Data Types – Reference Types

Objective

• To understand what reference types are

• The need to study reference types

• To understand Java standard packages

• To differentiate between Java defined types and user defined types

• To have a deeper understanding of the String class

• To get acquainted with the Math class

• To get acquainted with the Wrapper classes

Data Types – Reference Types

• Reference types - any data type that is composed of primitive data types as its

based type.

• In other words, it is an aggregate of primitive types.

• Reference types in Java are:

The array data structure, and

The class data type.

• In this section we will briefly discuss the concept of array.

• We will discuss the class type to the extent of the fundamental classes of Java.

Reference Types - Array

• Array

• An array is a set of storage locations set aside to hold one type of data.

• It is simply a means where by we can store values of the same type by using

one generic name.

• The list of items are stored linearly

• Hence the items can be accessed by their relative position in the list.

• Arrays are real objects in Java.

• Storage space is allocated for an array during execution time.

• The concept will be discussed fully in future lesson

Reference Types - Array

10 20 30 40 50 60 70 80

arr

indeces

0 1 2 3 4 5 6 7

Data

This is the most we will say about arrays for now

Reference Types - Class

• The concept of class as you know is the fundamental construct upon which

Java is built.

• A class serves as a blueprint or a template for a certain type of object.

• All data types other than the primitive type or the array must be addressed in

terms of class.

• This is evident in the way that we have been using class.

• For instance, going back to the class Book.

• In order to use this class we had to declare variables of the type:

• That is, Book b1; for instance

Reference Types - Standard Packages

• Java has a vast and rich collection of classes.

• These classes are stored in respective directories and subdirectories.

• Each directory or subdirectory is called, a package.

• Programmers can use these classes in their specific applications.

• There are several packages in Java.

• Two of the most important packages in the language are java and javax.

Java Standard Packages

java and javax packages, along with their sub-packages. Some of the sub-packages also contain sub-packages of their own

•The sub-package lang is a special package is for general programming

•The sub-package javax is designed for Graphical User Interface programming (GUI)

Java Standard Package- lang

• The java.lang package contains classes that are fundamental to Java.

• All of the classes in this package are available in your program automatically.

• That is, as soon as the Java compiler is loaded, all of these classes are loaded

into every Java source file.

Java Standard Package- lang

Java Standard Package- lang

• Chief among these fundamental classes are the following: Object String Math System

• The wrapper classes - for each primitive type there is a corresponding class. Byte Short Integer Long Character Float Double Boolean Void

The class Object

• See the Application Programmers’ Interface (API)

• Object

• String

• Math• Wrapper classes - Byte, Short, Integer, Long, Character, Float, Double,

Boolean, and Void.

Using == with Reference Types

1. class Test2. {3. public static void main(String[] arg)4. {5. String s1 = new String("Hello");6. String s2 = new String("Hello");7.8. System.out.println(s1 == s2);

9. }10. }

Answer: __________________

Using == with Reference Types

1. class Test2. {3. public static void main(String[] arg)4. {5. String s1 = new String("Hello");6. String s2 = new String("Hello");7.8. System.out.println(s1.equals(s2));9. }10. }

Input and Output Operations

• There are three other important operations that are performed on data.

• Input operations

• Output operations, and

• Formatting operations.

• We have been generating outputs already but it was not dealt with formally.

• Input Operations - There are several ways to input data into a program:

Java Input classes

The DOS prompt

The Scanner Class

JOptionPane Class

JOptionPane Input/Output Dialog Boxes

Capabilities

1. Create input dialog boxes

2. Create output windows

JOptionPane Input Dialog Boxes

There are four kinds of standard dialogs:

1. Message dialog shows a message and waits for the user to click OK.

2. Confirmation dialog shows a question and ask for confirmation such as OK

or Cancel.

3. Option dialog shows a question and gets the user’s response from a set of

options.

4. Input dialog shows a question and gets the user’s input from a text field, a

combo box, or list.

Enhance Your Output with Scrollable Window

JOptionPane – Input Dialog

• You must import the class JOptionPane

import javax.swing.JOptionPane;

• The class contains: Several constructors, and Several class methods that deals with dialog boxes.

• The general construct of these methods is as follows:

showXXXXDialog( parameter_list)

• Where XXXX is any of the four dialog box types. We will only be concerned with the input dialog type.

Using JOptionPane for Input

1. import javax.swing.JOptionPane;2. class optionPane3. {4. public static void main(String[] arg)5. {6. String str = JOptionPane.showInputDialog( "Read data” );7. }8. }

Using JOptionPane for Input

• Type the value 123.45

• This value is returned as a String

String str = JOptionPane.showInputDialog( "Read data” );

That is, str = “123.45”

Using the Wrapper class - Double

• Convert the string value to a double by using the parseDouble method

1. import javax.swing.JOptionPane;

2. class convert_string

3. {

4. public static void main(String[] arg)

5. {

6. String str = JOptionPane.showInputDialog("Read data");

7. double x = Double.parseDouble(str);

8. }

9. }

Build a Single Class to be used for Input

1. import javax.swing.JOptionPane;2. class GetData3. {4. static String str;5. static double getDouble(String s)6. {7. str = JOptionPane.showInputDialog(s);8. return Double.parseDouble(str);9. }10. static int getInt(String s)11. {12. str = JOptionPane.showInputDialog(s);13. return Integer.parseInt(str);14. }15. static String getWord(String s)16. {17. return JOptionPane.showInputDialog(s);18. }19. }

Build a Single Class for Input (Another view)

1. import javax.swing.JOptionPane;2. class GetData3. {4. static double getDouble(String s)5. {6. return Double.parseDouble( getWord(s) );7. }8. static int getInt(String s)9. {10. return Integer.parseInt(getWord(s));11. }12. static String getWord(String s)13. {14. return JOptionPane.showInputDialog(s);15. }16. }

Using the GetData Class

1. class TestGetData2. {3. public static void main(String[] arg)4. {

5. int x = getData.getInt("Type an integer value");

6. double y = getData.getDouble("Type a double value");

7. String name = getData.getWord("Enter a Name");

8. System.out.println("Your name is: " + name + "\nYour age is: " 9. + x + "\nYou have $" + y);10. }11. }

Three Dialog Boxes

• Dialog box when Line 5 is executed.

• Dialog box when Line 6 is executed.

• Dialog box when Line 7 is executed.

The output

Output Operation

• We have seen output operations using System.out.println()

• Now we will look at output operations using the JOPtionPane class.

• We use the showMessageDialog method to out result.

• The general for of the showMessageDialog is shown in Figure below

Output Operation

1. import javax.swing.JOptionPane;2. class output_pane3. {4. public static void main(String[] arg)5. {6. JOptionPane.showMessageDialog(null, "Your \noutput string", 7. "Your title", JOptionPane.INFORMATION_MESSAGE);8. }9. }

Notice the keywords, constants and features: 1. null signifies that this dialog box stands alone.2. Your \noutput string - The string to be displayed in the message dialog box.3. Your title – The title you want to give to the output.4. JOptionPane.INFORMATION_MESSAGE – This specifies the type of dialog

box. Notice the letter cases. They must be written as shown here.

Output Operation – Only String can be Accommodated

1. import javax.swing.JOptionPane;2. class OutputPane3. {4. public static void main(String[] arg)5. {6. int x = getData.getInt("Type an integer value");7. double y = getData.getDouble("Type a double value");8. String name = getData.getWord("Enter a Name");9. 10. String s = "Your name is: " + name + "\nYour age is: " + x + 11. "\nYou have $" + y;12. JOptionPane.showMessageDialog(null, s, "Personal Data", 13. JOptionPane.INFORMATION_MESSAGE);14. }15. }

Output Operation – Only String can be Accommodated

Output Operation

• In this situation the value to be outputed must be a string.

• This means that you must first construct the string before calling the method.

• This requires you to:

Know the format of the output ahead of time, and

Be able to use the string concatenation features along with tabs, new line,

and space to make the entire string.

Using a Scrollable Pane

• The above method has a major disadvantage

• A very long string will create a long the dialog box.

• Some data maybe lost.

• To avid this from happening, do the following:

1. Place the string (text) in a JTextArea object

2. Place the JTextArea in a JScrollPane object

3. Finally, place the JScrollPane object in the showMessageDialog

method of the JOptionPane class.

1. import javax.swing.JOptionPane;

2. import javax.swing.JTextArea;

3. import javax.swing.JScrollPane;

4. class OutputPane

5. {

6. public static void main(String[] arg)

7. {

8. int x = getData.getInt("Type an integer value");

9. double y = getData.getDouble("Type a double value");

10. String name = getData.getWord("Enter a Name");

11. String s = "Your name is: " + name + "\nYour age is: " + x +

12. "\nYou have $" + y;

13. s = s + "\nThat other person does not remember his name\nnor his age";

14. s = s + "\nlet alone how much money he has";

15. JTextArea text = new JTextArea(s, 10, 20);

16. JScrollPane pane = new JScrollPane(text);

17. JOptionPane.showMessageDialog(null, pane, "Personal Data",

18. JOptionPane.INFORMATION_MESSAGE);

19. }

20. }

Using a Scrollable Pane