen?

• Implicitly: arithmetic (numeric) promotion– occurs automatically when the operands of a binary arithmetic

operator (note “=“ is not one) are of different types• the promotion uses widening conversion, i.e.,

– if either operand is double, the other is converted to double– otherwise, if either operand is float, the other is converted to float– otherwise, if either operand is long, the other is converted to long– otherwise, both operands are converted to int

Examples: - 4.0 / 8 (which / is it: double/double, float/float, int/int) - 4 / 8.0 (which / is it: double/double, float/float, int/int) - 4 + 5 / 9 + 1.0 + 5 / 9 / 10.0 (what is the value?)

How Do Data Conversions Happen?

• Implicitly: string conversion– applies only to the operands of the operator + – occurs automatically: when one of the operands is a

string, the other operand is converted to a string

Examples:

System.out.println( “Year: “ + 2000 ); System.out.println( “Year: “ + (2000 + 5) ); System.out.println( “Year: “ + 200 + 5); System.out.println( 200 + 5 + “Year: “ );

See Addition.java

How Do Data Conversions Happen?

• Implicitly: assignment conversion– occurs automatically when the value of an expression is assigned

to a variable of another type: • only widening primitive conversions are allowed

• or a special case of narrowing primitive conversions:– the expression is a constant of type int: e.g., 2 * 200– the type of the variable is byte, short or char– the compiler can check to be sure that the value is representable by the

type of the variable

Examples:

byte theAnswer = 42; // is this OK?

float myFloat1 = 2; // is this OK?

float myFloat1 = 2.0; // is this OK?

float myFloat3 = theAnswer; // is this OK?

How Do Data Conversions Happen?

• Explicitly: Casting• Casting is the most powerful and dangerous technique

for data conversions• Both widening and narrowing conversions can be

accomplished by explicitly casting a value• To cast, the type is put in parentheses in front of the

value/variable being converted– for example, if total and count are integers, but we want a

floating point result when dividing them, we can explicitly cast total to be a float:

result = (float) total / count;

Casting

• Pay particular attention when you forcefully cast a narrowing conversion, e.g.,– when you cast a float/double to an int, the float/double

is just truncated to an int (e.g., 0.99 is truncated to 0), not rounded to an int (e.g., 0.5 will be rounded to 1)

Example:

double myDouble = 2.4 / 3.0; int myInt = (int) myDouble;

// what is the value of myInt ?

Using Math Rounding

• In the example of slide 10, if you want myInt to be 1, i.e., 0.8 is rounded to the nearest integer, use the Math.round() method:

double myDouble = 2.4 / 3.0;

int myInt = (int) Math.round( myDouble );

Using Math Rounding

• We invoke Math.round() from the Math class without creating an object

• This is because round is defined as a static method in the class of Math

• Static methods are called class methods– we invoke a static method through the class name, instead of

through an instance (i.e., an object) of the class

• The Math class defines many other static methods, providing various mathematical functions, such as absolute value, trigonometry functions, square root, etc.

Math.abs(), Math.cos(), Math.sqrt()

Conditional Statements

• A conditional statement lets us choose which statement will be executed next

• Conditional statements give us the power to make basic decisions

• Java's conditional statements:– the if and if-else statements– the conditional operator– the switch statement

33

The if Statement• The if statement has the following syntax:

if ( condition ) statement;

ifif is a Java is a Javareserved wordreserved word

The condition must be a The condition must be a boolean expressionboolean expression..e.g., a boolean variable, a == b, a <= b. e.g., a boolean variable, a == b, a <= b. It must evaluate to either true or false.It must evaluate to either true or false.

If the condition is true, the statement is executed.If the condition is true, the statement is executed.If it is false, the statement is skipped.If it is false, the statement is skipped.

The if Statement: Examples

• An example:

String str = “good”;if (GPA > 3.0) str = “excellent”;System.out.println (“Your GPA is " + str);

First, the condition is evaluated. The value of First, the condition is evaluated. The value of GPAGPAis either greater than the value of 3.0, or it is not.is either greater than the value of 3.0, or it is not.

If the condition is true, the assignment statement is executed.If the condition is true, the assignment statement is executed.If it is not, the assignment statement is skipped.If it is not, the assignment statement is skipped.

Either way, the call to println is executed next.Either way, the call to println is executed next.

Logic of an if statement

conditionevaluated

false

statement

true

36

The if-else Statement

• An else clause can be added to an if statement to make it an if-else statement:

if ( condition ) statement1;else statement2;

• If the condition is true, statement1 is executed; if the condition is false, statement2 is executed

• One or the other will be executed, but not both

Logic of an if-else statement

true false

conditionevaluated

Statement 1 Statement 2

38

What if I Want to Run Several Statements: Block Statements

• Several statements can be grouped together into a block statement

• A block is delimited by braces ( { … } )• A block statement can be used wherever a

statement is called for in the Java syntax• For example, in an if-else statement,

the if portion, or the else portion, or both, could be block statements

39

Boolean Expressions: Basics

• A condition of an if statement often uses one of Java's equality operators (==, !=) or relational operators (<, >, <=, >=), which all return boolean results:

== equal to!= not equal to< less than> greater than<= less than or equal to>= greater than or equal to

• Note the difference between the equality operator (==) and the assignment operator (=)

if ( condition ) statement1;else statement2;

40

More Complex (Compound) Boolean Expressions: Logical Operators

• Boolean expressions can also use the following logical operators:

! Logical NOT

&& Logical AND

|| Logical OR

• They all take boolean operands and produce boolean results

• Logical NOT is a unary operator (it has one operand), but logical AND and logical OR are binary operators (they each have two operands)

Loop Statements

while statement

do statement

for statement

while ( condition ) statement;

do { statement list;} while ( condition );

for ( initialization ; condition ; increment ) statement;

Flowchart of a while Loop

statement

true

conditionevaluated

false

while ( condition ) statement;

Example

// set initial value of month so that the while condition // below is false initially

int month = -1;

while (month < 1 || month > 12){

System.out.print( “Enter a month (1 to 12): “);

month = scan.nextInt();

}

System.out.print( “Enter a month (1 to 12): “);

int month = scan.nextInt();

while (month < 1 || month > 12){

System.out.println( month + “ is not a valid month.” );

System.out.print( “Enter a month (1 to 12): “);

month = scan.nextInt();

}

The do Statement: Syntax

do{ statement;}while ( condition );

BothBoth dodo and andwhilewhile

arearereservedreserved

wordswords

The The statementstatement is executed once initially, then the is executed once initially, then the conditioncondition is evaluated is evaluated

The The statementstatement is repetitively executed until the is repetitively executed until the conditioncondition becomes false becomes false

Flowchart of a do Loop

truetrue

conditionevaluated

statement

falsefalse

do{ statement;}while ( condition );

Comparing the while and do Loops

statement

truetrue

conditionevaluated

falsefalse

while loopwhile loop

truetrue

conditionevaluated

statement

falsefalse

do loopdo loop

A do loop is similar to a while loop, except that the condition is evaluated after the body of the loop is executed Therefore the body of a do loop will execute at least once

Example

int month; // no need to initialize month

do

{

System.out.print( “Enter a month (1 to 12): “);

month = scan.nextInt();

} while (month < 1 || month > 12);

// beginning of the next statement

The for Statement: Syntax

for ( initialization ; condition ; increment ) statement;

ReservedReservedwordword

The The initializationinitialization portion portionis executed onceis executed once

before the loop beginsbefore the loop begins

The statement isThe statement isexecuted until theexecuted until the

conditioncondition becomes false becomes false

The The incrementincrement portion is executed at the end of each iteration portion is executed at the end of each iteration

Both semi-colons are always Both semi-colons are always requiredrequired

The for Statement: Syntax

• Each expression in the header of a for loop is optional– if the initialization is left out, no initialization is

performed

– if the condition is left out, it is always considered to be true

– if the increment is left out, no increment operation is performed

Flowchart of a for loop

statement

truetrue

conditionevaluated

falsefalse

increment

initialization

for ( initialization ; condition ; increment ) statement;

The for Statement

• A for loop is equivalent to the following while loop structure:

initialization;while ( condition ){ statement; increment;}

for ( initialization ; condition ; increment ) statement;

The for Statement: Example

counter++

Establish initial value of control variable.

Determine if final value of control variable has been reached.

counter <= max sum+= counter

true

false

int counter = 1

Body of loop (this may be multiple statements)

Increment the control variable.

int sum = 0;

for (int counter = 1; counter <= max; counter++)

sum += counter;

// beginning of the next statement