Java Access Modifiers: Understanding public and private Classes

1. Review… Yong Choi BPA CSUB

2. Access Modifier • public class HelloWorldApp • 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) • If you omit the keyword, public, you limit the access to this class - private

3. Class Name • The access modifier is followed by the word, class, and the class name. • The class name (“HelloWorldApp”) 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. • class X vs. class GrandTotal

4. abstract boolean break byte case catch char class const continue default do int interface long native new package private protected public return short static A Key (reserved) words list • strictfp • super • switch • synchronized • this • throw • throws • transient • try • void • volatile • while • double • else • extends • final • finally • float • for • goto • if • implements • import • instanceof

5. 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 values: true, false, or null.

6. 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. - ClassName • The Java compiler expects the file name to match the same class name that you assigned at the beginning of your program. • Java is case sensitive. – The compiler considers differently.

10. 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.

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

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

13. Declaring a Variable • Java declaration: • Variable-Type Variable-Name • Example of declaration: • float fltDollarAmt; • int intNum = 23; • Multiple declarations of the same data type can be made in a single de declaration: • float fltDollarAmt, fltCurrBalance, fltNewTotal; • Multiple declarations of the different data type can NOT be made in a single de declaration: • float fltDollarAmt, int intTotal; - incorrect

14. 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.

15. 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: currentBalance • Good idea to include data type in name: fltCurrentBalance • An ending semicolon

16. Assignment Statements • An assignment statement changes the value that is held in a variable. public class Example5 { public static void main ( String[] args ) { long payAmount; //a declaration without an initial value payAmount = 123; //an assignment statement System.out.println("The variable contains: " + payAmount ); } }

17. Syntax of assignment Statements • variableName = expression; • The equal sign "=" means "assignment operator. • variableName 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 righthand expression evaluates to.

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

19. The Assignment Operator • We’ve already used this operator to initialize variables. • float fltCurrBalance = 1000.0395F; • fltNewTotal = fltCurrBalance; • It can also be used to change the value of an existing variable.

20. 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.

21. 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.

22. 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. i = d; - Java will refuse to compile an assignment statement requiring narrowing conversion. - so need Casting

23. Casting • Casting is the process of performing a deliberate change of data type. • Java will automatically perform widening conversion. • fltCurrBalance = intLastBalance; • The integer will automatically be converted to floating point.

24. 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!!

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

26. 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 adjacent to the variable.

27. 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 );

28. 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 );

29. 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 );

30. 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!

31. 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.

32. Expression Operation Example Result x-- use the value, then subtract 1 int x = 10;int y;y = x-- ; x is 9; y is 10 --x subtract 1, then use the value int x = 10;int y;y = --x ; x is 9; y is 9 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.

33. 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!