EE2E1. JAVA Programming

EE2E1. JAVA Programming Lecture 3 Java Programs and Packages

Contents • Java source files and class files • Packages • public/package/private scope • The CLASSPATH environment variable

Java source files and class files • Java progams execute on the Java Virtual Machine (JVM) which is usually implemented as an interpreter and executed with the java command • Java source code is compiled to bytecode by the compiler executed with the javac command

Java source file Test.java public class Test{ public static void main(String[] args) { System.out.println(“Hi!”);} } Java compiler Java class file Test.class Java bytecode Hi! Java interpreter (JVM)

The name of the source file (Test.java)matches the name of the public class(Test) and produces a bytecode file Test.class • Only one public class per file is allowed (see later - public/package/private scope) • A source file can contain any number of classes and compilation produces a .class file for each class in the source file

Packages • Java groups classes into packages • The standard Java library is distributed over a number of packages including java.lang, java.util indicating sub-packages • java.lang automatically imported into your programs • Contains a lot of the basic classes we use (String,System etc) • We can create our own packages using the package keyword • In the following example, classes A and B are in the package my_package

package my_package; class A { // A’s public & private members } class B { // B’s public & private members }

The default package • If a source file does not begin with the package statement, the classes contained in the source file reside in the default package • The java compiler automatically looks in the default package to find classes (see later) • It then searches all imported packages to find the remaining classes (see later)

Importing packages • The import statement imports packages by directing the compiler where to look • The import statement in a Java program must occur before any class definition • Either the whole package or a sub-package can be imported • Either all of the classes in a package can be imported or just a particular class from the package

import my_package.*; // Imports all of the classes class C { // Can refer directly to classes A & B private A a; private B b; } import my_package.A; // Imports class A only class C { // Can refer directly to class A only private A a; private my_package.B b; }

public/package/private scope • Scope is concerned with the visibility of program elements such as classes and members • Both classes and class members (methods or instance fields) can be defined with public, package or private scope • A class with public scope means it is visible outside its containing package • Allows Java to find a class (maybe with the help of CLASSPATH (see later)) as it must be in a file with the same name as the public class

A class member with publicscope means it is visible anywhere its class is visible • A class member with privatescope means it is visible only within its encapsulating class • A class/class member with package scope means it is visible only inside its containing package

Example 1 package my_package; class A // package scope { // A’s public & private members } public class B // public scope { // B’s public and private members }

class A has no private/public label and thus has, by default, package scope • class B has public scope • class A is visible only within the package but class B is visible outside the package also

package another_package; import my_package.*; class C { // C’s public & private members // class C ‘knows’ about class B private B b; // OK – class B has public scope }

package my_package; class D { // D’s public & private members // Class D ‘knows’ about classes A and B private B b; // OK – class B has public scope private A a; // OK – class A has package scope }

Example 2 package my_package; class A { int get() { return data; } // package scope public A(int d) { data=d;} // public scope private int data; // private scope } class B { void f() { A a=new A(d); // OK A has package scope int d=a.get(); // OK – get() has package scope int d1=a.data; // Error! – data is private } }

Even though member function A() has public scope, because class A only has package scope, the class and hence all of its member functions are only visible inside the package • If member functions are to be publicly visible, the class itself has to have public scope • In general, classes and their members should be given the narrowest possible scope appropriate to their functionality

The CLASSPATH environment variable • The compiler and runtime interpreter know how to find standard packages such as java.lang and java.util • The CLASSPATH environment variable is used to direct the compiler and interpreter to where programmer defined imported packages can be found • This then allows the public class in the file to be found • The CLASSPATH environment variable is an ordered list of directories and files

.;C:\java;C:\java\my_jar_file.jar • Example CLASSPATH’s • Windows • UNIX • A jar file is a Java archive file and is a collection of java class files (with the .class extension) • ‘.’ means the current directory. In some Java implementations, this is part of the CLASSPATH by default but normally has to be added .; /home/java; /home/java/my_jar_file.jar

The above directories and jar files are searched in the order they appear in the CLASSPATH list for imported packages • The CLASSPATH list can be overridden with the –classpath option to the compiler : (javac –classpath …) and the interpreter (java –classpath …) • All files in a package must be in a sub-directory that matches the full package name • Eg. all files in package my_package.sub-package are in a sub-directory my_package/sub-package (Unix) which branches off one of the directories in the CLASSPATH

// source file “my_package/A.java” package my_package; public class A { int get() { return data; } public A(int d) { data=d;} private int data; } // source file “TestA.java” import my_package.A; public class TestA { public static void main(String[] args){ A a=new A(100); } }

class TestA is in the default package • Package/sub-package directory matches the directory/sub-directory structure Package . (current directory) Default package Directory Package my_package Sub-directory ./my_package Sub-package

Example • We can create a Polygon class which represents a polygon as an array of points (vertices) • The Point class is imported as part of a (programmer defined) geometrical package geom • A method perimeter() computes the polygon’s perimiter • class PolygonTest contains a main method to test the Polygon class

package geom; public class Point { public Point(int x, int y) { xpos=x; ypos=y;} public int getX() { return xpos;} public int getY() { return ypos; } private int xpos, ypos; }

import geom.Point; public class Polygon { public Polygon(int n, Point[] v) { numVertices=n; vertices=new Point[n]; System.arraycopy(v,0,vertices,0,n); } public double perimeter() { double pm=0; int nn; for (int n=1; n<numVertices; n++) { int xv1= vertices[n-1].getX(); int yv1=vertices[n-1].getY(); nn=n%numVertices; int xv=vertices[nn].getX(); int yv=vertices[nn].getY(); pm+=(Math.sqrt((xv1-xv)*(xv1-xxv)+(yv1-yv)*(yv1-yv))); } return pm; } private Point[] vertices; private int numVertices; }

import geom.Point; public class PolygonTest { public static void main(String[] args) { Point[] pointArray={new Point(0,0), new Point(0,1), new Point(1,1), new Point(1,0)}; Polygon square=new Polygon(4,pointArray); double perim=square.perimeter(); System.out.println(“The perimeter is ” + perim); } }

class Point is in the geom package whereas classes Polygon and PolygonTest are in the default package • File Point.java will reside in a sub-directory geom of the current directory • class Point is made public so it is visible outside of its package • The geom package could be expanded to include other basic geometrical entities (line, curve etc) Default package Polygon.java PolygonTest.java Directory ‘.’ Sub-package geom Sub-directory geom Point.java

And finally…. • When writing seriously large Java applications, you will create many classes and several packages • Related classes will be grouped into packages • When you use a Java API, you normally have to set up the CLASSPATH environment variable so that its classes can be imported • Its important to understand the use of CLASSPATH • However, for your lab exercises, keeping all of your public classes in files with the same name as the class in the current directory (default package) will ensure all public classes are visible

EE2E1. JAVA Programming