The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie COMP 14 - 03 Introduction to Programming...

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Adrian Ilie

COMP 14 - 03Introduction to Programming

Adrian IlieAdrian IlieJuly 13, 2005July 13, 2005


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Today in Comp 14

• Review Classes and Methods♦ Rectangle class♦ ReverseString class


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Data ScopeExample

public class Rectangle{

// variables declared here are class-level// available in all methods in Rectangle class

public int computeArea() {

// variables declared here are method-level// only available in computeArea()

}

public void print() {

// variables declared here are method-level// only available in print()

}

}


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• Overloading - the process of using the same method name for multiple methods

• The signature of each overloaded method must be unique♦ Number of parameters♦ Type of the parameters♦ not the return type of the method, though

• The compiler determines which version of the method is being invoked by analyzing the parameters

ReviewOverloading Methods


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Constructors

public Rectangle (int l, int w){ length = l; width = w;}


private int length;private int width; Rectangle r2 = new

Rectangle (5, 10);public Rectangle (){ length = 0; width = 0;}


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• Create an object:Rectangle r;r = new Rectangle(2, 3);ORRectangle r = new Rectangle(2, 3);

• Use the object and the dot operator to access methods:r.setLength(5);r.setWidth(10);

r2500

25002 3

Creating and Using Objects

r

2 3

25002500

5 10


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Exercise

Write a method for the Rectangle class called printBox that will print the rectangle as a box made of %

example: length = 3, width = 5

%%%%%

% %

%%%%%


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public void printBox (){

for (int i = 1; i <= width; i++)System.out.print("%");

System.out.println( );

for (int i = 1; i <= length - 2; i++) { System.out.print("%");

for (int j = 1; j <= width - 2; j++) System.out.print(" ");

System.out.println("%");}

for (int i = 1; i <= width; i++) System.out.print("%"); System.out.println( );}

%%%%%%%%

length = 3, width = 8

% %%%%%%%%%


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public void printBox (){

for (int i = 1; i <= width; i++)System.out.print("%");

System.out.println( );

for (int i = 1; i <= length - 2; i++) { System.out.print("%");

for (int j = 1; j <= width - 2; j++) System.out.print(" ");

System.out.println("%");}

for (int i = 1; i <= width; i++) System.out.print("%"); System.out.println( );}

%%%

length = 8, width = 3

% %% %% %% %% %% %%%%


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public class RectangleTester{ public static void main (String[] args) { Rectangle r1 = new Rectangle(); Rectangle r2 = new Rectangle (20, 30); r1.setWidth(5); r1.setLength(10); r1.print(); r2.print();

System.out.println(r1);System.out.println(r2);

}

} // end of RectangleTester class

What if we just wanted to output r1 and r2 using System.out.println?

Testing


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The toString Method

• Special method in Java classes• Produces a String object based on

the current object, suitable for printing

• Mapped to the '+' operator• Also called when the object is a

parameter in a print() or println() method

• There is a default toString method, but it's better if we write our own

System.out.println(r1);

System.out.println(r1.toString());


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Rectangle.java

public String toString()

{

String result = "";

result += "length: " + length + "\n";

result += "width: " + width;

return (result);

}Rectangle r = new Rectangle (2,3);

System.out.println (r);

length: 2width: 3


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Rectangle.java

• What if we wanted to print out the box when we say System.out.println(r1)?


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Rectangle.java


private int length;private int width;

public Rectangle (int length, int width){

length = length; width = width;

}

Which length and which width??


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The Reference this

• Reserved word

• Refers to instance variables and methods of a class

• Allows you to distinguish between member variables and local variables with the same name


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Rectangle.java


private int length;private int width;

public Rectangle (int length, int width){

this.length = length; this.width = width;

}


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ReverseString.java

• Available on course web page♦ Fully commented

• ReverseTester.java♦ Driver class : Tests the

ReverseString class♦ Also available online


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Review Classes and Methods ReverseString.java

• When do you use dot operator (.) to access methods?ReverseString r1 = new ReverseString( keyboard.readLine() );

// Output the reverse of the entered stringSystem.out.println("The reverse of the string is " + r1.getReverse() );

Inside ReverseTester.javaDot operator to access method getReverse()


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• When do you use dot operator (.) to access methods?ReverseString r1 = new ReverseString( keyboard.readLine() );

// Output the reverse of the entered stringSystem.out.println("The reverse of the string is " + r1.getReverse() );

public ReverseString(String s){original = s;computeReverse();

}

Inside ReverseString.java

No dot operator to access computeReverse()


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• When do you use dot operator (.) to access methods?♦ Must use dot operator to access

method that is in a different class♦ To access a method in ReverseString

from ReverseTester, use dot operator

♦ To access a method in ReverseString from ReverseString, NO dot operator


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• When do you use public vs. private?

private void computeReverse(){

// method body}

public boolean isPalindrome(){

// method body}

Where are these methods called from? (What class are they accessed in?)

Only called from ReverseString class

Called from ReverseTester class


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• When do you use public vs. private?♦ Use private:

• When the method or data member is only going to be accessed inside the same class

♦ Use public:• When the method or data member is going

to be accessed outside the class• Anything that is called, or used, inside

ReverseTester class must be public


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• Why did we not use static in ReverseString class?

When and why do we use static?

How do we access static methods or variables from another class?


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• Why did we not use static in ReverseString class?When and why do we use static?

How do we access static methods or variables from another class?

classname.methodname()

Example: Integer.parseInt(“48”);

public static int parseInt(String s)

This method must be declared static inside class classname

Dot operator


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• Why did we not use static in ReverseString class?When and why do we use static?• Do not need object to use

method/variable

• Consistent among all objects

• Shared among all objects of the class


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• Why did we not use static in ReverseString class?When and why do we use static?• Want all objects of class to share one copy of

data

• Do not need method/variable to be object specificNOTE:

• Can't call a non-static method from a static method• Can't access non-static variables from a static method


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• Why did we not use static in ReverseString class?

NOTE:• Can't call a non-static method from a static method• Can't access non-static variables from a static methodpublic static void main (String[]

args) {myMethod();

}

main method is static, therefore myMethod() must also be static within same class

public static void myMethod ( ) {// method body

}


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• Why was there no main method in ReverseString class?♦ Only a driver class (a class that tests

your program) has a main method♦ General rule:

• For each program you write, there will only be one main method

• Here, it was given in ReverseTester class


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• Why was there no keyboard input in ReverseString class (keyboard.readLine())? ♦ Keyboard input already performed in

ReverseTester

♦ How did we give input to ReverseString class??


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• Why was there no keyboard input in ReverseString class (keyboard.readLine())? ♦ How did we give input to ReverseString

class?? ParametersSpecifically, in this example, parameter to the

constructor

public ReverseString(String s){

original = s;computeReverse();

}

Formal Parameter

Inside ReverseStrin

g class


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• Why was there no keyboard input in ReverseString class (keyboard.readLine())? ♦ How did we give input to ReverseString

class?? Parameters

public ReverseString(String s){

original = s;computeReverse();

}

Formal Parameter

Inside ReverseStrin

g class

ReverseString r1 = new ReverseString( keyboard.readLine() );

Inside ReverseTester class Actual Parameter


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• Why didn’t we use System.out.println in the ReverseString class?♦ Output to the screen already performed

in ReverseTester

♦ How did we give information to ReverseTester class for output?


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• Why didn’t we use System.out.println in the ReverseString class?♦ How did we give information to ReverseTester

class for output?

Return from MethodsSpecifically, in this example:

System.out.println("The reverse of the string is " + r1.getReverse() );

public String getReverse(){

return reverse;}

Inside ReverseStrin

g class

Inside ReverseTester class return type

Call to getReverse() will be replaced with a String


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• Why didn’t we use System.out.println in the ReverseString class?♦ How did we give information to ReverseTester

class for output?Return from MethodsSpecifically, in this example:

System.out.println("The reverse of the string is " + r1.getReverse() );

public String getReverse(){

return reverse;}

Inside ReverseStrin

g class

Inside ReverseTester class

Replaced with the value of reverse


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Communication between Classes

• Methods provide communication between methods♦ Parameters provide input into class♦ Return value provides outputpublic returnType methodName ( formal

parameters ) Output from method

Input to method


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Question

• Write a program that takes in 5 int values, computes the sum, and then prints the values in reverse order.


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int num0, num1, num2, num3, num4;int sum;

System.out.println("Enter five integers, one per line ");

num0 = Integer.parseInt(keyboard.readLine());num1 = Integer.parseInt(keyboard.readLine());num2 = Integer.parseInt(keyboard.readLine());num3 = Integer.parseInt(keyboard.readLine());num4 = Integer.parseInt(keyboard.readLine());

sum = num0 + num1 + num2 + num3 + num4;

System.out.println("The sum of the numbers = " + sum);System.out.println(num4 + " " + num3 + " " + num2 + " “

+ num1 + " " + num0);

Solution

What if we wanted to add and print 100 values?

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie COMP 14 - 03 Introduction to Programming...

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