Java Syntax and Semantics

1. Java Syntax and Semantics • Syntax • The formal rules governing how valid instructions are written in a programming language • Semantics • The set of rules that determines the meaning of instructions written in a programming language

2. Data Types • Data is stored internally in memory, externally on disk or tape or input from an input device • Data type determines how the data is represented in the computer and the kinds of processing the computer can perform on it.

3. Data Types • Standard or built-in data types • Used frequently and provided by Java • integer number, real number, characters and boolean • User-defined data types • These are referred to as classes

4. The char Data Type • Describes data consisting of one alphanumeric character – a letter, a digit or a special symbol • Examples of values of type char • ‘A’ ‘’ ‘a’ ‘8’ ‘+’ ‘?’ ‘ ‘ • Each character is enclosed in single quotes • How does Java represent the single quote? • ‘’’ – Not valid , syntax error • Java provides an escape sequence - the sequence of the two characters \’ is treated as a single character – ‘\’’ • How would you represent the backslash character?

5. Classes and objects • Class • A description of behavior of a group of objects with similar properties and behaviors • A pattern for an object • Contains fields or data values and methods. (Method: A subprogram that defines one aspect of the behavior of the class) • Object • An entity or thing that is relevant in the context of a problem • An instance of a class

6. Classes and Objects • How do we create an object from a class? Instantiation • Use of an operator called new, takes the class name and returns an object of the class type. • Object that is returned is an instance of the class

7. Class Declaration Modifiers ImportDeclaration;... Modifiers... ClassIdentifier { ClassDeclaration... } private . . . public class DoNothing { }

8. The String Class • A string is a sequence of characters enclosed in double quotes. • In Java, a string is an object, an instance of the class String. • Examples • “Introduction to” “Program” “ Design” • A string must be typed entirely on one line • Quotes are not considered parts of the string

9. The String Class • “amount” – character string made up of letters a, m, o, u, n, and t. • “12345” – character string made up of characters 1, 2, 3, 4, and 5 in that order • 12345 is an integer quantity that can be used in calculations. • “” – Empty strings contains no characters, not even spaces

10. The String Class • Java provides operations for • Joining strings • Comparing strings • Copying portions of strings • Changing the case of letters in strings • Converting strings to numbers

11. Declarations – defining terms • A statement that associates a name (an identifier) with a description of an element in a Java program. - elements : field, a method, a class or a package sot that the programmer can refer to that item by name. int minAB; • Compiler picks a location in memory to be associated with identifier

12. Declarations • In Java, the identifier must be declared before it is used. • Allows compiler to verify that the use of the identifier is consistent with what it is declared to be. • Java is strongly typed. A variable can only contain a value of the type or class specified in its declaration • Fields can be variable or constant

13. Variables char myChar; Variable Identifier myChar (memory location 111001010101) ? VARIABLE (char) VALUE DATA TYPE

14. Variables • A variable is a location in memory, referenced by an identifier or name, that contains a data value that can be changed • Variable declaration Modifiers TypeName Identifier, Identifier...; • TypeName – Name of class or type such as char or String char letter, middleInitial, ch; OR char letter; char middleInitial; char ch;

15. Variables String firstName; // person first name String lastName; //person’s last name String title; //person’s title char middileInitial; // person’s initial

16. Constants • Something whose value never changes • ‘A’, ‘@’, “Hello World” are constants • Use of the actual value of a constant is the use of a literal value. • Literal value - any constant value written in a program • Alternative to a literal value is the named constant which is introduced in a declaration statement • A named constant - a location in memory, referenced by an identifier, that contains a data value that cannot be changed

17. Constant Declaration ModifiersfinalTypeName Identifier = LiteralValue; • final modifier tells Java compiler that this value is the last and only value that this field will have. Examples final String LINE_OF_STARS = “*******”; final char BLANK = ‘ ‘; final String MESSAGE = “Error Condition”; (Programming convention: Named constant in uppercase, to distinguish between it and variables)

18. Matters of Style – Capitalization of Identifiers • Variables and methods begin with a lowercase letter and capitalize each successive English word. • lenghtInYards middleInitial hours • Class names begin with an upper case letter but are capitalized the same as variable names thereafter. • PayRollFrame String MyDataType • Identifiers representing named constants are all upper case with underscores used to separate the English words • BOOK_TITLE OVERTIME MAX_LENGTH

19. Executable Statements • Provide ways of acting or performing operations on data • Assignment statement • String expressions • Initialization of Fields • Outputting data to the screen

20. Assignment statement • A statement that stores the value of an expression into a variable. variable = expression; variable is set equal to the value of the expression variable gets the value of the expression Expression • An arrangement of identifiers, literals and operators that can be evaluated to compute a value of a given type

21. Assignment Statement • Only one variable can be on the left-hand side of an assignment statement. • NOT like the math = (e.g. x + y = z + 4) • The expression on the RHS is evaluated and the result is then stored into the single variable on the left of the operator. • The value assigned to a variable must be of the same type as the variable.

22. Declaration & Assignment Example String firstName, middleName, lastName; String title; char middleInitial, myChar;

23. String Expressions • Concatenation – A special string operation that uses the + operator • Concatenating two strings results in a new string containing characters from both strings String courseCode, phrase1,phrase2; phrase1 = “CS”; phrase2 = “51Q”; courseCode = phrase1 + phrase2; courseCode now stores string “CS51Q”

24. String Expressions • String literals that are too long to fit on one line can be broken into smaller string literals and concatenated. longSentence = “ this is a long sentence ” + “which cannot fit on one line“ + “. Here is the last segment.”; • To extend an existing string: String courseCode; courseCode = “CS”; courseCode = courseCode + “51Q”;

25. String Expressions • Concatenation only works with values of type String • Java converts built-in types to its equivalent string if one attempts to concatenate it to a String. String result; result = “The square of 12 is “ + 144; “The square of 12 is 144” is assigned to the variable result

26. Fields • Java does not distinguish between the declaration of a variable and a constant. • final keyword is used to make the necessary distinction and the field is given an initial value when declared. • Generally, fields can be initialized following this template ModifierTypeName Identifier = Expression, Identifier = Expression;

27. Output – System.out • System.out is an object that represents an output device. • By default, the output device is the screen. • Messages to these objects state what should be printed to the screen • Messages sent by applying methods printand println

28. System.out • Method print is invoked by placing the method name next to the object with a dot in between. • The “something” to be printed is called a parameter is placed within the braces. System.out.print(“Computer” + “ “ + “Science”); System.out.print(“Computer”); System.out.print(“ “); System.out.print(“Science”): Computer Science

29. System.out • print – successive messages printed next to each other • println – used to go to a new line after a string has been printed

30. Comments • Statements entered by the programmer that are ignored by the compiler • Useful for documenting program • Two forms • /* This statement is ignored */ • //Everything from the double forward slash to the //end of the line is ignored. • Use comments at the beginning of programs, after defining variables, to explain methods, to keep track of starting and ending of blocks of code.