Transcript Data Types – Reference Types

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
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Reference types - any data type that is composed of primitive data types as its
based type.
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In other words, it is an aggregate of primitive types.
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Reference types in Java are:
 The array data structure, and
 The class data type.
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In this section we will briefly discuss the concept of array.
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We will discuss the class type to the extent of the fundamental classes of Java.
Reference Types - Array
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Array
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An array is a set of storage locations set aside to hold one type of data.
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It is simply a means where by we can store values of the same type by using
one generic name.
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The list of items are stored linearly
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Hence the items can be accessed by their relative position in the list.
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Arrays are real objects in Java.
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Storage space is allocated for an array during execution time.
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The concept will be discussed fully in future lesson
Reference Types - Array
Data
arr
10
0
20
1
30
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50
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60
5
indeces
This is the most we will say about arrays for now
70
6
80
7
Reference Types - Class
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The concept of class as you know is the fundamental construct upon which
Java is built.
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A class serves as a blueprint or a template for a certain type of object.
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All data types other than the primitive type or the array must be addressed in
terms of class.
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This is evident in the way that we have been using class.
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For instance, going back to the class Book.
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In order to use this class we had to declare variables of the type:
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That is, Book b1; for instance
Reference Types - Standard Packages
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Java has a vast and rich collection of classes.
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These classes are stored in respective directories and subdirectories.
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Each directory or subdirectory is called, a package.
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Programmers can use these classes in their specific applications.
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There are several packages in Java.
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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 subpackages 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
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The java.lang package contains classes that are fundamental to Java.
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All of the classes in this package are available in your program automatically.
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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
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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
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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
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class Test
{
public static void main(String[] arg)
{
String s1 = new String("Hello");
String s2 = new String("Hello");
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System.out.println(s1 == s2);
}
Answer: __________________
Using == with Reference Types
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class Test
{
public static void main(String[] arg)
{
String s1 = new String("Hello");
String s2 = new String("Hello");
System.out.println(s1.equals(s2));
}
}
Input and Output Operations
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There are three other important operations that are performed on data.
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Input operations
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Output operations, and
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Formatting operations.
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We have been generating outputs already but it was not dealt with formally.
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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
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Create input dialog boxes
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Create output windows
JOptionPane Input Dialog Boxes
There are four kinds of standard dialogs:
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Message dialog shows a message and waits for the user to click OK.
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Confirmation dialog shows a question and ask for confirmation such as OK
or Cancel.
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Option dialog shows a question and gets the user’s response from a set of
options.
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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
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You must import the class JOptionPane
import javax.swing.JOptionPane;
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The class contains:
 Several constructors, and
 Several class methods that deals with dialog boxes.
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The general construct of these methods is as follows:
showXXXXDialog( parameter_list)
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Where XXXX is any of the four dialog box types. We will only be concerned
with the input dialog type.
Using JOptionPane for Input
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import javax.swing.JOptionPane;
class optionPane
{
public static void main(String[] arg)
{
String str = JOptionPane.showInputDialog( "Read data” );
}
}
Using JOptionPane for Input
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Type the value 123.45
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This value is returned as a String
String str = JOptionPane.showInputDialog( "Read data” );
That is, str = “123.45”
Using the Wrapper class - Double
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Convert the string value to a double by using the parseDouble method
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import javax.swing.JOptionPane;
class convert_string
{
public static void main(String[] arg)
{
String str = JOptionPane.showInputDialog("Read data");
double x = Double.parseDouble(str);
}
}
Build a Single Class to be used for Input
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import javax.swing.JOptionPane;
class GetData
{
static String str;
static double getDouble(String s)
{
str = JOptionPane.showInputDialog(s);
return Double.parseDouble(str);
}
static int getInt(String s)
{
str = JOptionPane.showInputDialog(s);
return Integer.parseInt(str);
}
static String getWord(String s)
{
return JOptionPane.showInputDialog(s);
}
}
Build a Single Class for Input (Another view)
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import javax.swing.JOptionPane;
class GetData
{
static double getDouble(String s)
{
return Double.parseDouble( getWord(s) );
}
static int getInt(String s)
{
return Integer.parseInt(getWord(s));
}
static String getWord(String s)
{
return JOptionPane.showInputDialog(s);
}
}
Using the GetData Class
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class TestGetData
{
public static void main(String[] arg)
{
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int x = getData.getInt("Type an integer value");
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double y = getData.getDouble("Type a double value");
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String name = getData.getWord("Enter a Name");
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System.out.println("Your name is: " + name + "\nYour age is: "
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+ x + "\nYou have $" + y);
}
}
Three Dialog Boxes
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Dialog box when Line 5 is executed.
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Dialog box when Line 6 is executed.
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Dialog box when Line 7 is executed.
The output
Output Operation
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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
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import javax.swing.JOptionPane;
class output_pane
{
public static void main(String[] arg)
{
JOptionPane.showMessageDialog(null, "Your \noutput string",
"Your title", JOptionPane.INFORMATION_MESSAGE);
}
}
Notice the keywords, constants and features:
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null signifies that this dialog box stands alone.
Your \noutput string - The string to be displayed in the message dialog box.
Your title – The title you want to give to the output.
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
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import javax.swing.JOptionPane;
class OutputPane
{
public static void main(String[] arg)
{
int x = getData.getInt("Type an integer value");
double y = getData.getDouble("Type a double value");
String name = getData.getWord("Enter a Name");
String s = "Your name is: " + name + "\nYour age is: " + x +
"\nYou have $" + y;
JOptionPane.showMessageDialog(null, s, "Personal Data",
JOptionPane.INFORMATION_MESSAGE);
}
}
Output Operation – Only String can be Accommodated
Output Operation
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In this situation the value to be outputted must be a string.
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This means that you must first construct the string before calling the method.
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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.
Format Decimal values
In order to format decimal values do the following:
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Import the following classes:
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java.text.NumberFormat;
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java.text.DecimalFormat;
Setup the format pattern:
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NumberFormat nf = NumberFormat.getInstance();
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DecimalFormat df = (DecimalFormat)nf;
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df.applyPattern("#,##0.0000; -#,##0.0000");
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import java.text.NumberFormat;
import java.text.DecimalFormat;
import javax.swing.JOptionPane;
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class Format
{
public static void main(String arg[])
{
double
pi = Math.PI;
String s = "Value of PI \nBefore formatting: " + pi;
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NumberFormat nf = NumberFormat.getInstance();
DecimalFormat df = (DecimalFormat)nf;
df.applyPattern("#,##0.0000; -#,##0.0000");
s = s + "\nAfter formatting: " + df.format(pi);
display(s);
}
static void display(String s)
{
JOptionPane.showMessageDialog(null, s, "Format numbers",
JOptionPane.INFORMATION_MESSAGE);
}
}
Output
Using a Scrollable Pane
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The above method has a major disadvantage
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A very long string will create a long the dialog box.
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Some data maybe lost.
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To avoid this from happening, do the following:
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Place the string (text) in a JTextArea object
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Place the JTextArea in a JScrollPane object
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Finally, place the JScrollPane object in the showMessageDialog
method of the JOptionPane class.
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import javax.swing.JOptionPane;
import javax.swing.JTextArea;
import javax.swing.JScrollPane;
class OutputPane
{
public static void main(String[] arg)
{
int x = getData.getInt("Type an integer value");
double y = getData.getDouble("Type a double value");
String name = getData.getWord("Enter a Name");
String s = "Your name is: " + name + "\nYour age is: " + x +
"\nYou have $" + y;
s = s + "\nThat other person does not remember his name\nnor his age";
s = s + "\nlet alone how much money he has";
JTextArea text = new JTextArea(s, 10, 20);
JScrollPane pane = new JScrollPane(text);
JOptionPane.showMessageDialog(null, pane, "Personal Data",
JOptionPane.INFORMATION_MESSAGE);
}
}
Using a Scrollable Pane