Data and Variables Data Types Expressions Operators Precedence - - PowerPoint PPT Presentation

Data and Variables

• Data Types
• Expressions

– Operators – Precedence

• String Concatenation
• Variables

– Declaration – Assignment – Shorthand operators

Review

– class

• All code in a java file is written in a

class

• public class <ClassName> { ….. }

– Static methods

• public static void <methodName> () {

….. }

• Made up of statements;
• Used in procedural programming

– Decompose large tasks into smaller subtasks – Create a method for each unique subtask to eliminate redundancy – Errors

• Syntax (won’t compile)
• Runtime (won’t run)
• Logic (incorrect result)

– Identifiers

• Must begin with a letter

, underscore, or $

• Must contain only letters,

digits, underscores, or $

• Can not be a Java

keyword/reserved word

Data Types

• Java is a “strongly typed” language.

– Each piece of data has a specific “type”

• Java supports two categories of data types

– Primitive data – Objects (covered later)

Primitive Data Types

• Integer (Whole Number)
• Java keywords for Integer Types
• int

(4 bytes)

• byte (1 byte)
• short (2 bytes)
• long (8 bytes)
• We will use the int type

– minimum value -2,147,483,648 – maximum value 2,147,483,647

• Examples:

0, 35, +148, -250

Primitive Data Types (cont.)

• Real (Floating point, Decimal)
• Java keywords for Real Number Types
• double(8 bytes) (double precision)
• float (4 bytes)
• We will use the double type

– minimum value 4.94065645841246544e-324 – maximum value 1.79769313486231570e+308

• Examples: 23. .03 -5.5 10.0583 34e-20

Primitive Data Types (cont.)

• Character (Single Character)
• Java keyword for Character Type

– char (2 bytes)

• Enclosed in single quotes ‘ ’
• Examples: ‘a’ ‘&’ ‘X’ ‘8’
• Use escape sequence for

– Single Quote ‘\’’ – Backslash ‘\\’

Primitive Data Types (cont.)

• boolean (Logical Values)
• Java keyword

– boolean

• Only two different values (Java keywords)

– true – false

• Covered Later

Practice

• Which of the following are legal int

literals?

• 5.
• -1
• 22
• ‘7’
• 1.5
• -6875309
• 10.0
• 2.3

Practice

• What primitive data type would you use

to store…

• a person’s middle initial?
• number of people in class?
• cost of lunch?
• distance to class?
• number of siblings a person has?
• your grade in a class?

Expressions

• Expression – A simple value or a set of
• perations that produces a value.
• Evaluation – The process of obtaining the

value of an expression.

• Expressions have values, statements do not.
• Two types

– Arithmetic – Boolean (covered later)

Expressions (cont.)

• A simple value or a set of operations that

produce a value

– Literal Value

• 42
• 365 0 +9812
• 28.9 0.24 207. 0.0 -.98
• true false
• ‘a’

‘m’ ‘X’ ‘!’ ‘\\’ ‘\’’

– Operations: combining values

• (5 * 6) + 32
• Use expressions in print statements

– System.out.println(4); – System.out.println(2 + 2);

Arithmetic Operators and Operands

• Operators: indicate the operation to be

performed

– Addition (+) (5 + 2) – Subtraction (-) (5 - 2) – Multiplication (*) (5 * 2) – Division ( / ) (5 / 2) (5.0 / 2.0) – Remainder (mod) (%) (5 % 2)

• Operands: values used in the expression

– Literal – Expression (3 + 2) * (6 / 2)

Division

• Integer Division - result is an integer

, the remainder is dropped (truncation).

– examples:

• 22 / 4 = 5
• 116 / 5 = 23
• Floating Point Division – “normal division”

.

– examples:

• 22.0 / 4. = 5.5
• 116.0 / 5.0 = 23.2

Remainder (Mod)

• % - The remainder operator

– Returns the remainder of division – examples:

• 22 % 4 = 2
• 22. % 4.0 = 2.0
• 5.2 % 2.4 = 0.4
• 1 % 5 = 1
• 0 % 5 = 0
• 5 % 0 = undefined (runtime error)
• Useful applications

– T esting for even/odd (number % 2 = 0 means even) – Extract final digit (number % 10)

Precedence

• Precedence – The order of evaluating

expressions

• Multiplication, Division, and Modulo take

precedence over Addition and Subtraction.

• Unary operators (+, -) (pos, neg) take

precedence over all 5 operators.

• Within same level of precedence, evaluate

from left to right

• Override precedence with parentheses.

Precedence Examples

• 8 * 4 / 10 + (4 + 2) * 5 % 2
• 46 % 8 * 2 / 7 + 11 / 4 * 3

Mixing Types

Promotion: a copy of a value is converted to a “higher” type.

– Does not lose information about the value – Integer to a double – Result of operation between integer and double is a double (integer is promoted to a double to perform the operation) – 6.4 + 8 = 14.4

Casting: a copy of a value is converted to another type

– Double to integer – Loses the fraction part – Requires a cast – put the name of the type you want in parentheses in front of the value you want to cast – (int) 5.62 (result is 5) – (int) 5.0 / 2.0 (result is 5 / 2.0 = 2.5) – (int) (5.0 / 2.0) (result is 2)

More Casting

• Only casts value immediately following cast For example:

– 23 / 2 = 11 – (double) 23 / 2 = 11.5 (23. / 2, 23 / 2., 23. / 2.) – (double) (23 / 2) = 11.0

• Example of when we may want to use casting:

– We have some books that are 0.15 feet wide and we want to know how many of them will fit in a bookshelf that is 2.5 feet wide. – Then, 2.5 / 0.15 = 16.666 books.

• How are we going to put 2/3 of a book on the shelf?

– Instead, we need to see how many whole books can fit

• n the shelf: (int) (2.5 / 0.15) = 16

String Concatenation

• Combining several strings into a single string, or

combining a string with other data into a new , longer string

– Addition operator (+) – Result of adding a String literal and a primitive data type is a String literal (primitive type is promoted to a String literal to perform the operation) – Examples:

• "hello" + 42

is "hello42"

• 1 + "abc" + 2

is "1abc2"

• "abc" + 1 + 2 is "abc12"
• 1 + 2 + "abc"

is "3abc"

• "abc" + 9 * 3 is

"abc27"

• "12" + 3 is

"123"

Expression Practice

• Consider the data type in the answers. For example, 5 (an int) is

totally different from 5.0 (a double)

• 5.5 + 2 / 3
• 2 + 3.5 * -2
• 5 * 10 / 5.0 + 3 * (int) 2.5 / 10
• 8 + 4 + "3" + 4 + 10 / 2 + "6" + (7 + 8)

In-Class Exercise

• 1. 2 * 3 - 4 % 2 / 2 + 10 / (double)(10 / 4)
• 2. 42 % 6 + 35 / 5 % 4 - 16. / (12 % 8)
• 3. (int) 5.0 / 2.0 + (4 + 6) / 5 * 2
• 4. 3 * (5 - 3) + 3 - 3 * 2
• 5. 9 / 2.0 - 2 - 10 / (int) (5 * 0.5)
• 6. "1" + 2 / 3 + "four" + 5 % 6 + 7 + (8 + 9)

Variables

• Variable – A memory location with a name and a type

that stores a value.

• Declaration – A request to set aside a new variable

with a given type and name.

– <type> <name>;

• Variable names are identifiers

– Style guidelines: start with lowercase letter , capitalize each subsequent word

• Examples

int age; double weight; char firstInitial;

Variable Assignment

• Assignment – Giving a value to a variable.
• A value is assigned to a variable using an assignment statement:

<variable> = <expression>;

• Equal sign (=) is the operator for assignment
• The value of the expression on the right-hand side of the

assignment is stored in the variable on the left-hand side of the assignment and is the result of the assignment operation

• Examples

double height = 70; double weight = 195; double bmi = weight / (height * weight) * 703; char firstInitial = ‘M’;

Initialization

• Giving a variable an initial value is known

as the initialization of the variable

int age; //uninitialized age = 35; //now initialized

Declaration/Assignment Variations

//declare and assign a value to //a variable in one statement <type> <name> = <expression>; – int value = 10; – int answer = 5 * 6; //declare multiple variables of //the same type in one statement <type> <name1>, <name2>, …, <name3>; – int value, answer; – int number = 5, sum; – int age1 = 5, age2 = 8;

Practice

• Which of the following choices is the

correct syntax for declaring a real number variable named ‘grade’ and initializing its value to 4.0?

• 1. 4.0 = grade;
• 2. double grade = 4.0;
• 3. grade = double 4.0;
• 4. 4.0 = double grade;
• 5. int grade = 4.0;

Variables (cont.)

Changing the value of a variable using “assignment”: What will the values of x, y, and z be after the following statements are executed?

int x = 3, y = 7, z; z = x + y; x = x + 2; x = y – 5; y = z – x;

Variables (cont.)

Changing the value of a variable using a “shorthand” method: – Special Assignment Operators that increment or decrement a value by a set amount Standard Assignment x = x + 1; y = y – 7; z = z * 2; a = a / 3; Shorthand Assignment x += 1; y -= 7; z *= 2; a /= 3;

Variables (cont.)

Changing the value of a variable using increment/decrement (++/--) operators

Increment a value by 1 i++; (post increment) ++i; (pre increment) Decrement a value by 1 i--; (post decrement)

• -i; (pre decrement)

Post versions evaluate to the older (original) value – “evaluate using current value of i, then increment i” Pre versions evaluate to the new (final) value – “increment i, then evaluate using new i“

Pre/Post Increment/Decrement Examples

What are the values of age1, age2, age3, and years after the following statements are executed? int age1 = 21; int age2 = 50; int years; int age3 = age2 – age1++; years = ++age1 + age2--; years++;

Common Syntax errors

• A variable can't be used until it is assigned a

value.

int x; System.out.println(x); // ERROR: x has no value

• You may not declare the same variable twice.

int x; int x; // ERROR: x already exists int x = 3; int x = 5; // ERROR: x already exists

Printing an expression or variable value

• Use + to print a String and an expression value on one line

System.out.println("Grade: " + (95.1 + 71.9) / 2);

• Output: Grade: 83.5

System.out.println("Grade: " + (95.1 + 71.9) + 2);

• Output: Grade: 167.02
• Use + to print a String and a variable's value on one line.

double grade = (95.1 + 71.9 + 82.6) / 3.0; System.out.println("Your grade was " + grade);

• Output: Your grade was 83.2

int students = 11 + 17 + 4 + 19 + 14; System.out.println("There are " + students + " students in the course.");

• Output: There are 65 students in the course.

Practice

• What does the following program output?

public class Variables { public static void main(String[] args) { int num1, num2, num3; num1 = 4; num2 = 12; num3 = num1; num2 /= 3; num1 = num2++ + 4; num3 += num1; int num4 = --num1; num4++; System.out.print("num1 = " + --num1 + "\nnum2 = " + ++num2 + "\nnum3 = " + num3++); System.out.println(" num4 = " + num4--); } }

In-Class Assignment

• Meadowdale Dairy Farm sells organic brown eggs

to local customers. They charge $3.25 for a dozen eggs, or 45 cents for individual eggs that are not part of a dozen.

• Write a class that includes a variable for the

number of eggs in the order and assign the value 27 to this variable.

• Calculate and display the amount owed with a full

explanation as follows: You ordered 27 eggs. That’s 2 dozen at $3.25 per dozen and 3 loose eggs at 45 cents each for a total of $7.85.

• Save the class as Eggs.java.

In-Class Assignment

• Write a class that calculates and displays the

conversion of an integer variable storing a number of dollars into currency denominations—20s, 10s, 5s, and 1s. Assign the value of 57 to the variable. Print the resulting conversion as shown below (the

• utput values must be calculated, not

printed out using literal values).

• Output:

$57 converts to: 2 20s, 1 10s, 1 5s, and 2 1s.

• Save the class as Dollars.java.