Introduction to Java Written by John Bell for CS 342, Spring 2018

Transcription

1 Introduction to Java Written by John Bell for CS 342, Spring 2018 Based on chapters 1 to 6 of Learning Java by Patrick Niemeyer and Daniel Leuck, with additional material from other sources. History I - C C was developed in 1972, in order to write UNIX. Personal computers didn t exist. Mainframes were relatively slow and small, so every CPU cycle and byte of space were precious. Programmers went to great lengths to make their programs small and fast. Few people had access to computers Mostly at universities and (government) research labs. No Internet. ( Computers first connected in ) C was written to be very fast and efficient, with little to no error checking. 1

2 History II C++ C++ was developed in 1979 by Bjarne Stroustrup. Computer networks were still limited to university and governmental use only. There was no public or commercial Internet. Most programmers loved C, and few knew anything about object-oriented programming. To be accepted, C++ had to be backwardscompatible with C. C++ compilers had to compile and run C program just as fast as their C versions. C++ introduced very few new keywords or rules to C. History III - Java Java was developed in The Internet was in full swing, with all that entails. Programmers started in high school or younger. Programmers had accepted and embraced the objectoriented paradigm. Computers were much faster and larger, with continuing rapid growth and improvement. Program size and complexity increased exponentially. Programmer time and reducing errors became more important than speed and efficiency. A new language was developed, for the Internet Age, with different priorities and no backwards compatability. 2

3 A Few Java Rules The name of a class must match it s file name. Every class in Java inherits from java.lang.object, either directly or indirectly. ( More about that later. ) Java does not allow any procedures outside of classes. All code must be part of a method which is a member of a class. This includes main( )! Therefore main( ) must be made a static member of a class. ( Static class members exist exactly once, independently of any objects of the class. ) Hello World, Java Style File HelloWorld.java import java.io.*; // To use the I/O services public class HelloWorld { public static void main( String [ ] args ) { } // end of main } // end of class System.out.println( "Hello World.\n" ); return; 3

4 A Book Class Scenario For the initial functionality of a Book, a program should be able to create a Book object by providing its title, and to then print the book to the screen, which will entail printing out the given book title. File BookTester.java import java.io.*; The BookTester Class public class BookTester { public static void main( String args[ ] ) { System.out.println( Testing the Book class. Written by JBell. \n" ); Book b = new Book( "Learning Java" ); System.out.print( "b = " ); b.print( ); System.out.println( "\n" ); } } return; 4

5 First Attempt at Compiling BookTester I:\ temp>javac BookTester.java BookTester.java:9: error: cannot find symbol Book b = new Book( "Learning Java" ); ^ symbol: class Book location: class BookTester BookTester.java:9: error: cannot find symbol Book b = new Book( "Learning Java" ); ^ symbol: class Book location: class BookTester 2 errors Minimum Possible Book Class, in file Book.java File Book.java public class Book { } 5

6 Trying Again to Compile BookTester.java I:\ temp>javac BookTester.java BookTester.java:9: error: constructor Book in class Book cannot be applied to given types; Book b = new Book( "Learning Java" ); ^ required: no arguments found: String reason: actual and formal argument lists differ in length BookTester.java:11: error: cannot find symbol b.print( ); ^ symbol: method print() location: variable b of type Book 2 errors A Constructor Has the Same Name as the Class, and no Return Type File Book.java public class Book { public Book( String s ) { return; } } public void print( ) { return; } 6

7 Now BookTester compiles and runs, ( but not properly. ) I:\ temp>javac BookTester.java I:\ temp>java BookTester Testing the Book class. Written by JBell. b = Okay, so now let s fix Book.java // Book.java - Written Spring 2018 by John Bell for CS342 import java.io.*; public class Book { private String title; public Book( String s ) { System.out.println( "Book( " + s + " ) called\n" ); title = s; return; } // Book( String ) public void print( ) { System.out.print( title ); return; } // print( ) } // Book class 7

8 Completion of the Desired Scenario I:\ temp>javac Book.java I:\ temp>java BookTester Testing the Book class. Written by JBell. Book( Learning Java ) called b = Learning Java Java Arrays of Objects - Allocate the array, then the objects In BookArrayTester.java Book [ ] shelf = new Book[ 3 ]; shelf[ 0 ] = new Book( "Book 1" ); shelf[ 1 ] = new Book( "Book 2" ); shelf[ 2 ] = new Book( "Book 3" ); for( int i = 0; i < 3; i++ ) { System.out.print ( "shelf[ " + i + " ] = " ); shelf[ i ].print( ); System.out.println( ); } 8

9 Output From Previous Code Book( Book 1 ) called Book( Book 2 ) called Book( Book 3 ) called shelf[ 0 ] = Book 1 shelf[ 1 ] = Book 2 shelf[ 2 ] = Book 3 Java Tools JRE - Java Runtime Environment - java, web apps JDK - Java Development Kit - javac, javap, jar Java Virtual Machine Compiling java code generates Java byte code. Java byte code runs on a Java Virtual Machine Separate JVMs run on different kinds of HW Eclipse - Integrated Development Environment ( IDE ) - See Appendix A Javadoc - /**... */ comments. See pg 85+ jar - An archiving tool for grouping Java classes, similar to tar or zip. 9

10 Java References to Objects Java accesses all Objects as reference types. As opposed to C++, which can have either objects or pointers to objects. ( See comparison. ) Strings are first-class objects in Java, also accessed with reference variables. String constants get automatically converted to String objects as needed. This is termed boxing / unboxing. Basic types may also be automatically (un)boxed from/to basic types. Double d = ; A Few Important Java Classes java.lang.object All Java classes inherit from Object, either directly or indirectly. Note tostring( ), clone( ), equals( ) java.util.arraylist A variable-length Array class Add items to the end of a ( growing ) array Generated from a template ( generic ) for different types, depending on the intended contents, e.g. ArrayList<Book> shelf = new ArrayList<Book>( ); 10

11 Constructors Constructors are called by the system when space is allocated for a class object. ( Possibly before main( ) is run. ) Used to initialize class objects. Constructors have the same name as their class. Java provides a default no-argument constructor, IF no other constructors are provided. Constructors may call other constructors of the same class, only as their first statement. Constructors eventually call the constructor of their super class ( parent ) directly or automatically. Destructors C++ has issues with dynamic memory, which often requires a destructor to clean up allocated memory. Java has a garbage collector to automatically clean up allocated memory, so destructors are used less frequently. However Java classes all have a finalize( ) method inherited from Object, which gets called when space for an object is deallocated. finalize usually closes files, breaks network connections, etc. 11

12 Inheritance in Java In Java the keyword extends indicates a child class inheriting from a parent. Example: public class Car extends Vehicle {... Java classes that do not specifically name a parent extend Object. Java classes may both inherit from a parent class and implement one or more interfaces: public class Car extends Vehicle implements Runnable { 12

13 The instanceof Operator The keyword instanceof is an operator, that determines if a variable is an instance of a type. ( Or one of its subtypes. If true, then the variable can be safely assigned or typecast to that type. ) Example: Question q = myexam.getnextquestion( ); if ( q instanceof MCQuestion ) { MCQuestion mcq = ( MCQuestion ) q; // Work with mcq as a MCQuestion } The Scanner Class The Scanner class processes input data, and converts strings of characters into ints, etc. Scanners can be connected to any data source, such as the keyboard, a String, or a data file. Examples: Scanner keys = new Scanner( System.in ); Scanner file = new Scanner( new File( input.txt ) ); String line = keys.nextline( ); while( file.hasnextint( ) ) { int n = file.nextint( ); // use n 13

14 The final keyword The keyword final is roughly equivalent to the C++ keyword const. final methods cannot be overridden by subclasses. final data fields cannot be changed. Examples: static final int MAXQUESTIONS = 100; static final int CLUBS = 1, HEARTS = 2, DIAMONDS = 3, SPADES = 4, NOTRUMP = 5, UNDEFINED = -1; int suit = Cards.UNDEFINED; Exceptions in Java Exceptions are the preferred means of handling exceptional or error situations in OOP. The basic idea is that the situation status is encapsulated into an object, and then passed to code prepared to handle the situation ( if any. ) C++ allows any kind of data to be thrown. Java only allows throwing of objects of type Throwable ( or descendents, i.e. Error, Exception ) All Throwable objects contain certain methods, such as printstacktrace( ), which are very helpful. 14

15 Exceptions in Java try / catch A try{ } block in Java contains code that may potentially throw Throwables, either directly or through a called method. A try block is followed by one or more catch( ) { } blocks, prepared to catch different kinds of Throwables. catch blocks are checked in order to find one prepared to catch a given Throwable. It is up to the programmer to order them from more specific to more general. Throwing in Java The throw( ) command in Java passes a Throwable to the system, which then looks for code to handle it. The argument to throw( ) must be a Throwable, which is often a new Exception ( or descendent ) when called from user code. Programmers can define new Exception types that extend any of the pre-defined types. Exceptions can be defined to encapsulate any needed information. Example: throw new NotAnsweredException(question); 15

16 Finding a catch Block When code throws a Throwable, the system first checks the immediately following catch blocks, looking for the first available match. Catch blocks will catch the named Throwable type, or any of its descendents. For example, catch( IOException e ) { will also catch FileNotFoundException or EOFException, but not TimeoutException. If no suitable catch block can be found, then the system looks elsewhere for a suitable match. ( See next slide. ) Uncaught Exceptions If a Throwable is thrown and there are no catch blocks immediately available prepared to catch that Throwable type, then the system starts looking up the call stack to find an appropriate catch block. If one can be found, then control transfers up to that catch block. Execution does NOT return to the point of the throw after the catch block completes. The printstacktrace( ) method prints a report of the state of the call stack when the throw was made. If no catch block can be found, then the program exits, with an Unhandled Exception error. 16

17 Rethrowing and Chaining Exceptions A catch( ) block may rethrow the Throwable that it caught, either after or instead of partially handling it. Rethrown Throwables will be shown as originating at the point where they were rethrown however, not their original source. A better alternative is to create a new Throwable, and encapsulate the original Throwable inside it: catch( Exception e ) { if( e instanceof FileNotFoundException ) throw new MyDefinedException( e ); The finally Block If a finally { } block is placed immediately after the catch( ) { } blocks associated with a try{ } block, then the code in the finally block will always run eventually, regardless of how control exits the try. If a local catch block is executed, then finally runs after that catch. If the system has to search the stack for a catch block, then finally runs before searching. If the try block finishes without throwing, finally runs anyway, even if return, break, or continue. 17

18 The throws Clause Java has two broad categories of Exceptions: Checked types are generally those that user code can recover from, such as FileNotFound. Unchecked types are generally those that are systemlevel problems, such as InsufficientMemory. Checked exceptions must either be caught within the method that throws them, or specified in a throws clause for the method in question. Example: void myfunc( ) throws EOFException { 18