Java for Interfaces and Networks (DT3010, HT10)

Transcription

1 Java for Interfaces and Networks (DT3010, HT10) More Basics: Classes, Exceptions, Garbage Collection, Interfaces, Packages Federico Pecora School of Science and Technology Örebro University Federico Pecora Java for Interfaces and Networks Lecture 2 1 / 31

2 Outline 1 Instance vs. Class variables and methods 2 Garbage collection 3 More on inheritance 4 Exceptions 5 Classes and interfaces 6 Packages Federico Pecora Java for Interfaces and Networks Lecture 2 2 / 31

3 Instance vs. Class variables and methods Outline 1 Instance vs. Class variables and methods 2 Garbage collection 3 More on inheritance 4 Exceptions 5 Classes and interfaces 6 Packages Federico Pecora Java for Interfaces and Networks Lecture 2 3 / 31

4 Instance vs. Class variables and methods Instance vs. Class variables Instance variable: not static as many copies as there are instantiated objects represents an attribute which all objects of the class have created when the object is instantiated (with new) Class variable: static one copy, regardless of the number of instantiated objects represents an attribute of the class of objects as a whole, not bound to a specific object created when class is loaded Federico Pecora Java for Interfaces and Networks Lecture 2 4 / 31

5 Instance vs. Class variables and methods Instance vs. Class methods Instance method: not static represents a capability of an instance of the class, all instances have this capaility has an implicit parameter this which is a reference to the object itself can access instance variables can access class variables Class method: static represents a capability associated to the class as a whole cannot access instance variables can access class variables Federico Pecora Java for Interfaces and Networks Lecture 2 5 / 31

6 Instance vs. Class variables and methods Constants: final Variables can be declared final, this makes them constants This also applies to method parameters final int test = 17; test = 4; // Compilation error! void amethod(final Circle circ) { circ = null; //Compilation error! } Similar to C++ s const attribute, but not as versatile class Foo { public: const int k; int fum; void amethod(const int* const p) const { int i; k = 17; //Error due to first const *p = 17; //Error due to second const p = &i; //Error due to third const fum = 17; //Error due to fourth const } }; Federico Pecora Java for Interfaces and Networks Lecture 2 6 / 31

7 Instance vs. Class variables and methods Example of static and final MyInteger.java 1 public class MyInteger { 2 private int number; 3 public void increment() { 4 ++number; 5 } 6 private static int numinstances = 0; 7 public final int instancenumber = numinstances++; 8 public static int convert(string s) { 9 return Integer.parseInt(s); 10 } 11 public static void main(string args) { 12 MyInteger i = new MyInteger(); 13 MyInteger.convert("13"); 14 i.convert("13"); 15 convert("13"); 16 i.increment(); 17 } //main 18 } //class MyInteger Federico Pecora Java for Interfaces and Networks Lecture 2 7 / 31

8 Instance vs. Class variables and methods Example: Complex numbers A class Complex, which describes complex numbers A class variable numinst which counts the number of instances Private instance variables re and im A simple instance method for printing A class method main Federico Pecora Java for Interfaces and Networks Lecture 2 8 / 31

9 Instance vs. Class variables and methods Example: Complex numbers Complex.java 1 public class Complex { 2 public static int numinst = 0; 3 private float re, im; 4 public Complex(float re, float im) { 5 this.re = re; 6 this.im = im; 7 Complex.numInst++; 8 } //Complex 9 public Complex() { this(0, 0); } 10 public void writenumber() { 11 System.out.print(re + " + " + im + "i" ); 12 } //writenumber 13 public static void main(string args) { 14 Complex carray = new Complex3; 15 for (int index = 0; index < 3; index++) { 16 carrayindex = new Complex(); 17 } //while 18 System.out.println(Complex.numInst + 19 " objects allocated."); 20 } //main 21 } //class Complex Federico Pecora Java for Interfaces and Networks Lecture 2 8 / 31

10 Instance vs. Class variables and methods Example: Complex numbers linux> javac Complex.java linux> java Complex 3 objects allocated. linux> Federico Pecora Java for Interfaces and Networks Lecture 2 8 / 31

11 Instance vs. Class variables and methods Example: Complex numbers linux> javac Complex.java linux> java Complex 3 objects allocated. linux> Problem: the instance counter only counts up... how do we make it decrement when an object ceases to exist? Federico Pecora Java for Interfaces and Networks Lecture 2 8 / 31

12 Garbage collection Outline 1 Instance vs. Class variables and methods 2 Garbage collection 3 More on inheritance 4 Exceptions 5 Classes and interfaces 6 Packages Federico Pecora Java for Interfaces and Networks Lecture 2 9 / 31

13 Garbage collection The finalize method Java allows to execute some operations upon object destruction These operations are specified in the finalize method (which overloads Object.finalize()) protected void finalize() throws Throwable { numinst--; super.finalize(); } //finalize Notice that it is not known when the JVM will actually call this method! Federico Pecora Java for Interfaces and Networks Lecture 2 10 / 31

14 Garbage collection The finalize method Java allows to execute some operations upon object destruction These operations are specified in the finalize method (which overloads Object.finalize()) protected void finalize() throws Throwable { try { //Your operations here } finally { super.finalize(); } } //finalize Notice that it is not known when the JVM will actually call this method! Federico Pecora Java for Interfaces and Networks Lecture 2 10 / 31

15 Garbage collection Example: Complex numbers (new main) Complex.java 1 public static void main(string args) { 2 Complex carray = new Complex3; 3 for (int index = 0; index < 3; index++) { 4 carrayindex = new Complex(); 5 } //while 6 { 7 Complex restrictedscope = new Complex(); 8 System.out.println(numInst + " in two scopes."); 9 } 10 System.out.println(numInst + " in one scope."); 11 } //main linux> javac Complex.java linux> java Complex 4 in two scopes. 4 in one scope. linux>... what is happening here? Federico Pecora Java for Interfaces and Networks Lecture 2 11 / 31

16 Garbage collection Example: Complex numbers (new main) Data from a loop of allocations of a Complex variable Federico Pecora Java for Interfaces and Networks Lecture 2 12 / 31

17 Garbage collection Garbage collection The finalize() method is called when deemed appropriate by the JVM this can occur at any time, also in the middle of a print statement Garbage collection is done manually in C/C++ (with free()/delete()) this is a notoriously difficult task Fully automatic in Java, no need to worry about memory leaks BUT, remember that you have no control on when garbage collection occurs Federico Pecora Java for Interfaces and Networks Lecture 2 13 / 31

18 More on inheritance Outline 1 Instance vs. Class variables and methods 2 Garbage collection 3 More on inheritance 4 Exceptions 5 Classes and interfaces 6 Packages Federico Pecora Java for Interfaces and Networks Lecture 2 14 / 31

19 More on inheritance Inheritance in Java A class can inherit only a single class in java multiple inheritance is not supported However, interfaces offer a way to do multiple inheritance It is also possible to block inheritance a class designer can choose to specify his/her class as final Federico Pecora Java for Interfaces and Networks Lecture 2 15 / 31

20 More on inheritance Final classes A class declared final cannot be extended java.lang.string is such a class No one can subclass String and introduce anomalous behavior If a class is final, all its methods are implicitly final the method is guaranteed not to be overridden This is useful to enforce immutability If you must use final classes (and methods), document why Federico Pecora Java for Interfaces and Networks Lecture 2 16 / 31

21 More on inheritance The Object class All classes implicitly extend the Object class Object is the root of the class hierarchy All objects, including arrays, implement the methods of this class Federico Pecora Java for Interfaces and Networks Lecture 2 17 / 31

22 More on inheritance Access level modifiers Determine where a particular field or method are accessible Modifier Class Package Subclass World public Y Y Y Y protected Y Y Y N Y Y N N private Y N N N Use the most restrictive access level that makes sense for a particular member Use private unless you have a good reason not to; avoid public variables except for constants NB: the absence of a modifier is not equivalent to public! Federico Pecora Java for Interfaces and Networks Lecture 2 18 / 31

23 Exceptions Outline 1 Instance vs. Class variables and methods 2 Garbage collection 3 More on inheritance 4 Exceptions 5 Classes and interfaces 6 Packages Federico Pecora Java for Interfaces and Networks Lecture 2 19 / 31

24 Exceptions Error handling in C/C++ Error handling in C/C++ mostly dependends on programmer e.g., writing code to check return values unsafe.c 1 #include <stdio.h> 2 int main(int argc, char* argv) { 3 printf("hello, world!\n"); 4 return 0; 5 } /* main */ Federico Pecora Java for Interfaces and Networks Lecture 2 20 / 31

25 Exceptions Error handling in C/C++ Error handling in C/C++ mostly dependends on programmer e.g., writing code to check return values safer.c 1 #include <stdio.h> 2 #include <stdlib.h> 3 int main(int argc, char* argv) { 4 if (printf("hello, world!\n")!= 14) { 5 fprintf(stderr, "Failure writing text!\n"); 6 return EXIT_FAILURE; 7 } 8 if (fclose(stdout)!= 0) { 9 fprintf(stderr, "Failure writing file!\n"); 10 return EXIT_FAILURE; 11 } 12 return 0; 13 } /* main */ Federico Pecora Java for Interfaces and Networks Lecture 2 20 / 31

26 Exceptions Exceptions in Java Java throws an exception when something is wrong Exceptions can be caught, potentially in a completely different part of the code from where they are thrown Some exceptions must be caught, others don t have to (unchecked exceptions) most exceptions you will ever write should not be unchecked An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions Designing your own exceptions consists in deciding how to react to the disruption Federico Pecora Java for Interfaces and Networks Lecture 2 21 / 31

27 Exceptions Exceptions in Java: example 1 private void drinkcoffee() { 2 //Sluuuurp... 3 } 4 private void eatdinner() { 5 eatappetizer(); 6 eatmaincourse(); 7 eatdessert(); 8 drinkcoffee(); 9 } 10 public static void main(string args) { 11 Dinner d = new Dinner(); 12 d.eatdinner(); 13 System.out.println("Eating done"); 14 System.out.println("(hopefully...)"); 15 } //main What if something goes wrong in the methods called by eatdinner()? Federico Pecora Java for Interfaces and Networks Lecture 2 22 / 31

28 Exceptions Exceptions in Java: example 1 private boolean drinkcoffee() { 2 if (coffeepot == null) return false; 3 else { /* Sluuurp... */ return true; } 4 } 5 private boolean eatdinner() { 6 if (!eatappetizer()) return false; 7 if (!eatmaincourse()) return false; 8 if (!eatdessert()) return false; 9 if (!drinkcoffee()) return false; 10 return true; 11 } 12 public static void main(string args) { 13 Dinner d = new Dinner(); 14 if (!d.eatdinner()) System.out.println("Error!"); 15 } //main Notice that error handling code is obfuscating the orignial implementation! Federico Pecora Java for Interfaces and Networks Lecture 2 22 / 31

29 Exceptions Exceptions in Java: example 1 private void drinkcoffee() throws NoCoffeePotException { 2 if (coffeepot == null) throw new NoCoffeePotException(); 3 else { /* Sluuurp... */ } 4 } 5 private void eatdinner() throws NoCoffeePotException { 6 eatappetizer(); 7 eatmaincourse(); 8 eatdessert(); 9 drinkcoffee(); 10 } 11 public static void main(string args) { 12 Dinner d = new Dinner(); 13 try { d.eatdinner(); } 14 catch (NoCoffeePotException e) { 15 System.out.println("No coffee pot!"); 16 } 17 catch (Exception e) { System.out.println("Error!"); } 18 } //main Federico Pecora Java for Interfaces and Networks Lecture 2 22 / 31

30 Exceptions Exceptions in Java: example 1 private void drinkcoffee() throws NoCoffeePotException { 2 if (coffeepot == null) throw new NoCoffeePotException(); 3 else { /* Sluuurp... */ } 4 } 5 private void eatdinner() throws NoCoffeePotException { 6 eatappetizer(); 7 eatmaincourse(); 8 eatdessert(); 9 drinkcoffee(); 10 } 11 public static void main(string args) { 12 Dinner d = new Dinner(); 13 try { d.eatdinner(); } 14 catch (NoCoffeePotException e) { 15 System.out.println("No coffee pot!"); 16 } 17 catch (Exception e) { System.out.println("Error!"); } 18 } //main Federico Pecora Java for Interfaces and Networks Lecture 2 22 / 31

31 Exceptions Throwable objects NoCoffeePotException.java class NoCoffeePotException extends Exception { //Body can be empty... } Federico Pecora Java for Interfaces and Networks Lecture 2 23 / 31

32 Exceptions Throwable objects Unchecked exceptions Error: thrown by the JVM when a dynamic linking failure or other hard failure occurs (e.g., ClassFormatError, OutOfMemoryError) RunTimeException: reserved for exceptions that indicate incorrect use of an API (e.g., ArithmeticException, NullPointerException) Checked exceptions Exception: normal problems, not serious system failures (e.g., your own exceptions, IllegalAccessException, NegativeArraySizeException) Federico Pecora Java for Interfaces and Networks Lecture 2 23 / 31

33 Exceptions Extending the Exception class: example Maintain an object num of type Number Loop over keyboard input At each iteration read an integer If odd: add to num If even: divide num by read integer handle case when given number is zero with exception thrown by Number If input = 99: exit Federico Pecora Java for Interfaces and Networks Lecture 2 24 / 31

34 Exceptions Extending the Exception class: example Number.java 1 import java.io.bufferedreader; 2 import java.io.inputstreamreader; 3 class Number { 4 private int num = 0; 5 public Number(int n) { 6 this.num = n; 7 } //Number 8 public void add(int n) { 9 this.num += n; 10 } //add 11 public void divide(int n) throws DivZeroException { 12 if (n == 0) throw new DivZeroException(); 13 this.num = this.num / n; 14 } // divide 15 public int value() { return this.num; } 16 //Continues... Federico Pecora Java for Interfaces and Networks Lecture 2 24 / 31

35 Exceptions Extending the Exception class: example Number.java (cont) 17 public static void main(string args) { 18 BufferedReader kbd_reader = /*... */ 19 int n = 1; String buf; Number num = new Number(14); 20 System.out.println("Starting with: " + num.value()); 21 try { while (n!= 99) { 22 System.out.print("Input a number (99 to quit): "); 23 buf = kbd_reader.readline(); 24 n = Integer.parseInt(buf); 25 if (n % 2 == 1) num.add(n); 26 else try { num.divide(n); } 27 catch (DivZeroException e) { 28 System.out.println("Division by 0 skipped."); 29 } 30 System.out.println("Number is: " + num.value()); 31 } } //while - try 32 catch (Exception e) { System.out.println("Error!"); } 33 } //main 34 } //class Number Federico Pecora Java for Interfaces and Networks Lecture 2 24 / 31

36 Exceptions Extending the Exception class: example Number.java (cont) 17 public static void main(string args) { 18 BufferedReader kbd_reader = /*... */ 19 int n = 1; String buf; Number num = new Number(14); 20 System.out.println("Starting with: " + num.value()); 21 try { while (n!= 99) { 22 System.out.print("Input a number (99 to quit): "); 23 buf = kbd_reader.readline(); 24 n = Integer.parseInt(buf); 25 if (n % 2 == 1) num.add(n); 26 else try { num.divide(n); } 27 catch (DivZeroException e) { 28 System.out.println("Division by 0 skipped."); 29 } 30 System.out.println("Number is: " + num.value()); 31 } } //while - try 32 catch (Exception e) { System.out.println("Error!"); } 33 } //main 34 } //class Number Federico Pecora Java for Interfaces and Networks Lecture 2 24 / 31

37 Exceptions Extending the Exception class: example DivZeroException.java 1 class DivZeroException extends Exception { 2 /* 3 Can embed information into 4 exception object... 5 See Java API for methods to override 6 */ 7 } Your exception object can have information mebedded into it The Exception class allows you to specify a String detail message Message can be retrieved by invoking getmessage() on the exception object Federico Pecora Java for Interfaces and Networks Lecture 2 24 / 31

38 Exceptions Extending the Exception class: example You can give programmers that use the exception object the possibility to specify the detail message MyException.java 1 class MyExecption extends Exception { 2 MyExecption() { } 3 MyExecption(String msg) { super(msg); } 4 } Federico Pecora Java for Interfaces and Networks Lecture 2 24 / 31

39 Classes and interfaces Outline 1 Instance vs. Class variables and methods 2 Garbage collection 3 More on inheritance 4 Exceptions 5 Classes and interfaces 6 Packages Federico Pecora Java for Interfaces and Networks Lecture 2 25 / 31

40 Classes and interfaces Interfaces in Java An interface is a specification, it does not implement anything Example: an object that implements this interface should have a function foo (defined by its signature) A class can implement an interface A class can implement multiple interfaces! Interface an abstract class that does not implement anything Rationale: an interface should be use to implement many, potentially very different classes Federico Pecora Java for Interfaces and Networks Lecture 2 26 / 31

41 Classes and interfaces Interfaces: example An interface called Function which specifies how to write math functions... namely, the need to specify the function, its derivative and its integral function... and supports the development of an integral solver Federico Pecora Java for Interfaces and Networks Lecture 2 27 / 31

42 Classes and interfaces Interfaces: example Function.java 1 public interface Function { 2 public float f(float x); 3 public float derivative(float x); 4 public float integralfunction(float x); 5 } // interface Function Federico Pecora Java for Interfaces and Networks Lecture 2 27 / 31

43 Classes and interfaces Interfaces: example Now, let s implement the function f (x) = sin(x) Sine.java 1 public class Sine implements Function { 2 public float f(float x) { 3 return (float)math.sin(x); 4 } 5 public float derivative(float x) { 6 return (float)math.cos(x); 7 } 8 public float integralfunction(float x) { 9 return -(float)math.cos(x); 10 } 11 } //class Sine Federico Pecora Java for Interfaces and Networks Lecture 2 27 / 31

44 Classes and interfaces Interfaces: example Now, let s implement the function f (x) = x x Polynomial.java 1 public class Polynomial implements Function { 2 public float f(float x) { 3 return x*x - x + 3.0f; 4 } 5 public float derivative(float x) { 6 return 2.0f*x - 1.0f; 7 } 8 public float integralfunction(float x) { 9 return x*x*x/3.0f - x*x/2.0f + 3.0f*x; 10 } 11 } //Polynomial Federico Pecora Java for Interfaces and Networks Lecture 2 27 / 31

45 Classes and interfaces Interfaces: example Now, let s implement the integral solver, i.e., b a f (x)dx = f (x)dx b f (x)dx a and a test class Integral.java 1 public class Integral { 2 public float integrate(function fn, float a, float b) { 3 return fn.integralfunction(b) - 4 fn.integralfunction(a); 5 } 6 } // Integral NB: interfaces are similar to function pointers in C! Federico Pecora Java for Interfaces and Networks Lecture 2 27 / 31

46 Classes and interfaces Interfaces: example Finally, a test class IntegralTest.java 1 import java.io.*; 2 public class IntegralTest { 3 public static void main(string args) { 4 Sine sin = new Sine(); 5 Polynomial poly = new Polynomial(); 6 Integral solver = new Integral(); 7 System.out.println("Integral of sin(x): " + 8 solver.integral(sin, 1.0f, 3.0f)); 9 System.out.println("Integral of x - x^2 + 3: " + 10 solver.integral(poly, 1.0f, 3.0f)); 11 } // main 12 } //IntegralTest Federico Pecora Java for Interfaces and Networks Lecture 2 27 / 31

47 Classes and interfaces Interfaces vs. abstract classes An interface implementation may be added to any existing third party class a third party class must be rewritten to extend only from the abstract class Interfaces are used to describe the peripheral abilities of a class, not its central identity a Book which implements the Stackable interface (which is not the central goal of a book) An abstract class defines the core identity of its descendants a Dog which defines the basic aspects of Dalmations, Poodles, GermanShepherds, etc. Federico Pecora Java for Interfaces and Networks Lecture 2 28 / 31

48 Packages Outline 1 Instance vs. Class variables and methods 2 Garbage collection 3 More on inheritance 4 Exceptions 5 Classes and interfaces 6 Packages Federico Pecora Java for Interfaces and Networks Lecture 2 29 / 31

49 Packages Packages in Java Provide a means to modularize applications using directory structure Group classes, variables, properties, etc. Built-in examples java.io input/output e.g., java.io.bufferedreader java.lang basic elements of the java language e.g., java.lang.object (imported automatically) java.awt graphical user interfaces (old, not very good) javax.swing graphical user interfaces (new, good) If you import apackage.*; then you can access apackage.aclass just by writing AClass Federico Pecora Java for Interfaces and Networks Lecture 2 30 / 31

50 Packages More Basics: Classes, Exceptions, Garbage Collection, Interfaces, Packages Thank you! Federico Pecora Java for Interfaces and Networks Lecture 2 31 / 31