Introduction to Java for C/C++ Programmers. Kevin Squire Department of Computer Science Naval Postgraduate School

Transcription

1 Introduction to Java for C/C++ Programmers Kevin Squire Department of Computer Science Naval Postgraduate School

2 Homework Download and install Java 6.0 Eclipse I ll introduce Eclipse sometime this week 2

3 The First Program class Hello { // main: generate some simple output public static void main (String[] args) { System.out.println ("Hello, world."); // Save as Hello.java // Compile with javac Hello.java // Run with java Hello 3

4 The First Program class CLASSNAME { public static void main (String[] args) { STATEMENTS 4

5 Variables and Types Some types are the same as C int hour, minute; float temperature; Some (e.g., Strings) are (somewhat) new: String firstname; String lastname; 5

6 Printing Variables class Hello { public static void main (String[] args) { String firstline; firstline = "Hello, again!"; System.out.println (firstline); 6

7 Operators Mostly the same int hour, minute; hour = 11; minute = 59; System.out.print ("Number of minutes since midnight: "); System.out.println (hour*60 + minute); System.out.print ("Fraction of the hour that has passed: "); System.out.println (minute/60); 7

8 Operators for Strings Some new: int i = 5; String hello = "" + i + " Hello, " + "world."; puts the following string in the variable hello "5 Hello, world." 8

9 Math Methods (Functions) import static java.lang.math.sqrt; double root = sqrt (17.0); double angle = 1.5; double height = Math.sin (angle); double degrees = 90; double angle = degrees * 2 * Math.PI / 360.0; int x = (int)math.round (Math.PI * 20.0); double x = Math.exp (Math.log (10.0)); 9

10 static keyword In addition to (instance) members, a Java class can include static members that are attached to the class rather than instances of the class. Equivalent to C/C++ functions which are not class methods Since they are not attached to objects, they do not have access to any particular object s data members All Math functions (Math.sin, Math.sqrt, etc.) are static member functions of the Math class. 10

11 Defining New Static Methods Form: public static void NAME ( LIST OF PARAMETERS ) { STATEMENTS Example public static void newline () { System.out.println (""); 11

12 Class With Only static Methods class NewLine { public static void newline () { System.out.println (""); public static void threeline () { newline (); newline (); newline (); public static void main (String[] args) { System.out.println ("First line."); threeline (); System.out.println ("Second line."); 12

13 String Objects Java has two kinds of variables: primitive types objects Strings are the only object type we've talked about so far Questions we might ask: What is the data contained in a String object? What are the methods we can invoke on String objects? 13

14 Aside: Java Documentation All Java documentation is downloadable or available online. Try: search Google for java 5.0 String 14

15 Reminder: chars char fred = c ; if (fred == c ) { System.out.println (fred); 15

16 String charat method The following code: String fruit = "banana"; char letter = fruit.charat(1); System.out.println (letter); yields a Why? 16

17 String charat method The following code: String fruit = "banana"; char letter = fruit.charat(1); System.out.println (letter); yields a Why? To get the first letter, use char letter = fruit.charat(0); 17

18 Length What's wrong with the statements below? int length = fruit.length(); char last = fruit.charat (length); 18

19 Length What's wrong with the statements below? int length = fruit.length(); char last = fruit.charat (length); Correct version: int length = fruit.length(); char last = fruit.charat (length-1); 19

20 Indices String fruit = "banana"; int index = fruit.indexof( a ); int index = fruit.indexof( a, 2); // start at // position 2 20

21 Other Notes on Strings Strings are immutable Strings are not comparable with ==, >, etc. 21

22 Comparing Strings String name1 = "Alan Turing"; String name2 = "Ada Lovelace"; if (name1.equals (name2)) { System.out.println ("The names are the same."); int flag = name1.compareto (name2); if (flag == 0) { System.out.println ("The names are the same."); else if (flag < 0) { System.out.println ("name1 comes before name2."); else if (flag > 0) { System.out.println ("name2 comes before name1."); 22

23 Interesting Objects Packages Contain built-in Java classes java.lang is available by default (includes all of the basic parts of the language) Must be imported, e.g. import java.awt.*; imports all classes in the java.awt package. 23

24 Point Objects import java.awt.*; // goes at top of file, outside main()... Point blank; blank = new Point (3, 4); Rectangle box = new Rectangle (0, 0, 100, 200); // Access coordinate of blank int x = blank.x; System.out.println (blank.x + ", " + blank.y); int distance = blank.x * blank.x + blank.y * blank.y; 24

25 Objects as Parameters public static void printpoint (Point p) { System.out.println ("(" + p.x + ", " + p.y + ")"); // System.out.println(blank) gives: java.awt.point[x=3,y=4] public static double distance (Point p1, Point p2) { double dx = (double)(p2.x - p1.x); double dy = (double)(p2.y - p1.y); return Math.sqrt (dx*dx + dy*dy); 25

26 Objects as Return Types public static Point findcenter (Rectangle box) { int x = box.x + box.width/2; int y = box.y + box.height/2; return new Point (x, y); 26

27 Objects are Mutable public static void moverect (Rectangle box, int dx, int dy) { box.x = box.x + dx; box.y = box.y + dy;... Rectangle box = new Rectangle (0, 0, 100, 200); moverect (box, 50, 100); System.out.println (box); // yields java.awt.rectangle[x=50,y=100,width=100,height=200] 27

28 Aliasing/References Rectangle box1 = new Rectangle (0, 0, 100, 200); Rectangle box2 = box1; box1 and box2 both refer to the same Rectangle. Alternatively, we can say that the given Rectangle has two aliases (box1 and box2) 28

29 null When you create an object variable, remember that you are creating a reference to an object. Until you make the variable point to an object, the value of the variable is null. null is a special value in Java (and a Java keyword) that is used to mean no object. The declaration Point blank; is equivalent to the initialization Point blank = null; 29

30 Null Pointer Exceptions Point blank = null; int x = blank.x; blank.translate (50, 50); // NullPointerException // NullPointerException 30

31 Garbage Collection Create a new Point: Point blank = new Point (3, 4); Set blank to null: blank = null; In C, this would cause a memory leak; the memory allocated in the first statement would be lost until the program exits. In contrast, Java keeps track of the memory allocated to the object, and reclaims it (deallocates it) when there are no more references to the object There is therefore no need for delete in Java (and it doesn t exist). 31

32 Creating Your Own Classes Defining classes in Java is slightly different than in C++ By convention, class names (and hence object types) always begin with a capital letter, which helps distinguish them from primitive types and variable names. You usually put one class definition in each file, and the name of the file must be the same as the name of the class, with the suffix.java. For example, the Time class is defined in the file named Time.java. In any program, one class is designated as the startup class. The startup class must contain a method named main, which is where the execution of the program begins. Other classes may have a method named main, but it will not be executed. Basic differences between Java and C++ class definition are highlighted on the following slides. 32

33 C++/Java Class Definition C++: Java: class CRectangle { int width, height; public: CRectangle (int, int); int area () { return (width*height); ; CRectangle::CRectangle (int a, int b) { width = a; height = b; public class CRectangle { private int width, height; public CRectangle (int a, int b) { width = a; height = b; public int area () { return (width*height); 33

34 C++/Java Class Definition C++: Java: class CRectangle { int width, height; public: CRectangle (int, int); int area () { return (width*height); ; // Note: ";" required here CRectangle::CRectangle (int a, int b) { width = a; height = b; public class CRectangle { private int width, height; public CRectangle (int a, int b) { width = a; height = b; public int area () { return (width*height); // Note: no ";" required here 34

35 C++/Java Class Definition C++: Java: class CRectangle { int width, height; public: CRectangle (int, int); int area () { return (width*height); ; CRectangle::CRectangle (int a, int b) { width = a; height = b; public class CRectangle { private int width, height; public CRectangle (int a, int b) { width = a; height = b; public int area () { return (width*height); 35

36 C++/Java Class Definition C++: Java: class CRectangle { int width, height; public: CRectangle (int, int); int area () { return (width*height); ; CRectangle::CRectangle (int a, int b) { width = a; height = b; public class CRectangle { private int width, height; public CRectangle (int a, int b) { width = a; height = b; public int area () { return (width*height); 36

37 C++/Java Class Definition C++: Java: public class CRectangle {... int main () { public static void main(string [] args) { CRectangle recta (3,4); CRectangle rectb (5,6); cout << "recta area: " << recta.area() << endl; cout << "rectb area: " << rectb.area() << endl; return 0; CRectangle recta = new CRectangle (3,4); CRectangle rectb = new CRectangle (5,6); System.out.println( "recta area: " + recta.area()); System.out.println( "rectb area: " + rectb.area()); 37

38 C++/Java Class Definition C++: Java: public class CRectangle { // main is... // inside a class def. int main () { public static void main(string [] args) { CRectangle recta (3,4); CRectangle rectb (5,6); cout << "recta area: " << recta.area() << endl; cout << "rectb area: " << rectb.area() << endl; return 0; CRectangle recta = new CRectangle (3,4); CRectangle rectb = new CRectangle (5,6); System.out.println( "recta area: " + recta.area()); System.out.println( "rectb area: " + rectb.area()); 38

39 C++/Java Class Definition C++: int main () { Java: public class CRectangle {... // main() s type is different public static void main(string [] args) { CRectangle recta (3,4); CRectangle rectb (5,6); cout << "recta area: " << recta.area() << endl; cout << "rectb area: " << rectb.area() << endl; return 0; CRectangle recta = new CRectangle (3,4); CRectangle rectb = new CRectangle (5,6); System.out.println( "recta area: " + recta.area()); System.out.println( "rectb area: " + rectb.area()); 39

40 C++/Java Class Definition Function overloading is the same as C++ Destructors are the same (~ClassName()), but are not needed as frequently (why?) 40

41 Inheritance and Polymorphism Inheritance and Polymorphism are very similar in Java and C++ To derive a subclass, the notation is class SUBCLASS extends SUPERCLASS { The constructor of the superclass is accessed via a call to the function super(). Note that all methods are virtual by default So, if a subclass overrides a method of a parent class, the right method will always be called 41

42 Code Example C++: Java: #include <iostream> using namespace std; class CPolygon { protected: int width, height; public: CPolygon(int a, int b) { width=a; height=b; virtual int area() {return 0; ; class CRectangle: public CPolygon { public: CRectangle(int a, int b): CPolygon(a,b) { In file Polygon.java: public class Polygon { protected int width, height; public Polygon(int a, int b) { width=a; height=b; public int area() {return 0; In file Rectangle.java: public class Rectangle extends Polygon{ public Rectangle(int a, int b) { super(a,b); int area () { return (width * height); ; public int area () { return (width * height); 42

43 Code Example C++: Java: #include <iostream> using namespace std; class CPolygon { protected: int width, height; public: CPolygon(int a, int b) { width=a; height=b; virtual int area() {return 0; ; class CRectangle: public CPolygon { public: CRectangle(int a, int b): CPolygon(a,b) { In file Polygon.java: public class Polygon { protected int width, height; public Polygon(int a, int b) { width=a; height=b; public int area() {return 0; In file Rectangle.java: public class Rectangle extends Polygon{ public Rectangle(int a, int b) { super(a,b); int area () { return (width * height); ; public int area () { return (width * height); 43

44 Code Example C++: Java: #include <iostream> using namespace std; class CPolygon { protected: int width, height; public: CPolygon(int a, int b) { width=a; height=b; virtual int area() {return 0; ; class CRectangle: public CPolygon { public: CRectangle(int a, int b): CPolygon(a,b) { In file Polygon.java: public class Polygon { protected int width, height; public Polygon(int a, int b) { width=a; height=b; public int area() {return 0; In file Rectangle.java: public class Rectangle extends Polygon{ public Rectangle(int a, int b) { super(a,b); int area () { return (width * height); ; public int area () { return (width * height); 44

45 Code Example C++: Java: #include <iostream> using namespace std; class CPolygon { protected: int width, height; public: CPolygon(int a, int b) { width=a; height=b; virtual int area() {return 0; ; class CRectangle: public CPolygon { public: CRectangle(int a, int b): CPolygon(a,b) { In file Polygon.java: public class Polygon { protected int width, height; public Polygon(int a, int b) { width=a; height=b; public int area() {return 0; In file Rectangle.java: public class Rectangle extends Polygon{ public Rectangle(int a, int b) { super(a,b); int area () { return (width * height); ; public int area () { return (width * height); 45

46 Code Example (cont.) C++: Java: In file Triangle.java: class CTriangle: public CPolygon { public: CTriangle(int a, int b): CPolygon(a,b) { int area () { return (width * height / 2); ; public class Triangle extends Polygon { public Triangle(int a, int b) { super(a,b); public int area () { return (width * height / 2); int main () { CRectangle rect(4,5); CTriangle trgl(4,5); cout << rect.area() << endl; cout << trgl.area() << endl; return 0; In file PolyTest.java: public class PolyTest { public static void int main () { Rectangle rect = new Rectangle(4,5); Triangle trgl = new Triangle(4,5); System.out.println(rect.area()); System.out.println(trgl.area()); 46

47 Code Example (cont.) C++: Java: In file Triangle.java: class CTriangle: public CPolygon { public: CTriangle(int a, int b): CPolygon(a,b) { int area () { return (width * height / 2); ; public class Triangle extends Polygon { public Triangle(int a, int b) { super(a,b); public int area () { return (width * height / 2); int main () { CRectangle rect(4,5); CTriangle trgl(4,5); cout << rect.area() << endl; cout << trgl.area() << endl; return 0; In file PolyTest.java: public class PolyTest { public static void int main () { Rectangle rect = new Rectangle(4,5); Triangle trgl = new Triangle(4,5); System.out.println(rect.area()); System.out.println(trgl.area()); 47

48 Code Example (cont.) C++: Java: In file Triangle.java: class CTriangle: public CPolygon { public: CTriangle(int a, int b): CPolygon(a,b) { int area () { return (width * height / 2); ; public class Triangle extends Polygon { public Triangle(int a, int b) { super(a,b); public int area () { return (width * height / 2); int main () { CRectangle rect(4,5); CTriangle trgl(4,5); cout << rect.area() << endl; cout << trgl.area() << endl; return 0; In file PolyTest.java: public class PolyTest { public static void int main () { Rectangle rect = new Rectangle(4,5); Triangle trgl = new Triangle(4,5); System.out.println(rect.area()); System.out.println(trgl.area()); 48

49 Polymorphism Example (cont.) C++: Java: In file Triangle.java: class CTriangle: public CPolygon { public: CTriangle(int a, int b): CPolygon(a,b) { int area () { return (width * height / 2); ; int main () { CPolygon *rect = new CRectangle(4,5); CPolygon *trgl = new CTriangle(4,5); cout << rect->area() << endl; cout << trgl->area() << endl; return 0; public class Triangle extends Polygon { public Triangle(int a, int b) { super(a,b); public int area () { return (width * height / 2); In file PolyTest.java: public class PolyTest { public static void int main () { Polygon rect = new Rectangle(4,5); Polygon trgl = new Triangle(4,5); System.out.println(rect.area()); System.out.println(trgl.area()); 49

50 C++ Pure Virtual Classes = Java Abstract Classes C++: #include <iostream> using namespace std; class CPolygon { protected: int width, height; public: CPolygon(int a, int b) { width=a; height=b; virtual int area() = 0; ;... int main () { CPolygon *rect = new CRectangle(4,5); CPolygon *trgl = new CTriangle(4,5); // illegal //CPolygon *poly = new CPolygon(4,5); cout << rect->area() << endl; cout << trgl->area() << endl; //cout << poly->area() << endl; return 0; Java: In file Polygon.java: public abstract class Polygon { protected int width, height; public Polygon(int a, int b) { width=a; height=b; public abstract int area();... In file PolyTest.java: public class PolyTest { public static void int main () { Polygon rect = new Rectangle(4,5); Polygon trgl = new Triangle(4,5); // illegal // Polygon poly = new Polygon(4,5); System.out.println(rect.area()); System.out.println(trgl.area()); // System.out.println(poly.area()); 50

51 Class Object In Java, all classes (built-in and user-defined) are subclasses or descendents of class Object. You ll notice this fact when using the online Java documentation Because of this, every class inherits a number of useful standard methods from class Object, including: tostring(), equal(), hashcode() It is often useful or necessary to override these methods. 51

52 Numerical Object Types For each primitive type in Java, there is a corresponding Object type Sometimes Object types are required when using built-in classes (e.g., ArrayList) Easy to convert between the two (just assign one to the other) Note on 8-bit types char is a character type in Java byte is an 8-bit numerical type in Java These are not equivalent Primitive Type char boolean byte int short long float double Object Type Character Boolean Byte Integer Short Long Float Double 52

53 Additional Type Notes All Java types are signed The following are illegal in Java: unsigned long biglong; unsigned double salary; // not legal in Java // not legal in Java The boolean type in Java is called boolean (cf. bool in C++) true and false are NOT equal to 1 and 0, as they are in C++ Casting between boolean and int types is not allowed: boolean abool = false; int anint = (int)abool; // not legal in Java // not legal in Java 53

54 Java Interfaces An interface in Java is a collection of method declarations and constants, with no data and no bodies. We say that a class implements an interface if it implements all of the methods declared in the interface. In general, interfaces define behaviors we desire in a wide variety of possibly unrelated classes. 54

55 Interface Example As an example, consider an interface for objects that can be sold: public interface Sellable { public String description(); public int listprice(); public int lowestprice(); Any class which implements this interface must define these three functions. 55

56 Interface Example Here's a class which implements Sellable: public class Photograph implements Sellable { private String descript; private int price; private boolean color; public Photograph(String desc, int p, boolean c) { descript = desc; price = p; color = c; public String description() { return descript; public int listprice() { return price; public int lowestprice() { return price/2; public boolean iscolor() { return color; 56

57 Interface Example Suppose we had another interface, Transportable: public interface Transportable { /** weight in grams */ public int weight(); /** whether the object is hazardous */ public boolean ishazardous(); Note that, as before, the interface defines no code, only function prototypes. 57

58 Multiple Inheritance We can then create a class which implements both interfaces. This is called multiple inheritance. public class BoxedItem implements Sellable, Transportable {... public String description() { return descript; public int listprice() { return price; public int lowestprice() { return price/2; public int weight() { return weight; public boolean ishazardous() { return haz; public int insuredvalue() { return price*2;... 58

59 Using Interfaces When using objects which define interfaces, we can treat them very much like classes: Sellable[] itemsforsale = new Sellable[10];... itemsforsale[0] = new Photograph( Monterey Bay, 100, true); itemsforsale[1] = new BoxedItem( TV,...);... System.out.println(itemsForSale[0].description() + : $ + itemsforsale[0].listprice() + +, sale price: + itemsforsale[0].lowestprice(); 59

60 Pre-defined Java Interfaces Java has a large number of useful interfaces predefined, many of which we'll see throughout the quarter Some of them include Comparable: allows objects of an implementing class to be compared and sorted Runnable: used in threads programming Java Collections Framework interfaces, including List: list of items Queue: FIFO list Map: collection of <key,value> pairs Set: collection of objects Collection: any of the above, and more Interfaces used in GUI programming 60

61 Inheritance versus Interface The Java interface is used to share common behavior (only method headers) among the instances of different classes. Inheritance is used to share common code (including both data members and methods) among the instances of related classes. In your program designs, remember to use the Java interface to share common behavior. Use inheritance to share common code. If an entity A is a specialized form of another entity B, then model them by using inheritance. Declare A as a subclass of B. 61

62 C++ Templates Java Generics Java includes a generics framework, similar to C++ Templates, which allows us to define a class in terms of formal type parameters. public class Pair<K,V> {... We instantiate a class using actual type parameters. Pair<String,Integer> pair1 = new Pair<String,Integer>(); 62

63 Generics Example public class Pair<K, V> { private K key; private V value; public void set(k k, V v) { key = k; value = v; public K getkey() { return key; public V getvalue() { return value; public String tostring() { return "[" + getkey() + ", " + getvalue() + "]"; 63

64 Generics Example public static void main (String[] args) { // Create a String-Integer Pair Pair<String,Integer> pair1 = new Pair<String,Integer>(); pair1.set(new String("height"), new Integer(36)); System.out.println(pair1); // Create a Student-Double Pair Pair<Student,Double> pair2 = new Pair<Student,Double>(); pair2.set(new Student("A5976","Sue",19), new Double(9.5)); System.out.println(pair2); 64

65 Generics Example In the previous example, K and V could be instantiated as arbitrary types. We can also restrict their types: public class Pair2<K extends Person, V extends Number> { private K key; private V value;... Generics will be useful for further discussion on lists and other abstract data types (ADTs). 65

66 Eclipse First Run Checklist First run: 1. Select/create a workspace directory (when eclipse starts) 66

67 Eclipse First Run Checklist 1. Change formatting to NOT use tab characters Preferences -> Java -> Code Style -> Formatter For active profile, select Java Conventions [built-in] 67

68 Eclipse First Run Checklist Best: edit this profile, rename it to Java Conventions [spaces only] and select Spaces Only as the Tab policy. 68

69 Eclipse First Run Checklist Make sure JRE is selected Preferences -> Java -> Installed JREs Add a new JRE if necessary 69

70 Eclipse First Run Checklist Make sure JRE is selected Preferences -> Java -> Installed JREs Add a new JRE if necessary 70

71 Eclipse Project Checklist Create a new Java project 71

72 Eclipse Project Checklist Type <Your_Last_Name>_Prog_<x>_<Name> for the project name Select Use project folder as root for sources and class files. Click Finish. 72

73 Eclipse Project Checklist Right click on the new project, and create a new class 73

74 Eclipse Project Checklist For the new class, fill in a package name (same as Project, but first letter should be lower case), class name, and select appropriate options at the bottom. Click Finish. 74

75 Eclipse Project Checklist Your project should look like this. Finish it up and turn it in! 75