CSM-Java Programming-I Spring,2005

CSM-Java Programming-I Spring,2005

Class Design

Lesson - 4

CSM-Java Programming-I Lesson-1

Objectives• Review of last class

• Method Overloading

• Constructor Overloading

• Static revisited

• finalize

• Recursion

• Command-Line Arguments

• Arrays

• Class Design


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.


Method Overloading

public class Example {

public void show() {

// does something;

}

public double show (double i) {

// does something else

return i/2.0;

}

}


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;…….

}


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();


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.


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.


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().


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


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


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.


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;

}}


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.


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.


Cohesion

• The public interface of a class should be cohesive (all features should be closely related to the single concept that the class represents).


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; }


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;}

}


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; }


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; }}


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.


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.


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).


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.


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.


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.……

}


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;

}


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.


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

package packagename;

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.


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.


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



• 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



• Step 7: Run your program from the base directory

cd CIS381/Assignments

java homework1.BankAccountTest

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CSM-Java Programming-I Spring,2005

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