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CSM-Java Programming-I Spring,2005

Class Design Lesson - 4. CSM-Java Programming-I Spring,2005. Objectives Review of last class Method Overloading Constructor Overloading Static revisited finalize Recursion Command-Line Arguments Arrays Class Design. CSM-Java Programming-I Lesson-1.

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CSM-Java Programming-I Spring,2005

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  1. Class Design Lesson - 4 CSM-Java Programming-I Spring,2005

  2. Objectives • Review of last class • Method Overloading • Constructor Overloading • Static revisited • finalize • Recursion • Command-Line Arguments • Arrays • Class Design CSM-Java Programming-I Lesson-1

  3. Method Overloading • In Java, it is possible for two or more methods to have the same name, as long as they have different number or types of parameters. This is called method overloading. • When an overloaded method is invoked, the compiler compares the number and types of parameters to find the method that best matches the available signatures. • The return type and the exceptions thrown are not counted for overloading of methods. CSM-Java Programming-I Lesson-1

  4. Method Overloading public class Example { public void show() { // does something; } public double show (double i) { // does something else return i/2.0; } } CSM-Java Programming-I Lesson-1

  5. Constructor Overloading public class Box { int height; int width; int depth; public Box () { height = 0; …. } public Box(int bheight, int bwidth, int bdepth) { height = bheight; ……. } CSM-Java Programming-I Lesson-1

  6. Static Revisited • When a member is declared static, it can be accessed before any objects of its class are created. • Both methods and variables can be declared static. • A static member is a member that is only one per class, rather than one in every object created from that class. • A static method can access only static variables and static methods of the class. • There is no this reference because there is no specific object being operated on. • classname.staticmethod(); CSM-Java Programming-I Lesson-1

  7. Garbage collection and finalize • Java performs garbage collection and eliminates the need to free objects explicitly. • When an object has no references to it anywhere, except in other objects that are also unreferenced, its space can be reclaimed. • Before the object is destroyed, it might be necessary for the object to perform some actions. For example closing an opened file. In such a case define a finalize() method with the actions to be performed before the object is destroyed. CSM-Java Programming-I Lesson-1

  8. finalize • When a finalize method is defined in a class, Java run time calls finalize() whenever it is about to recycle an object of that class. protected void finalize() { // code } • A garbage collector reclaims objects in any order or never reclaim them. CSM-Java Programming-I Lesson-1

  9. Command-Line Arguments • The data that follows the program name on the command-line when it is executed is a command-line argument. • The command-line arguments are stored as strings in the String array passed to the main(). CSM-Java Programming-I Lesson-1

  10. Command-Line Arguments public class Example { public static void main(String args[]) { for (int i=0; i < args.length; i++) System.out.println(“args[“ + i + “]: “ + args[i]); } } > java Example my test 1 args[0]: my args[1]: test args[2]: 1 CSM-Java Programming-I Lesson-1

  11. Arrays • An array is a fixed-length collection of variables of the same type. • Arrays can have one or more dimensions. • An element in an array is accessed by its index. E.g.: a[i] int[] data = new int[10]; • Arrays have a length field that says how many elements the array contains. In the above example: data.length =10 CSM-Java Programming-I Lesson-1

  12. Arrays • An array range is between 0 and length-1. An indexOutofBoundsException is thrown if you try to access elements outside the range of the array. • An array can be initialized when they are declared. Eg: int singleDigitEvenNumbers[] = {2, 4, 6, 8 }; • In the above example there is no need to do “new”. An array will be created large enough to hold all the elements that are initialized. CSM-Java Programming-I Lesson-1

  13. Recursion Recursion is defining something in terms of itself (a method to call itself). public class Factorial { public int fact(int n) { int result; if (n == 1) { return 1; } result = fact(n-1) * n; return result; } } CSM-Java Programming-I Lesson-1

  14. Class Design • Classes are collections of objects. • Objects are entities not actions. • Methods are actions. • A class should be a single concept. • Begin designing your class by identifying objects and the classes to which they belong. CSM-Java Programming-I Lesson-1

  15. Class Design • Some examples of classes can be, Student, Employee, etc., • Very occasionally, a class has no objects, but it contains a collection of related static methods and constants. -Math class is an example of such a class. It is called a utility class. • If you can’t tell from the class name what an object of the class is supposed to do, then you are probably not on the right track. CSM-Java Programming-I Lesson-1

  16. Cohesion • The public interface of a class should be cohesive (all features should be closely related to the single concept that the class represents). CSM-Java Programming-I Lesson-1

  17. Example /** A purse computes the total value of a collection of coins. */ public class Purse { private double total; /** Constructs an empty purse. */ public Purse() { total = 0; } CSM-Java Programming-I Lesson-1

  18. Example /** Add a coin to the purse. @param aCoin the coin to add */ public void add(Coin aCoin) { total = total + aCoin.getValue(); } /** Get the total value of the coins in the purse. @return the sum of all coin values */ public double getTotal() { return total; } } CSM-Java Programming-I Lesson-1

  19. Example /** A coin with a monetary value. */ public class Coin { private double value; private String name; /** Constructs a coin. @param aValue the monetary value of the coin. @param aName the name of the coin */ public Coin(double aValue, String aName) { value = aValue; name = aName; } CSM-Java Programming-I Lesson-1

  20. Example /** Gets the coin value. @return the value */ public double getValue() { return value; } /** Gets the coin name. @return the name */ public String getName() { return name; } } CSM-Java Programming-I Lesson-1

  21. Coupling • A class depends on another class if it uses objects of that other class. • If many classes of a program depend on each other, then we say that the coupling between classes is high. • It is a good practice to minimize coupling between classes. • Follow a consistent scheme for the method names and parameters. CSM-Java Programming-I Lesson-1

  22. Accessor and Mutator Methods • A method that accesses an object and returns some information about it, without changing the object, is called an accessor method. Eg: getBalance() method in BankAccount class. • A method whose purpose is to modify the state of an object is called a mutator method. Eg: withdraw() or deposit() • Some classes are designed to have only accessor methods. Eg: String. – Immutable class. CSM-Java Programming-I Lesson-1

  23. Preconditions • A precondition is a requirement that the caller of a method must obey. • For example, the deposit method of the BankAccount class has a precondition amount >= 0 (amount parameter must not be negative) • If a method violates this precondition it is not responsible for computing a correct result (might throw an exception). CSM-Java Programming-I Lesson-1

  24. Postconditions • Postcondition makes a statement about the object's state after the deposit method is called. • For example, the deposit method of the BankAccount class has a postcondition getBalance() >= 0 (balance returned after deposit is not negative). • As long as the precondition is fulfilled, this method guarentees that the balance after the deposit is not negative. CSM-Java Programming-I Lesson-1

  25. Scope • The scope of local variables extends from the point of declaration to the end of the block that encloses it. • The scope of a variable in a for loop extends to the for loop but not beyond. for (int i = 1; i <= years; i++) { …… } // scope of i ends here. CSM-Java Programming-I Lesson-1

  26. Scope • In Java, you cannot have two local variables with overlapping scope. Rectangle r = new Rectangle (5, 10, 20, 30); if (x >= 0) { double r = Math.sqrt(x); // Error- Can’t declare another variable called r here. …… } CSM-Java Programming-I Lesson-1

  27. Scope of Class Memebrs • In Java, the scope of a local variable and an instance variable can overlap. You can solve this problem by using the this reference. public class Coin { double value; String name; public Coin (double value, String name) { this.value = value; this.name = name; } CSM-Java Programming-I Lesson-1

  28. Calling One Constructor from Another public class Coin { public Coin( double value , String name) { this.value = value; this.name = name; } public Coin() { this (1, “dollar”); } ….. } Such a constructor call can occur only as the first line in another constructor. CSM-Java Programming-I Lesson-1

  29. Packages • A package is a set of related classes. They provide a structuring mechanism. • To put classes in a package, you must place a line packagepackagename; package com.hotsmann.bigjava; as the first line in the source file containing classes. • If you did not include package statement at the top of your source file, its classes are placed in the default package. CSM-Java Programming-I Lesson-1

  30. Importing packages • The import directive lets you refer to a class of a package by its class name, without the package prefix. import java.awt.Color; • There is no need to import the classes in the java.lang package explicitly. This package contains most basic Java classes. • There is no need to import the other classes in the same package. CSM-Java Programming-I Lesson-1

  31. How to program with packages • Step 1: Come up with a package name : Say homework1. • Step 2: Pick a base directory: Say C:\CIS381\Assignments • Step 3: Make a subdirectory from the base directory that matches your package name. mkdir C:\CIS381\Assignments\homework1 • Step 4: Place your source files into the package subdirectory. Say your homework1 consists of BankAccount.java and BankAccountTest.java, then put them in C:\CIS381\Assignments\homework1 CSM-Java Programming-I Lesson-1

  32. How to program with packages • Step 5: Use the package statement in each of your source files. package homework1; as the first line of code in your BankAccount.java and BankAccountTest.java. • Step 6: Change to the base directory to compile your files. cd \CIS381\Assigments javac homework1\BankAccount.java javac homework1\BankAccountTest.java CSM-Java Programming-I Lesson-1

  33. How to program with packages • Step 7: Run your program from the base directory cd CIS381/Assignments java homework1.BankAccountTest CSM-Java Programming-I Lesson-1

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