2 Topics of theReview • 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
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CSE 3 Some SalientCharacteristics 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
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4 Java Processing andExecution • 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
6 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
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7 Grouping Classes: TheJava 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)
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8 A Little Exampleof 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
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9 Processing and RunningHelloWorld • javac HelloWorld.java • Produces HelloWorld.class (byte code) • java HelloWorld • Starts the JVM and runs the main method
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10 References and PrimitiveData 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”
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11 Primitive Data Types •Represent numbers, characters, boolean values • Integers: byte, short, int, and long • Real numbers: float and double • Characters: char
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12 Primitive Data Types Datatype Range of values byte -128 .. 127 (8 bits) short -32,768 .. 32,767 (16 bits) int -2,147,483,648 .. 2,147,483,647 (32 bits) long -9,223,372,036,854,775,808 .. ... (64 bits) float +/-10-38 to +/-10+38 and 0, about 6 digits precision double +/-10-308 to +/-10+308 and 0, about 15 digits precision char Unicode characters (generally 16 bits per char) boolean True or false
16 Type Compatibility andConversion • 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
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17 Declaring and SettingVariables • 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)
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18 Referencing and CreatingObjects • 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″;
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19 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
22 Methods • A Javamethod 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
24 Escape Sequences • Anescape sequence is a sequence of two characters beginning with the character • A way to represents special characters/symbols
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25 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
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26 Comparing Objects • Youcan’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!)
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27 The StringBuffer Class •Stores character sequences • Unlike a String object, you can change the contents of a StringBuffer object
29 Wrapper Classes forPrimitive 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.
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30 Defining Your OwnClasses • 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
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31 Defining Your OwnClasses (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
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32 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; ...
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33 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; }
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34 The Person Class(3) // modifier and accessor for givenName public void setGivenName (String given) { this.givenName = given; } public String getGivenName () { return this.givenName; }
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35 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; }
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36 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”; }
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37 The Person Class(6) // same Person? public boolean equals (Person per) { return (per == null) ? false : this.IDNumber.equals(per.IDNumber); }
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38 Arrays • In Java,an array is also an object • The elements are indexes and are referenced using the form arrayvar[subscript]
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39 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);
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40 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; }
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41 Input/Output using ClassJOptionPane • Java 1.2 and higher provide class JOptionPane, which facilitates display • Dialog windows for input • Message windows for output
43 Converting Numeric Stringsto Numbers • A dialog window always returns a reference to a String • Therefore, a conversion is required, using static methods of class String:
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44 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
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45 Opening and UsingFiles: 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) { ... }
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46 Opening and UsingFiles: 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();
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47 Alternate Ways toSplit 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)
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48 Opening and UsingFiles: 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<<i); // like C ps.format(“%2d %12d%n”, i, 1<<i); // same // closing output streams is very important! ps.close();
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49 Summary of theReview • 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
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50 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
![8 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](https://image.slidesharecdn.com/introductiontojava-250807091600-9cd7dea2/75/Introduction-to-Java-basic-understanding-ppt-8-2048.jpg)
![14 Operators 1. subscript [ ], call ( ), member access . 2. pre/post-increment ++ --, boolean complement !, bitwise complement ~, unary + -, type cast (type), object creation new 3. * / % 4. binary + - (+ also concatenates strings) 5. signed shift << >>, unsigned shift >>> 6. comparison < <= > >=, class test instanceof 7. equality comparison == != 8. bitwise and & 9. bitwise or |](https://image.slidesharecdn.com/introductiontojava-250807091600-9cd7dea2/75/Introduction-to-Java-basic-understanding-ppt-14-2048.jpg)
![38 Arrays • In Java, an array is also an object • The elements are indexes and are referenced using the form arrayvar[subscript]](https://image.slidesharecdn.com/introductiontojava-250807091600-9cd7dea2/75/Introduction-to-Java-basic-understanding-ppt-38-2048.jpg)
![39 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);](https://image.slidesharecdn.com/introductiontojava-250807091600-9cd7dea2/75/Introduction-to-Java-basic-understanding-ppt-39-2048.jpg)
![40 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; }](https://image.slidesharecdn.com/introductiontojava-250807091600-9cd7dea2/75/Introduction-to-Java-basic-understanding-ppt-40-2048.jpg)
![45 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) { ... }](https://image.slidesharecdn.com/introductiontojava-250807091600-9cd7dea2/75/Introduction-to-Java-basic-understanding-ppt-45-2048.jpg)
![47 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)](https://image.slidesharecdn.com/introductiontojava-250807091600-9cd7dea2/75/Introduction-to-Java-basic-understanding-ppt-47-2048.jpg)
![48 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<<i); // like C ps.format(“%2d %12d%n”, i, 1<<i); // same // closing output streams is very important! ps.close();](https://image.slidesharecdn.com/introductiontojava-250807091600-9cd7dea2/75/Introduction-to-Java-basic-understanding-ppt-48-2048.jpg)
