Download ppt - Introduction to Java Chapter 5 - Arrays, Strings, File Access, and Plotting1 Chapter 5 Arrays, File Access, and Plotting

Chapter 5 - Arrays, Strings, File Access, and Plotting 1

Introduction to Java

Chapter 5

Arrays, File Access, and Plotting



Arrays

• An array is a special object containing:– A group of contiguous

memory locations that all have the same name and same type

– A separate instance variable containing the number of elements in the array

a[0]

a[1]

a[2]

a[3]

a[4]

C omputerM emory A rray a

5 a.length



Declaring Arrays

• An array must be declared or created before it can be used. This is a two-step process:– First, declare a reference to an array.

– Then, create the array with the new operator.

• Example: double x[]; // Create reference

x = new double[5]; // Create array object

• These steps can be combined on a single line: double x[] = new double[5]; // All together



Using Arrays

• An array element may be used in any place where an ordinary variable of the same type may be used.

• An array element is addressed using the array name followed by a integer subscript in brackets: a[2]

• Arrays are typically used to perform the same calculation on many different values

• Example: for ( int i = 0; i < 100; i++ ) {

a[i] = Math.sqrt(a[i]); // sqrt of 100 values

}



Initializing Arrays

• When an array object is created with the new operator, its elements are automatically initialized to zero.

• Arrays can be initialized to non-zero values using array initializers, which are comma-separated list enclosed in braces

• Array initializers only work in declaration statements• Example:

int a[] = {1, 2, 3, 4, 5}; // Create & initialize array



Out-of-Bounds Subscripts

• Each element of an array is addressed using the name of the array plus the subscripts 0, 1, …, n-1, where n is the number of elements in the array.

• Subscripts < 0 or n are illegal, since they do not correspond to real memory locations in the array.

• These subscripts are said to be out-of-bounds.• Reference to an out-of-bounds subscript produces

an out-of-bounds exception, and the program will crash if the exception is not handled.



Note failure when access to an out-of-bounds subscript (5) is attempted.

Out-of-Bounds Subscripts (2)

• Example: // Declare and initialize array int a[] = {1,2,3,4,5}; // Write array (with an error!) for ( int i = 0; i <= 5; i++ ) System.out.println("a[" + i + "] = " + a[i]);

• Result: C:\book\java\chap5>java TestBounds a[0] = 1 a[1] = 2 a[2] = 3 a[3] = 4 a[4] = 5 java.lang.ArrayIndexOutOfBoundsException: 5 at TestBounds.main(TestBounds.java:15)



Reading and Writing Data to Files• Disk files are a convenient way to store large amounts

of data between uses.• The simplest way to read or write disk files is with

command-line redirection.– The file to read data from is listed after a < on the command

line. All input data comes from this file.

– The file to write data from is listed after a > on the command line. All output data goes to this file.

• Example:– D:\>java Example < infile > outfile



Reading and Writing Data to Files

• Command line redirection is relatively inflexible, since all input must be in a single file and all output must go to a single file.

• It is better to use the Java I/O system to open and close files as needed when a program is running.

• The Java I/O system is very complex, and discussion is postponed to Chapter 14.

• We introduce 2 convenience classes FileIn and FileOut for easy Java I/O.



Reading Files with Class FileIn

• Class chapman.io.FileIn is designed to read numeric data from an input file.

• To open a file for reading, import package chapman.io, and create a new FileIn object with the name of the file as a calling parameter.

FileIn in = new FileIn("inputFile");

• Then read input data using the readDouble(), readFloat(), readInt(), or readLong() methods.



Using Class FileInimport chapman.io.*;public class TestFileIn { public static void main(String arg[]) {

...

// Open file FileIn in = new FileIn("infile");

// Check for valid open if ( in.readStatus != in.FILE_NOT_FOUND ) {

// Read numbers into array while ( in.readStatus != in.EOF ) { a[i++] = in.readDouble(); } nvals = i;

// Close file in.close();

... (further processing) }

// Get here if file not found. Tell user else { System.out.println("File not found: infile"); } }}

Open file by creating object

Check for valid file open

Import package

Close file when done

Read data one value at a time

Report errors if they exist



Writing Files with Class FileOut

• Class chapman.io.FileOut is designed to write formatted data from an output file.

• To open a file for writing, import package chapman.io, and create a new FileOut object with the name of the file as a calling parameter.

FileOut out = new FileOut("outFile");

• Then write output data using the printf() method, and close file using the close() method.



import chapman.io.*;public class TestFileOut {

public static void main(String arg[]) {

// Test open without append FileOut out = new FileOut("outfile");

// Check for valid open if ( out.writeStatus != out.IO_EXCEPTION ) {

// Write values out.printf("double = %20.14f\n",Math.PI); out.printf("long = %20d\n",12345678901234L); out.printf("char = %20c\n",'A'); out.printf("String = %20s\n","This is a test."); }

// Close file out.close(); }}

Using Class FileOut

Open file by creating object

Check for valid file open

Import package

Close file when done

Write data one value at a time



Introduction to Plotting

• Support for graphics is built into the Java API.– Support is in the form of low-level graphics classes and

methods

– These methods are discussed in Chapter 11

• Meanwhile, we will use the convenience class chapman.graphics.JPlot2D to create 2D plots.



Introduction to Class JPlot2D

• Class JPlot2D supports many types of plots:– Linear plots

– Semilog x plots

– Semilog y plots

– Logarithmic plots

– Polar Plots

– Bar Plots

• Examples are shown on the following slides



Example JPlot2D Outputs (1)

Linear Plot Semilog x Plot




Log-log PlotSemilog y Plot




Bar PlotPolar Plot



Creating Plots

• To create plots, use the template shown on the following two pages

• Create arrays x and y containing the data to plot, and insert it into the template

• The components of this program will be explained in Chapters 11 and 12; meanwhile we can use it to plot any desired data

• See Table 5.5 for a list of plotting options



import java.awt.*;import java.awt.event.*;import javax.swing.*;import chapman.graphics.JPlot2D;public class TestJPlot2D {

public static void main(String s[]) {

// Create data to plot ...

// Create plot object and set plot information. JPlot2D pl = new JPlot2D( x, y ); pl.setPlotType ( JPlot2D.LINEAR ); pl.setLineColor( Color.blue ); pl.setLineWidth( 3.0f ); pl.setLineStyle( JPlot2D.LINESTYLE_SOLID ); pl.setMarkerState( JPlot2D.MARKER_ON ); pl.setMarkerColor( Color.red ); pl.setMarkerStyle( JPlot2D.MARKER_DIAMOND ); pl.setTitle( "Plot of y(x) = x^2 - 10x + 26" ); pl.setXLabel( "x" ); pl.setYLabel( "y" ); pl.setGridState( JPlot2D.GRID_ON );

Using Class JPlot2D (1)

Create data to plot here

Create JPlot2D obj

Import packages

Set the desired plotting styles

Set data to plot



// Create a frame and place the plot in the center of the // frame. Note that the plot will occupy all of the JFrame fr = new JFrame("JPlot2D ...");

// Create a Window Listener to handle "close" events WindowHandler l = new WindowHandler(); fr.addWindowListener(l);

fr.getContentPane().add(pl, BorderLayout.CENTER); fr.setSize(500,500); fr.setVisible( true ); }}

// Create a window listener to close the program.class WindowHandler extends WindowAdapter {

// This method implements a simple listener that detects // the "window closing event" and stops the program. public void windowClosing(WindowEvent e) { System.exit(0); };}

Using Class JPlot2D (2)

Create listener (see Chap 11)

Set size of plot

Create frame to hold plot

Window listener (see Chap 11)

Add plot to frame



Strings• A String is an object containing one or more

characters, treated as a unit.• Strings are types of objects. Once a string is

created, its contents never change. • The simplest form of string is a string literal,

which is a series of characters between quotation marks.

• Example:"This is a string literal."



Creating Strings• To create a String:

– First, declare a reference to a String.

– Then, create the String with a string literal or the new operator.

• Examples: String s1, s2; // Create references

s1 = "This is a test."; // Create array object s2 = new String(); // Create array object

• These steps can be combined on a single line: String s3[] = "String 3."; // All together



Substrings• A substring is a portion of a string.

• The String method substring creates a new String object containing a portion of another String.

• The forms of this method are:s.substring(int st); // From "st" s.substring(int st, int en); // "st" to "en"

• This method returns a String object containing the characters from st to en (or the end of the string).



Substrings (2)

• Examples:String s = "abcdefgh";

String s1 = s.substring(3);

String s2 = s.substring(3,6);

• Substring s1 contains "defgh", and substring s2 contains "def".

• Note that the indices start at 0, and that the substring contains the values from st to en-1.

"abcdefgh"

"defgh"

"def"

s.substring(3,6)s.substring(3)

s

s1

s2



Concatenating Strings• The String method concat creates a new String object containing the contents of two other strings.

• The form of this method is:s1.concat(String s2); // Combine s1 and s2

• This method returns a String object containing the contents of s1 followed by the contents of s2.



Concatenating Strings (2)

• Example:String s1 = "abc";String s2 = "def";

// Watch what happens here!System.out.println("\nBefore assignment:");System.out.println("s1 = " + s1);System.out.println("s2 = " + s2);s1 = s1.concat(s2);System.out.println("\nAfter assignment:");System.out.println("s1 = " + s1);System.out.println("s2 = " + s2);

Before assignment:s1 = abcs2 = def

After assignment:s1 = abcdefs2 = def

s1

s2

References Objects

s1

s2

Object"abc"

Object"def"

Object"abc"

Object"def"

Object"abcdef"

After

Before

New object created.



Selected Additional String Methods

Method Descriptionint compareTo(String s) Compares the string object to another string lexicographically.

Returns: 0 if string is equal to s <0 if string less than s >0 if string greater than s

boolean equals(Object o) Returns true if o is a String, and o contains exactly the samecharacters as the string.

boolean equalsIgnoreCase( String s)

Returns true if s contains exactly the same characters as thestring, disregarding case.

int IndexOf(String s) Returns the index of the first location of substring s in thestring.

int IndexOf(String s, int start)

Returns the index of the first location of substring s at or afterposition start in the string.

String toLowerCase() Converts the string to lower case.String toUpperCase() Converts the string to upper case.String trim() Removes white space from either end of the string.