CMSC 341 Java Packages Classes Variables Expressions Flow Control and Exceptions Sun s Naming Conventions Classes and Interfaces StringBuffer Integer MyDate Identifiers for methods fields and variables name getName setName isName birthDate Packages java lang java util proj1 Constants PI MAX NUMBER 8 3 2007 UMBC CMSC 341 Java 2 2 Comments Java supports three types of comments C style multi liner comments C style one liner comments Javadoc This is an example of a javadoc comment These comments can be converted to part of the pages you see in the API 8 3 2007 UMBC CMSC 341 Java 2 3 The final modifier Constants in Java are created using the final modifier final int MAX 9 Final may also be applied to methods in which case it means the method can not be overridden in subclasses Final may also be applied to classes in which case it means the class can not be extended or subclassed as in the String class 8 3 2007 UMBC CMSC 341 Java 2 4 Packages Only one package per file Packages serve as a namespace in Java and create a directory hierarchy when compiled Classes are placed in a package using the following syntax in the first line that is not a comment package packagename package packagename subpackagename 8 3 2007 UMBC CMSC 341 Java 2 5 Packages cont Classes in a package are compiled using the d option On the following slide you will find the command to compile the code from the Proj1 src directory to the Proj1 bin directory 8 3 2007 UMBC CMSC 341 Java 2 6 Packages cont It is common practice to duplicate the package directory hierarchy in a directory named src and to compile to a directory named bin Proj1 src The following command is run from the src directory proj1 gui javac d bin proj1 gui Example java Example java bin proj1 gui Example class 8 3 2007 UMBC CMSC 341 Java 2 7 Packages cont By default all classes that do not contain a package declaration are in the unnamed package The fully qualified name of a class is the packageName ClassName java lang String To alleviate the burden of using the fully qualified name of a class people use an import statement found before the class declaration import java util StringBuffer import java util 8 3 2007 UMBC CMSC 341 Java 2 8 Fields and Methods In Java you have fields and methods A field is like a data member in C Method is like a member method in C Every field and method has an access level The public private and protected keywords have the same functionality as those in C public protected private package 8 3 2007 UMBC CMSC 341 Java 2 9 Access Control Modifier Same class Same package Subclass Universe private default protected public 8 3 2007 UMBC CMSC 341 Java 2 10 Access Control for Classes Classes may have either public or package accessibility Only one public class per file Omitting the access modifier prior to class keyword gives the class package accessibility 8 3 2007 UMBC CMSC 341 Java 2 11 Classes In Java all classes at some point in their inheritance hierarchy are subclasses of java lang Object therefore all objects have some inherited default implementation before you begin to code them String toString boolean equals Object o 8 3 2007 UMBC CMSC 341 Java 2 12 Classes cont Unlike C you must define the accessibility for every field and every method In the following code the x is public but the y gets the default accessibility of package since it doesn t have a modifier public int x int y 8 3 2007 UMBC CMSC 341 Java 2 13 Instance and Local Variables Unlike C you must define everything within a class Like C variables declared outside of method are instance variables and store instance or object data The lifetime of the variable is the lifetime of the instance variables declared within a method including the parameter variables are local variables The lifetime of the variable is the lifetime of the method 8 3 2007 UMBC CMSC 341 Java 2 14 Static Variables A class may also contain static variables and methods Similar to C Static variables store static or class data meaning only one copy of the data is shared by all objects of the class Static methods do not have access to instance variables but they do have access to static variables Instance methods also have access to static variables 8 3 2007 UMBC CMSC 341 Java 2 15 Instance vs Static Methods Static methods have static as a modifier can access static data can be invoked by a host object or simply by using the class name as a qualifier Instance methods can access static data can access instance data of the host object must be invoked by a host object contain a this reference that stores the address of host object 8 3 2007 UMBC CMSC 341 Java 2 16 Pass By Value or By Reference All arguments are passed by value to a method However since references are addresses in reality they are passed by reference meaning Arguments that contain primitive data are passed by value Changes to parameters in method do not effect arguments Arguments that contain reference data are passed by reference Changes to parameter in method may effect arguments 8 3 2007 UMBC CMSC 341 Java 2 17 Constructors Similar to C Java will provide a default no argument constructor if one is not defined in the class Java however will initialize all fields object or instance data to their zero values as in the array objects Like C once any constructor is defined the default constructor is lost unless explicitly defined in the class 8 3 2007 UMBC CMSC 341 Java 2 18 Constructors cont Similar to C constructors in Java have no return value have the same name as the class initialize the data and are typically overloaded Unlike C a Java constructor can call another constructor using a call to a this method as the first line of code in the constructor 8 3 2007 UMBC CMSC 341 Java 2 19 Expressions and Control Flow Java uses the same operators as C Only differences are sign can be used for String concatenation logical and relative operators return a boolean Same control flow constructs as C but expression must return a boolean Conditional if boolean expression else if boolean expression else switch variable case 1 break default 8 3 2007 Variable must be an integral primitive type of size int or smaller or a char UMBC CMSC 341 Java 2 20 Control Flow Constructs cont Iterative while boolean expression do while boolean expression for initialize boolean expression update break and continue work in the same way as in C May use labels with break and continue as in C 8 3 2007 UMBC CMSC 341 Java 2 21 Control Flow Constructs cont Enhanced for loop