Review Yong Choi BPA CSUB Access Modifier public

Review… Yong Choi BPA CSUB

Access Modifier • public class Hello. World. App – More detailes of Java key (reserved) word – The keyword, public is called an access modifier and Java code usually begin with an access modifier – public, indicates that this code can be accessed by all objects and can be extended, or used, as a basis for another class. (opposite to private)

Class Name • The access modifier is followed by the word, class, and the class name. • The class name (“Hello. World. App”) is assigned by a programmer. • It should be a user-friendly word that is not on the list of key words. • By using understandable and user-friendly word for class, objects, and variables, you can not only avoid confusions but also increase understandability of your program.

A Key (reserved) words list • • • abstract boolean break byte case catch char class const continue • • • double else extends finally float for goto if implements • • • interface long native new package private protected public return short • • • strictfp super switch synchronize d this throws transient try void

Requirements for the Class Name • A class name should begin with a letter of the alphabet. – includes any non-English letter, such as or , an underscore, or a dollar sign. • A class name can contain only letters, digits, underscore, or a dollar signs. • A class name cannot be a Java programming key words such as public or class. • A class name cannot be one of following

More about Class Name • It is a Java language industry standard to begin class names with an upper case letter and employ uppercase letters as need to improve readability. Class. Name • The Java compiler expects the file name to match the same class name that you assigned at the beginning of your program.

Using Curly Braces • Programmers enclose the contents of all classes within curly braces ({ and }). • For every opening curly brace in a java program, there must be a corresponding closing brace. • The placement of the opening and closing curly braces is not important to the compiler.

Precedence of Arithmetic Operator Meaning precedence - unary minus higest + unary plus higest * multiplication middle / division middle % remainder addition or concatenation subtraction middle + - low

More Arithmetic Operators • NEVER use the lower case 'l' because it is easily confused with a digit '1'. – 123 l (last one is L) vs. 1231

Declaring a Variable • Java declaration: – Variable-Type Variable-Name • Example of declaration: – float flt. Dollar. Amt; – int. Num = 23; • Multiple declarations of the same data type can be made in a single de declaration: – float flt. Dollar. Amt, flt. Curr. Balance, flt. New. Total; • Multiple declarations of the different data type can NOT be made in a single de declaration:

Syntax of Variable Declaration • Start with lower case letter • Remember: it’s case sensitive! – TOTAL and total are different names. • Must start with a letter, dollar sign, or underscore – Do not start with a digit. • Must contain only letters, dollar signs, underscores, or digits – Use only the characters 'a' through 'z', 'A' through 'Z', '0' through '9', character '_', and character '$'. • A name can not contain the space character.

Syntax of Variable Declaration • A name can be any length. • A name can not be a reserved word. • A name must not already be in use in this part of the program. – Must not be a reserved word – “Camelback” naming style: COBOL: Current-Balance Java: current. Balance – Good idea to include data type in name: flt. Current. Balance • An ending semicolon

Assignment Statements • An assignment statement changes the value that is held in a variable. public class Example 5 { public static void main ( String[] args ) { long pay. Amount; //a declaration without an initial value pay. Amount = 123; //an assignment statement System. out. println("The variable contains: " + pay. Amount ); } }

Syntax of assignment Statements • variable. Name = expression; – The equal sign "=" means "assignment operator. – variable. Name is the name of a variable that has been declared somewhere in the program. – expression is a collection of characters that calls for a value. – Errors may occur if the lefthand variable is not the same variable type that the

Syntax of assignment Statements • An assignment statement asks for the computer to perform two steps, in order: 1. Evaluate the expression (i. e. , calculate a value. ) 2. Store the value in the variable. • For example, the assignment statement: • sum = 32 + 8 ; asks for two actions: 1. Evaluate the Expression — 32 + 8 is calculated, yielding 40. 2. Puts the value (40) in the variable, which is

The Assignment Operator • We’ve already used this operator to initialize variables. – float flt. Curr. Balance = 1000. 0395 F; – flt. New. Total = flt. Curr. Balance; • It can also be used to change the value of an existing variable.

Expressions • An expression is a combination of literals, operators, variables, and parentheses used to calculate a value. This (slightly incomplete) definition needs some explanation: – literal — characters that directly mean a value, like: 3. 456 – operator — a symbol like plus ("+") or times ("*") that asks for doing arithmetic. – variable — a section of memory containing a value. – parentheses — "(" and ")". • When the expression on the right gets complicated you need to know the two steps to figure out what happens.

Expressions (con’t) • This might sound awful. Actually, this is stuff that you know from algebra, like: • (32 - y) / ( x + 5 ) , the character "/" means "division. " Not just any mess will work (of course). The following: • 32 - y) / ( x 5 + ) is not a syntactically correct expression. There are rules for this, but the best rule is that an expression must look OK as algebra.

Casting • What happens when a numeric value is assigned into a numeric variable of unlike type? Double d int i i = 45; - OK because int to int d = i; - Ok because int to double (automatic conversion – see next slide). The 45. 0 is stored in d.

Casting • Casting is the process of performing a deliberate change of data type. • Java will automatically perform widening conversion. – flt. Curr. Balance = int. Last. Balance; – The integer will automatically be converted to floating point.

Casting • One data type can be explicitly converted to another by a programmer. Double d int i i = (int) 3. 14; - i equals 3 • Must be careful to use!!

Casting Order of widening conversion: byte short int long float double

Increment Operator • The increment operator ++ adds one to a variable. – counter = counter + 1 ; // add one to counter – Counter ++ • Usually the variable is an integer type (byte, short, int, or long) but it can be a floating point type (float or double. ) • The two plus signs must not be separated. • Usually they are written immediately

How to Use Increment Operator • The increment operator can be used as part of an arithmetic expression, as in the following: int sum = 0; int counter = 10; sum = counter++ ; System. out. println("sum: "+ sum " + counter: " + counter );

Example of Increment Operator • The expression on the right of the = can be more complicated, as in the following fragment: int value = 10 ; int result = 0 ; result = value++ * 2 ; System. out. println("value: " + value + " result: " + result );

Example of Without Increment Operator • The following is same as previous example. int value = 10 ; int result = 0 ; result = value * 2 ; value = value + 1 ; System. out. println("value: " + value + " result: " + result );

Expression of Increment Operator • The increment operator must be applied to a variable. It cannot be applied to a larger arithmetic expression. The following is incorrect: int x = 15; int result; result = (x * 3 + 2)++ ; // Wrong!

Prefix Increment Operator • The increment operator ++ can be put in front of a variable. • When it is put in front of a variable (as in ++counter) it is called a prefix operator. • When it is put behind a variable (as in counter++) it is called a postfix operator. Both ways increment the variable. However: – ++countermeans increment before using. – counter++means increment after using.

Decrement Operator • The operator -- is a postfix and a prefix decrement operator. The postfix operator decrements a variable after using its value; the prefix operator increments a variable before using its value. Expression Operation x-- use the value, then subtract 1 --x subtract 1, then use the value Example int x = 10; int y; y = x-- ; int x = 10; int y; y = --x ; Result x is 9; y is 10 x is 9; y is 9

Example of Decrement Operator int x = 99; int y = 10; y = --x ; System. out. println("x: " + x + " y: " + y ); • Advice for using Prefix and Postfix Increments and Decrements – Don’t use them always. – Sometimes they look too confusing!