introduction to java

Cmp Sci 187:Introduction to Java : Cmp Sci 187: Introduction to Java Based on Appendix A of text (Koffmann and Wolfgang)

Topics of the Review : Topics of the Review Essentials of object-oriented programming, in Java Java primitive data types, control structures, and arrays Using some predefined classes: Math JOptionPane, I/O streams String, StringBuffer, StringBuilder StringTokenizer Writing and documenting your own Java classes

Some Salient Characteristics of Java : Some Salient Characteristics of Java Java is platform independent: the same program can run on any correctly implemented Java system Java is object-oriented: Structured in terms of classes, which group data with operations on that data Can construct new classes by extending existing ones Java designed as A core language plus A rich collection of commonly available packages Java can be embedded in Web pages

Java Processing and Execution : Java Processing and Execution Begin with Java source code in text files: Model.java A Java source code compiler produces Java byte code Outputs one file per class: Model.class May be standalone or part of an IDE A Java Virtual Machine loads and executes class files May compile them to native code (e.g., x86) internally

Compiling and Executing a Java Program : Compiling and Executing a Java Program

Classes and Objects : Classes and Objects The class is the unit of programming A Java program is a collection of classes Each class definition (usually) in its own .java file The file name must match the class name A class describes objects (instances) Describes their common characteristics: is a blueprint Thus all the instances have these same characteristics These characteristics are: Data fields for each object Methods (operations) that do work on the objects

Grouping Classes: The Java API : Grouping Classes: The Java API API = Application Programming Interface Java = small core + extensive collection of packages A package consists of some related Java classes: Swing: a GUI (graphical user interface) package AWT: Application Window Toolkit (more GUI) util: utility data structures (important to CS 187!) The import statement tells the compiler to make available classes and methods of another package A main method indicates where to begin executing a class (if it is designed to be run as a program)

A Little Example of import and main : A Little Example of import and main import javax.swing.*; // all classes from javax.swing public class HelloWorld { // starts a class public static void main (String[] args) { // starts a main method // in: array of String; out: none (void) } } public = can be seen from any package static = not “part of” an object

Processing and Running HelloWorld : Processing and Running HelloWorld javac HelloWorld.java Produces HelloWorld.class (byte code) java HelloWorld Starts the JVM and runs the main method

References and Primitive Data Types : References and Primitive Data Types Java distinguishes two kinds of entities Primitive types Objects Primitive-type data is stored in primitive-type variables Reference variables store the address of an object No notion of “object (physically) in the stack” No notion of “object (physically) within an object”

Primitive Data Types : Primitive Data Types Represent numbers, characters, boolean values Integers: byte, short, int, and long Real numbers: float and double Characters: char

Primitive Data Types : Primitive Data Types

Primitive Data Types (continued) : Primitive Data Types (continued)

Operators : Operators subscript [ ], call ( ), member access . pre/post-increment ++ --, boolean complement !, bitwise complement ~, unary + -, type cast (type), object creation new * / % binary + - (+ also concatenates strings) signed shift << >>, unsigned shift >>> comparison < <= > >=, class test instanceof equality comparison == != bitwise and & bitwise or |

Operators : Operators logical (sequential) and && logical (sequential) or || conditional cond ? true-expr : false-expr assignment =, compound assignment += -= *= /= <<= >>= >>>= &= |=

Type Compatibility and Conversion : Type Compatibility and Conversion Widening conversion: In operations on mixed-type operands, the numeric type of the smaller range is converted to the numeric type of the larger range In an assignment, a numeric type of smaller range can be assigned to a numeric type of larger range byte to short to int to long int kind to float to double

Declaring and Setting Variables : Declaring and Setting Variables int square; square = n * n; double cube = n * (double)square; Can generally declare local variables where they are initialized All variables get a safe initial value anyway (zero/null)

Referencing and Creating Objects : Referencing and Creating Objects You can declare reference variables They reference objects of specified types Two reference variables can reference the same object The new operator creates an instance of a class A constructor executes when a new object is created Example: String greeting = ″hello″;

Java Control Statements : Java Control Statements A group of statements executed in order is written { stmt1; stmt2; ...; stmtN; } The statements execute in the order 1, 2, ..., N Control statements alter this sequential flow of execution

Java Control Statements (continued) : Java Control Statements (continued)

Java Control Statements (continued) : Java Control Statements (continued)

Methods : Methods A Java method defines a group of statements as performing a particular operation static indicates a static or class method A method that is not static is an instance method All method arguments are call-by-value Primitive type: value is passed to the method Method may modify local copy but will not affect caller’s value Object reference: address of object is passed Change to reference variable does not affect caller But operations can affect the object, visible to caller

The Class Math : The Class Math

Escape Sequences : Escape Sequences An escape sequence is a sequence of two characters beginning with the character \ A way to represents special characters/symbols

The String Class : The String Class The String class defines a data type that is used to store a sequence of characters You cannot modify a String object If you attempt to do so, Java will create a new object that contains the modified character sequence

Comparing Objects : Comparing Objects You can’t use the relational or equality operators to compare the values stored in strings (or other objects) (You will compare the pointers, not the objects!)

The StringBuffer Class : The StringBuffer Class Stores character sequences Unlike a String object, you can change the contents of a StringBuffer object

StringTokenizer Class : StringTokenizer Class We often need to process individual pieces, or tokens, of a String

Wrapper Classes for Primitive Types : Wrapper Classes for Primitive Types Sometimes we need to process primitive-type data as objects Java provides a set of classes called wrapper classes whose objects contain primitive-type values: Float, Double, Integer, Boolean, Character, etc.

Defining Your Own Classes : Defining Your Own Classes Unified Modeling Language (UML) is a standard diagram notation for describing a class Class name Field values Class name Field signatures: type and name Method signatures: name, argument types, result type

Defining Your Own Classes (continued) : Defining Your Own Classes (continued) The modifier private limits access to just this class Only class members with public visibility can be accessed outside of the class* (* but see protected) Constructors initialize the data fields of an instance

The Person Class : The Person Class // we have omitted javadoc to save space public class Person { private String givenName; private String familyName; private String IDNumber; private int birthYear; private static final int VOTE_AGE = 18; private static final int SENIOR_AGE = 65; ...

The Person Class (2) : The Person Class (2) // constructors: fill in new objects public Person(String first, String family, String ID, int birth) { this.givenName = first; this.familyName = family; this.IDNumber = ID; this.birthYear = birth; } public Person (String ID) { this.IDNumber = ID; }

The Person Class (3) : The Person Class (3) // modifier and accessor for givenName public void setGivenName (String given) { this.givenName = given; } public String getGivenName () { return this.givenName; }

The Person Class (4) : The Person Class (4) // more interesting methods ... public int age (int inYear) { return inYear – birthYear; } public boolean canVote (int inYear) { int theAge = age(inYear); return theAge >= VOTE_AGE; }

The Person Class (5) : The Person Class (5) // “printing” a Person public String toString () { return “Given name: “ + givenName + “\n” + “Family name: “ + familyName + “\n” + “ID number: “ + IDNumber + “\n” + “Year of birth: “ + birthYear + “\n”; }

The Person Class (6) : The Person Class (6) // same Person? public boolean equals (Person per) { return (per == null) ? false : this.IDNumber.equals(per.IDNumber); }

Arrays : Arrays In Java, an array is also an object The elements are indexes and are referenced using the form arrayvar[subscript]

Array Example : Array Example float grades[] = new float[numStudents]; ... grades[student] = something; ... float total = 0.0; for (int i = 0; i < grades.length; ++i) { total += grades[i]; } System.out.printf(“Average = %6.2f%n”, total / numStudents);

Array Example Variations : Array Example Variations // possibly more efficient for (int i = grades.length; --i >= 0; ) { total += grades[i]; } // uses Java 5.0 “for each” looping for (float grade : grades) { total += grade; }

Input/Output using Class JOptionPane : Input/Output using Class JOptionPane Java 1.2 and higher provide class JOptionPane, which facilitates display Dialog windows for input Message windows for output

Input/Output using Class JOptionPane (continued) : Input/Output using Class JOptionPane (continued)

Converting Numeric Strings to Numbers : Converting Numeric Strings to Numbers A dialog window always returns a reference to a String Therefore, a conversion is required, using static methods of class String:

Input/Output using Streams : Input/Output using Streams An InputStream is a sequence of characters representing program input data An OutputStream is a sequence of characters representing program output The console keyboard stream is System.in The console window is associated with System.out

Opening and Using Files: Reading Input : Opening and Using Files: Reading Input import java.io.*; public static void main (String[] args) { // open an input stream (**exceptions!) BufferedReader rdr = new BufferedReader( new FileReader(args[0])); // read a line of input String line = rdr.readLine(); // see if at end of file if (line == null) { ... }

Opening and Using Files: Reading Input (2) : Opening and Using Files: Reading Input (2) // using input with StringTokenizer StringTokenizer sTok = new StringTokenizer (line); while (sTok.hasMoreElements()) { String token = sTok.nextToken(); ...; } // when done, always close a stream/reader rdr.close();

Alternate Ways to Split a String : Alternate Ways to Split a String Use the split method of String: String[] = s.split(“\\s”); // see class Pattern in java.util.regex Use a StreamTokenizer (in java.io)

Opening and Using Files: Writing Output : Opening and Using Files: Writing Output // open a print stream (**exceptions!) PrintStream ps = new PrintStream(args[0]); // ways to write output ps.print(“Hello”); // a string ps.print(i+3); // an integer ps.println(“ and goodbye.”); // with NL ps.printf(“%2d %12d%n”, i, 1<

Summary of the Review : Summary of the Review A Java program is a collection of classes The JVM approach enables a Java program written on one machine to execute on any other machine that has a JVM Java defines a set of primitive data types that are used to represent numbers, characters, and boolean data The control structures of Java are similar to those found in other languages The Java String and StringBuffer classes are used to reference objects that store character strings

Chapter Review (continued) : Chapter Review (continued) Be sure to use methods such as equals and compareTo to compare the contents of String objects You can declare your own Java classes and create objects of these classes using the new operator A class has data fields and instance methods Array variables can reference array objects Class JOptionPane can be used to display dialog windows for data entry and message windows for output The stream classes in package java.io read strings from the console and display strings to the console, and also support file I/O