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1001ICT:: lecture 2

1001ICT:: lecture 2. Introduction to Programming. 1. Friday, 24 October 2003. Variables. A variable is an item of data named by an identifier. An object stores its state in variables. You must explicitly provide a name and a type for each variable you want to use in your program.

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1001ICT:: lecture 2

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  1. 1001ICT:: lecture 2 • Introduction to Programming 1 Friday, 24 October 2003

  2. Variables • A variableis an item of data named by an identifier. • An object stores its state in variables. • You must explicitly provide a name and a type for each variable you want to use in your program. • The variable's name must be a legal identifier --an unlimited series of Unicode characters that begins with a letter. You use the variable name to refer to the data that the variable contains. The variable's type determines what values it can hold and what operations can be performed on it. 2 Friday, 24 October 2003

  3. Primitive & Reference Variables • To give a variable a type and a name, you write a variable declaration, which generally looks like this: • type name • Every variable must have a data type. A variable's data type determines the values that the variable can contain and the operations that can be performed on it. • The Java programming language has two categories of data types: primitive and reference. A variable of primitive type contains a single value of the appropriate size and format for its type: a number, a character, or a boolean value. 3 Friday, 24 October 2003

  4. Primitive variables • The following table lists, by keyword, all of the primitive data types supported by Java, their sizes and formats, and a brief description of each: 4 Friday, 24 October 2003

  5. You can put a literal primitive value directly in your code. For example, if you need to assign the value 4 to an integer variable you can write this: • int anInt = 4; • The digit 4 is a literal integer value. Here are some examples of literal values of various primitive types: 5 Friday, 24 October 2003

  6. Generally speaking, a series of digits with no decimal point is typed as an integer. You can specify a long integer by putting an 'L' or 'l' after the number. 'L' is preferred as it cannot be confused with the digit '1'. A series of digits with a decimal point is of type double. You can specify a float by putting an 'f' or 'F' after the number. A literal character value is any single Unicode character between single quote marks. The two boolean literals are simply true and false. 6 Friday, 24 October 2003

  7. Reference variables • Arrays*, classes, and interfaces* are reference types. The value of a reference type variable, in contrast to that of a primitive type, is a reference to (an address of) the value or set of values represented by the variable. • A reference is called a pointer, or a memory address in other languages. The Java programming language does not support the explicit use of addresses like other languages do. You use the variable's name instead. 7 *discussed later Friday, 24 October 2003

  8. Variable Names • A program refers to a variable's value by the variable's name. • In the Java programming language, the following must hold true for a variable name: • It must be a legal identifier. An identifier is an unlimited series of Unicode characters that begins with a letter. • It must not be a keyword, a boolean literal (true or false), or the reserved word null. • It must be unique within its scope. A variable may have the same name as a variable whose declaration appears in a different scope. In some situations, a variable may share the same name as another variable if it is declared within a nested block of code. (variable scope will be discussed next) 8 Friday, 24 October 2003

  9. By Convention :  Variable names begin with a lowercase letter, and class names begin with an uppercase letter. If a variable name consists of more than one word, the words are joined together, and each word after the first begins with an uppercase letter, like this: isVisible. The underscore character (_) is acceptable anywhere in a name, but by convention is used only to separate words in constants (because constants are all caps by convention and thus cannot be case-delimited). 9 Friday, 24 October 2003

  10. examples of variable declarations: • int years; • int days = 7; • long seconds; • double interestRate; • float sharePrice = 3.2456F; • boolean gameOver; • boolean accountOverdrawn = false; 10 Friday, 24 October 2003

  11. Short quiz • What is a variable? • What are the 2 categories of variable types? • What are the main primitive types? • Declare the following variables, giving them appropriate data types: • variabledescription • students stores number of students • speedOfLight stores speed of light (surprise!) • isAlive stores whether something is dead or alive • orderTotal stores currency value of order total 11 Friday, 24 October 2003

  12. Scope • A variable's scope is the region of a program within which the variable can be referred to by its simple name. Secondarily, scope also determines when the system creates and destroys memory for the variable. • The location of the variable declaration within your program establishes its scope and places it into one of these four categories: • member variable • local variable • method parameter • exception-handler parameter (discussed in 1104CIT programming II) 12 Friday, 24 October 2003

  13. 13 Friday, 24 October 2003

  14. A member variable is a member of a class or an object. It is declared within a class but outside of any method or constructor. • A member variable's scope is the entire declaration of the class. However, the declaration of a member needs to appear before it is used when the use is in a member initialisation expression. 14 Friday, 24 October 2003

  15. You declare local variables within a block of code. • In general, the scope of a local variable extends from its declaration to the end of the code block in which it was declared. • Parameters are formal arguments to methods or constructors and are used to pass values into methods and constructors. The scope of a parameter is the entire method or constructor for which it is a parameter. • Parameters will be discussed further when we discuss methods in more detail. 15 Friday, 24 October 2003

  16. Expressions & assignment • an expression is a statement which returns a value • consider the following code fragment: • int x = 3; • int y = 4; • int z; • z = x + y; • the last line is an expression, since it returns a value • the value produced is often called the return value • You can provide an initial value for a variable within its declaration by using the assignment operator (=) just as is done in lines 1 & 2 above 16 Friday, 24 October 2003

  17. assignment refers to assigning a value to a variable • consider again the following code fragment: • int x = 3; • int y = 4; • int z; • z = x + y; • z = z + x; • what does z equal? 17 Friday, 24 October 2003

  18. the right side of an assignment expression is always calculated before the assignment takes place: • int x = 5; • x = x + 5; • What is the value of x? • Consider the following statements: • int x, y, z; • x = y = z = 7; • x, y & z will all equal the value 7 18 Friday, 24 October 2003

  19. Short quiz • Consider the following assignment expressions: • int x, y, z; • y = 4; • z = 3; • z = z + y + 1; • y = y - 2 + z; • x = z - y; • x = x + 2; • What is the final value of the variable x? 19 Friday, 24 October 2003

  20. Which of the following are valid variable names? • int • anInt • i • i1 • 1 • thing1 • 1thing • ONE-HUNDRED • ONE_HUNDRED • something2do 20 Friday, 24 October 2003

  21. What is another word for a class? • a) Object • b) Template • c) Instance • What does an instance method of a class represent? • a) the attributes of that class • b) the behaviour of that class • c) the behaviour of an object created from that class 21 Friday, 24 October 2003

  22. Answers • 0 • 2, 3, 4, 6, 9, 10 • A class is an abstract template used to create objects that are similar to each other • instance methods refer to a specific object’s behaviour. Class methods refer to the behaviour of all objects belonging to that class. 22 Friday, 24 October 2003

  23. Example Program: Currency Converter • Problem statement: • create a program which converts Australian Dollars (AUD) to US Dollars (USD) 23 Friday, 24 October 2003

  24. Glossary • API • Application Programming Interface. The specification of how a programmer writing an application accesses the behavior and state of classes and objects. • applet • A component that typically executes in a web browser, but can execute in a variety of other applications or devices that support the applet programming model. • ASCII • American Standard Code for Information Interchange. A standard assignment of 7-bit numeric codes to characters. See also Unicode. 24 Friday, 24 October 2003

  25. bit • The smallest unit of information in a computer, with a value of either 0 or 1. • boolean • Refers to an expression or variable that can have only a true or false value. The Java programming language provides the boolean type and the literal values true and false. • byte • A sequence of eight bits. The Java programming language provides a corresponding byte type. 25 Friday, 24 October 2003

  26. char • A Java programming language keyword used to declare a variable of type character. • class • a template for an object that contains variables to describe how the object behaves. Classes can inherit variables and methods from other classes. • class method • a method that operates on a class itself rather than on specific instances of a class. Class methods are invoked without reference to a particular object. Class methods affect the class as a whole, not a particular instance of the class. Also called a static method. See also instance method. 26 Friday, 24 October 2003

  27. class variable • a variable that describes an attribute of class instead of specific instances of the class. A data item associated with a particular class as a whole--not with particular instances of the class. Class variables are defined in class definitions. Also called a static field. See also instance variable. • comment • In a program, explanatory text that is ignored by the compiler. In programs written in the Java programming language, comments are delimited using // or /*...*/. 27 Friday, 24 October 2003

  28. compilation unit • The smallest unit of source code that can be compiled. In the current implementation of the Java platform, the compilation unit is a class. • compiler • A program to translate source code into code to be executed by a computer. The Java compiler translates source code written in the Java programming language into bytecode for the Java virtual machine1. See also interpreter. • deprecation • Refers to a class, interface, constructor, method or field that is no longer recommended, and may cease to exist in a future version. 28 Friday, 24 October 2003

  29. double • A Java programming language keyword used to define a variable of type double. • encapsulation • The localization of knowledge within a module. Because objects encapsulate data and implementation, the user of an object can view the object as a black box that provides services. Instance variables and methods can be added, deleted, or changed, but as long as the services provided by the object remain the same, code that uses the object can continue to use it without being rewritten. See also instance variable, instance method. 29 Friday, 24 October 2003

  30. exception • An event during program execution that prevents the program from continuing normally; generally, an error. The Java programming language supports exceptions with the try, catch, and throw keywords. See also exception handler. • exception handler • A block of code that reacts to a specific type of exception. If the exception is for an error that the program can recover from, the program can resume executing after the exception handler has executed. 30 Friday, 24 October 2003

  31. field • A data member of a class. Unless specified otherwise, a field is not static. • float • A Java programming language keyword used to define a floating point number variable. • garbage collection • The automatic detection and freeing of memory that is no longer in use. The Java runtime system performs garbage collection so that programmers never explicitly free objects. 31 Friday, 24 October 2003

  32. GUI • Graphical User Interface. Refers to the techniques involved in using graphics, along with a keyboard and a mouse, to provide an easy-to-use interface to some program. • import • A Java programming language keyword used at the beginning of a source file that can specify classes or entire packages to be referred to later without including their package names in the reference. 32 Friday, 24 October 2003

  33. instance • the same thing as an object. Each object is an instance of some class. In programs written in the Java programming language, an instance of a class is created using the new operator followed by the class name. • instance method • a method of an object that operates on that object by manipulating the values of its instance variables. Because instance methods are much more common than class methods, they are often just called methods. 33 Friday, 24 October 2003

  34. instance variable • a variable that describes an attribute of an instance of a class instead of the class itself. Any item of data that is associated with a particular object. Each instance of a class has its own copy of the instance variables defined in the class. Also called a field. See also class variable. • int • A Java programming language keyword used to define a variable of type integer. 34 Friday, 24 October 2003

  35. interface • A Java programming language keyword used to define a collection of method definitions and constant values. It can later be implemented by classes that define this interface with the "implements" keyword. • interpreter • A module that alternately decodes and executes every statement in some body of code. The Java interpreter decodes and executes bytecode for the JavaTM virtual machine1. See also compiler, runtime system. 35 Friday, 24 October 2003

  36. JVM (Java Virtual Machine) • A software "execution engine" that safely and compatibly executes the byte codes in Java class files on a microprocessor (whether in a computer or in another electronic device). • local variable • A data item known within a block, but inaccessible to code outside the block. For example, any variable defined within a method is a local variable and can't be used outside the method. • long • A Java programming language keyword used to define a variable of type long. 36 Friday, 24 October 2003

  37. member • A field or method of a class. Unless specified otherwise, a member is not static. • method • a group of statements in a class that defines how the class’s objects will behave. Methods are analogous to functions in other languages but must always be located inside a class • new • A Java programming language keyword used to create an instance of a class. 37 Friday, 24 October 2003

  38. object • an instance of a class. Multiple objects that are instances of the same class have access to the same methods but often have different values for their instance variables. The principal building blocks of object-oriented programs. Each object is a programming unit consisting of data (instance variables) and functionality (instance methods). See also class. • object-oriented design • A software design method that models the characteristics of abstract or real objects using classes and objects. 38 Friday, 24 October 2003

  39. scope • A characteristic of an identifier that determines where the identifier can be used. Most identifiers in the Java programming environment have either class or local scope. Instance and class variables and methods have class scope; they can be used outside the class and its subclasses only by prefixing them with an instance of the class or (for class variables and methods) with the class name. All other variables are declared within methods and have local scope; they can be used only within the enclosing block. 39 Friday, 24 October 2003

  40. static • A Java programming language keyword used to define a variable as a class variable. Classes maintain one copy of class variables regardless of how many instances exist of that class. "static" can also be used to define a method as a class method. Class methods are invoked by the class instead of a specific instance, and can only operate on class variables. • short • A Java programming language keyword used to define a variable of type short. • static field • Another name for class variable. 40 Friday, 24 October 2003

  41. static method • Another name for class method. • Unicode • A 16-bit character set defined by ISO 10646. See also ASCII. All source code in the Java programming environment is written in Unicode. • variable • An item of data named by an identifier. Each variable has a type, such as int or Object, and a scope. See also class variable, instance variable, local variable. 41 Friday, 24 October 2003

  42. virtual machine • An abstract specification for a computing device that can be implemented in different ways, in software or hardware. You compile to the instruction set of a virtual machine much like you'd compile to the instruction set of a microprocessor. The Java virtual machine consists of a bytecode instruction set, a set of registers, a stack, a garbage-collected heap, and an area for storing methods. • void • A Java programming language keyword used in method declarations to specify that the method does not return any value. "void" can also be used as a nonfunctional statement. 42 Friday, 24 October 2003

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