Introduction to JavaOverviewHistoryCharacteristicsData TypesPrimitive Data TypesVariablesOperatorsExpressionsAnatomy of A Java ProgramCommentsPackageClassesReserved WordsModifiersBlocksStatementsMethodsmain MethodObjectsClassesClassesConstructorsExampleInheritanceExampleInterfaceExampleEncapsulationAbstract Classes and MethodsAbstract Classes vs. InterfacesPackage幻灯片编号 33Zhifu PeiCSCI5448 – Spring 2011Prof. Kenneth M. Anderson Introduction• History, Characteristics of Java language Java Language Basics• Data types, Variables, Operators and Expressions Anatomy of a Java Program• Comments, Packages, Classes, Reserved Words, Modifiers, Blocks, Statements, Methods and main Method Concepts of Object-Oriented Programming• Objects, Classes, Inheritance, Interface and Package Developed by James Gosling at Sun Microsystems. Released by Sun Microsystems in 1995. Hotjava – The first java-enabled web browser J2EE, J2ME and J2SE Simple, object-oriented, and familiar Robust and secure Architecture-neutral and portable High performance Interpreted, threaded, and dynamic A data type is a scheme for representing values.• Values are not just numbers, but any kind of data that a computer can process. A data type defines a kind of data that is represented by a variable Java data types are case sensitive.Data Type Size (byte) Rangebyte 1 -128 to 127boolean 1 true or falsechar 2 (Unicode) A-Z, a-z, 0-9, etc.short 2 -32768 to 32767int 4 (about) -2 million to 2 millionfloat 4 -3.4E38 to 3.4E18long 8 (about) -10E18 to 10E18double 8 -1.7E308 to 1.7E308• There are only eight primitive data types.• New primitive data types cannot be created by a programmer Variables are labels that describe a particular location in memory and associate it with a data type. A declaration of a variable is where the program allow memory for the variable.• instance variables: declared without “static”• class variables: declared with “static”• local variables and Parameters Operators are special symbols that perform specific operations on one, two, or three operands, and then return a result.• assignment, arithmetic, and unary operators“=”; “+”, “*”; “++”, “!”• equality, relational, and conditional operators“==”, “!=”, “<=”; “||”, “?:” • bitwise and bit shift operators“~”, “&”, “>>” An expression is a construct made up of variables, operators, and method invocations. An expression can return other types of values as well. A compound expression can be constructed from various smaller ones. A simple java Program Comments in Java are preceded in two types:• Line comments: Line comments are precede by two slashes “//” in a line.• Paragraph comments: Paragraph comments are enclosed between “/*” and “*/” in one or multiple lines. The second line of the program is the package name. It specifies the package name, org.Welcome for the class Welcome. The class is the essential Java construct. A class is a template. A program is defined by using one or more classes. Reserved words (Keywords) are words that have a specific meaning to the compiler and cannot be used for other purposes (name of variables, classes, methods…) in the program. Examples:• When the compiler sees the word class, it understands the word after class is the name for the class.• public, void, static are also reserved words. Certain reserved words are called modifies that specify the properties of the data, methods, and classes and how they can be used. Examples: • A public datum, method, or class can be accessed by other programs.• private, protected, static and void are also modifiers. A pair of braces in a program forms a block for a group components of a program. A statement represents an action or a sequence of actions. A statement ends with a semicolon “;”. A method is a collection of statements that performs a sequence of operations to display a message on the console. A method can be used even without fully understanding the details of how it works. A method is used by invoking a statement with a string argument which is enclosed within parentheses. The main method provides the control of the program flow.  The Java interpreter executes the application by invoking the main method. Objects are key to understand OO technology. An object is a structured block of data. An object may use many bytes of memory. An object stores its state in fields and exposes its behavior through methods. The data type of an object is its class. Creating an object is called instantiation. The class is the essential Java construct.  A class is a blueprint or prototype from which objects are created. A class is a description of a group of objects with similar properties and behaviors. A class is a pattern for an object. A class does not create any objects. A class consists of• a collection of fields, or variables, very much like the named fields of a struct.• all the operations (called methods) that can be performed on those fields.• can be instantiated. A class describes objects and operations defined on those objects. Classes should define one or more methods to create or construct instances of the class. Their name is the same as the class name. Constructors are differentiated by the number and types of their arguments. If none constructor is defined, a default one will be created. Constructors do not return anything. Classes are arranged in a hierarchy. Inheritance enables to define a new class based on a class that already exists. A class that is derived from another class is called a subclass. The class from which the subclass is derived is called a superclass. A class inherits fields and methods from all its superclasses, whether direct or indirect. A subclass can override methods that it inheritance, or it can hide fields or methods that it inherits. An interface defines a protocol of communication between two objects. An interface declaration contains signatures, but no implementations, for a set of methods, and might also contain constant definitions. A class that implements an interface must implement all the methods declared in the interface. An interface name can be used anywhere a type can be used. Encapsulation