Java Program Structure

Java Elements 1

Java Program Structure

• Execution begins with first statement in main() • Every Java program MUST have a static method called main( )!

public static void main(String[] args) { …}

Java Elements 2

Data and Data Types• all data must have a data type • data type determines:

– internal representation and storage size. – Range of values – processing/operations

Java Elements 3

Variables

• All Java identifiers must be declared before they are used

• Declarations - create and labels storage• Memory location assigned• Declare one variable per line• type name;

– int a;– int a,b;– int a; // preferable– int b;

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Primitive Types

Type Description Exampleint Integers

(whole numbers)

45, -6, 0, 29987

double Real numbers 7.86, -19.234

char Single characters

'a', 'X', '?'

boolean Logical values True, false

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integer data type• int• whole numbers and their negatives in the range

allowed by your computer, no decimal point – 5,-99,3456

examples:int x;int y;

int total; int keys;

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boolean

• true or false• Example:boolean done;done = true;

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char• one character

– a letter, a digit, or a special symbol• sample values:

– 'A', 'B', 'a', 'b', '1', '2', '+', '-', '$', '#', '?', '*', etc• Unicode• Each character is enclosed in single quotes. • The character '0' is different than the integer 0.

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char

• Examplechar letter;letter = 'A',

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Real numbers• Numbers with decimals• For very large numbers or very small fractions

3.67 * 1017  = 367000000000000000.0 = 3.67E17 5.89 * 10-6 = 0.00000589 = 5.89E-6

• Scientific notation/floating point notation– e stands for exponent and means "multiply by 10 to

the power that follows“• Examples:

5.274 .95 550. 9521e-3 -95e-1 95.213e2

• float – 4 bytes– -3.4E+38 – 3.4E+38– 6- 7 significant digits– Single precision– Use if size is an issue

• double – 8 bytes– -1.7E+308 – 1.7E+308– 15 significant digits– Double precision– Use if precision is an issue, i.e currency

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double

• Example:double price;double velocity;price = 10.6;velocity = 47.63555;

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Initialization

• give a variable a value to start with• can initialize in declaration

int a = 1;int alpha = 32;int stars = 15;int count = 0;// the following example is legal in java, but// violates security guidelinesint length, width = 5; // only width is initialized// and now we see why

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Constants

• cannot be changed• class constant – can be accessed anywhere

in the class• Make programs easier to read• Makes value easier to change• Generally declare outside of method

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Declaring constants

• final type name = value; //local

• public static final type name = value; //globalpublic static final double PI = 3.14159;public static final char BLANK = ' ';public static final double INT_RATE = 0.12;

• use all caps and underscore

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Assignment• variable = expression;• different than equality

– How it works: First the expression on the right-hand side is evaluated and then the resulting value is stored in the variable (in memory) on the left-hand side of the assignment operator.

• variable is the name of a physical location in computer memory

• an expression may be • a constant, or• a variable• a formula to be evaluated

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Expressions

• Simple value or set of operations that produces a value

• literal– 24 or -3.67

• Evaluation – obtain value of expression• Operator

– A symbol used to indicate an operation to be performed on one or more values

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Arithmetic operators

Operator Meaning Example Result

+ addition 2 + 2 4

- Subtraction 53 - 18 35

* Multiplication 3 * 8 24

/ Division 4.8 / 2.0 2.4

% Remainder 25 % 6 1

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Integer division

=> Rounds towards 03 /4 => 019 / 5 => 35/3 => 1• Division by zero is illegal and an ArithmeticException is

thrown• Security issue

– If the dividend is the negative integer of largest possible magnitude for its type, and the divisor is -1, then integer overflow occurs and the result is equal to the dividend.

– No exception is thrown in this case

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ModulusÞ Remainder28 % 7 => 019 % 5 => 425 % 2 => 1 Testing for even or odd

• Number % 2 = 0 for evens Finding individual digits of a number

• Number % 10 is the final digit Also works for floats

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Combined Assignment Operators

• number = number + 5; number += 5;• number = number * 10; number *= 10;• +=• -=• *=• /=• %=

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Increment and Decrement• Increment ++ x = x+ 1;

++x;x++;

• Decrement ‑‑--x; x--;

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Increment and Decrement• Prefix: ++x => increment x before using it

– Generally more efficient than postfix• Postfix: x++ => increment x after using value• Standalone, result is the same

– ++x;– x++;

• You see the behavior when used in expressionsx = 5;y = 5;System.out.println(++x + “ “ + y++); //outputs 6 and 5// final value of x and y is 6

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Precedence

• Orders of operation• "who goes first" when expressions have multiple

operators• rules are similar to rules used in algebra• () parentheses will override the precedence rules• When operators have same precedence, operations

are performed left to right

Java Elements 24

Java Operator Precedence

Description Operators

Unary operators +, -

Multiplicative operators *, /, %

Additive operators +, -

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Precedence

• 3 + 5 + 6 / 2 => 11• (3 + 5 + 6)/2 => 14• Left to right• 40 – 25 - 9 => (40-25) – 9 => 15 – 9 => 6EXAMPLE: 13 * 2 + 239 / 10 % 5 – 2 * 2 => 25

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Mixing Types and Casting

• explicit conversion of a value from one data type to another.

• (type) variable• Example:

– (int) 2.5 /.15 //converts 2.5 to int– (int) (2.5 /.15) //converts result to int

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Assignment examplesint stamp;int answer;Int widget;char letter;

stamp = 14; // valid14 = stamp; // invalidanswer = stamp;widget = stamp * 3;letter = 'a';letter = "alpha"; // invalid

Intro to Strings• String not built into Java• String is a class• Part of java.lang class• Automatically imported• Declare:

– String object; // note caps• String name = "Tom Jones";• String s1 = "Hello”;• String s2 = "World!";

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Strings• Strings can be created implicitly by using a quoted string

– String s1 = "Hello”;– String s2 = "World!";

• or, by using + on two String objects to create a new one – String combo = s1 + " " + s2;

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String methods• <variable>.<method name>(<expression>,<expression>,…,<expression>)

– any String method requiring an index will throw an IndexOutOfBoundsException if 0 > index > length()-1

– String s1 = "Hello”;– String s2 = "World!";– length(); - returns length of string– System.out.println("Length of s1 = " + s1.length()); Length of s1 = 5• Index – integer used to indicate location

• Zero-based-

– charAt(0) => returns 1st character

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Console Input• Scanner is a class• Need to import java.utilimport java.util.*; //allows use of Scanner classScanner console = new Scanner (System.in);• Creates object console (can use any identifier name)• Associates console with standard input device• System.in – standard input device

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Console input• console.nextInt() // retrieves next item as an integer• console.nextDouble() //retrieves next item as double• console.next() //retrieves next item as string• console.nextLine() // retrieves next item as string up to

newline

• ch = console.next().charAt(0); // reads a single character

• Inappropriate type -> exception

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import java.util.*;public class BMICalculator{ public static void main(String [] args) { double height; double weight; double bmi; Scanner console = new Scanner(System.in); System.out.println("Enter height"); height = console.nextDouble(); System.out.println("Enter weight"); weight = console.nextDouble(); bmi = weight/(height * height) * 703; System.out.println("Current BMI:"); System.out.println(bmi); }}

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Output

• System.out – standard output device• System.out.print(expression);• System.out.println(expression);//goes to next line• System.out.println();//blank line

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Packages, Classes, Methods, import

• Few operations defined in Java• Many methods & identifiers are defined in packages • Class – set of related operations, allows users to create own type• Method – set of instructions designed to accomplish a specific

task• Package – collection of related classes

– java.util, contains class Scanner and methods nextInt, etc.import packageName.*;Import java.util.*; // compiler determines relevant classesimport java.util.Scanner;

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Creating a Java Application program

• Program consists of one or more classes• Declare variables inside method• Declare named constants and input stream

objects outside of main

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Java application programimport statements if anypublic class ClassName{ declare names constants and/or stream objects

public static void main(String[] args) {

variable declarations

executable statements}

}

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Programming Style and Form

• Use of blanks– Separate numbers when data is input– Use blank lines to separate data and code

• All Java statements must end with a semicolon• Use uppercase for constants• Begin variables with lowercase• For run-together-words, capitalize each new

word

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Programming Style and Form, cont’d

• Use clearly written prompt linesSystem.out.println(“Please enter a number between 1 and 10 and “ + “press Enter”);

• Use comments to document• Use proper indentation and formatting