Vonique Leary

Okay, I think I just figured out my answer. If I alter the code like this:

public class Moo extends Zoo { public String coolMethod() { return "Hi baby";

and override the method, then the second coolMethod() inside Moo returns "Hi baby."

Got it. The little snippet in the SCJP book uses this.coolMethod() when it wasn't even necessary and that is what threw me off.

Thanks, all

Vonique

In my above example the this keyword doesn't seem necessary because if you take it out you get the same results. Why would the writers include it? And also, I understand that this.coolMethod() refers to the current object that is calling it so it returns "Wow baby" but can anyone explain in which situation would it return something different? I guess that is what I am struggling to understand.

Hi again to you Campbell!

class Zoo { public String coolMethod() { return "Wow baby"; } } class Moo extends Zoo { public void useMyCoolMethod() { // Does an instance of Moo inherit the coolMethod()? System.out.println("Moo says, " + this.coolMethod()); // The preceding line works because Moo can inherit the // public method // Can an instance of Moo invoke coolMethod() on an // instance of Zoo? Zoo z = new Zoo(); System.out.println("Zoo says, " + z.coolMethod()); // coolMethod() is public, so Moo can invoke it on a Zoo //reference } }


What is the point of the this keyword in this example from Bates and Sierra's book? I tried the example both with and without it and got the same result. So now I'm confused about when to use this inside a method. Can someone give me an example that actually makes a difference in the output?

Thanks, Vonique

Thanks for that explanation Jayesh. That really helps to clear up why the only place I've ever seen serialization examples is in the prep book for the test. Things make so much more sense to me now!

Vonique

Thanks everyone, for your replies. It's a lot clearer now but it makes me realize just how much I don't know about networking, etc. Well, from what I have read on this forum, serialization is not on the test, so I am going to limit my knowledge about it to what I have already learned.

Nice to see you too, Campbell!

Von

So, do most java programs that are accessed over a network use serialization or are there other ways to persist objects?

Thanks,
Vonique

I understand the concept of serialization and that it is used to transfer files over a network, but I'm a little confused about how often it is used. Aren't all or most java files transferred over a network? I mean if you are accessing a java program while surfing the internet or while playing an online game, aren't those files being transferred over a network? What other ways is a java program accessed if not over a network? I know that there are some applications that run on one's computer but I thought java was mainly an internet language.

I did it!!! I fixed everything and it now works!

Here is my fixed code:

import java.awt.*; import java.awt.event.*; import javax.swing.*; import java.text.DecimalFormat; public class Test5 extends JFrame { // JLabel and JTextField for price private JLabel startMPHJLabel; private JTextField startMPHJTextField; // JLabel and JTextField for down payment private JLabel stopMPHJLabel; private JTextField stopMPHJTextField; // JButton to initiate calculation private JButton calculateJButton; // JTextArea to display results private JTextArea chartJTextArea; // no-argument constructor public Test5() { createUserInterface(); } // create and position GUI components; register event handlers private void createUserInterface() { // get content pane and set layout to null Container contentPane = getContentPane(); contentPane.setLayout( null ); // set up startMPHJLabel startMPHJLabel = new JLabel(); startMPHJLabel.setBounds( 40, 24, 80, 21 ); startMPHJLabel.setText( "Starting MPH:" ); contentPane.add( startMPHJLabel ); // set up startMPHJTextField startMPHJTextField = new JTextField(); startMPHJTextField.setBounds( 184, 24, 56, 21 ); startMPHJTextField.setHorizontalAlignment( JTextField.RIGHT ); contentPane.add( startMPHJTextField ); // set up stopJLabel stopMPHJLabel = new JLabel(); stopMPHJLabel.setBounds( 40, 56, 96, 21 ); stopMPHJLabel.setText( "Ending MPH:" ); contentPane.add( stopMPHJLabel ); // set up stopJTextField stopMPHJTextField = new JTextField(); stopMPHJTextField.setBounds( 184, 56, 56, 21 ); stopMPHJTextField.setHorizontalAlignment(JTextField.RIGHT ); contentPane.add( stopMPHJTextField ); // set up calculateJButton and register its event handler calculateJButton = new JButton(); calculateJButton.setBounds( 92, 100, 94, 24 ); calculateJButton.setText( "Calculate" ); contentPane.add( calculateJButton ); calculateJButton.addActionListener( new ActionListener() // anonymous inner class { // event handler called when user clicks calculateJButton public void actionPerformed( ActionEvent event ) { calculateJButtonActionPerformed( event ); } } // end anonymous inner class ); // end call to addActionListener // set up chartJTextArea chartJTextArea = new JTextArea(); chartJTextArea.setEditable( false ); JScrollPane scrollJScrollPane = new JScrollPane( chartJTextArea ); scrollJScrollPane.setBounds( 28,130,232,290 ); contentPane.add( scrollJScrollPane ); // set properties of application's window setTitle( "Stopping Distance Calculator" ); // set title bar text setSize( 288, 502 ); // set window's size setVisible( true ); // display window } // end method createUserInterface // method called when user clicks calculateJButton private void calculateJButtonActionPerformed( ActionEvent event ) { int MPH = 0; int counter = 1; chartJTextArea.setText("MPH" + "\t" + "Feet to Stop"); double x = calcSpeed(Integer.parseInt(startMPHJTextField.getText()), Integer.parseInt(stopMPHJTextField.getText())); } // end method calculateJButtonActionPerformed public double calcSpeed(int startSpeed, int stopSpeed) { double feet = 0; int MPH = startSpeed; int miles = stopSpeed; //for loop to calculate int number = MPH; for (int counter = number; counter <= miles; counter++ ) { feet = (MPH * 2.25) + (MPH * MPH / 21); chartJTextArea.append("\n" + counter +"\t" + feet); MPH = MPH + 1; } return feet; } // main method public static void main( String[] args ) { Test5 application = new Test5(); application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE ); } // end method main }


I had various little mistakes like int where there should have been a double, and x where miles should have been! Whew! Just posting it here made me look at it with a keener eye.

Thanks everyone!
Vonique

[size=12]Thank you, Paul. Well, I fixed the braces and managed to get rid of that error message, but now I only get one stopping distance value and I need all the stopping distances for several starts and stops. This is where I am really stumped. Here is my fixed code:

import java.awt.*; import java.awt.event.*; import javax.swing.*; import java.text.DecimalFormat; public class Test4 extends JFrame { // JLabel and JTextField for price private JLabel startMPHJLabel; private JTextField startMPHJTextField; // JLabel and JTextField for down payment private JLabel stopMPHJLabel; private JTextField stopMPHJTextField; // JButton to initiate calculation private JButton calculateJButton; // JTextArea to display results private JTextArea chartJTextArea; // no-argument constructor public Test4() { createUserInterface(); } // create and position GUI components; register event handlers private void createUserInterface() { // get content pane and set layout to null Container contentPane = getContentPane(); contentPane.setLayout( null ); // set up startMPHJLabel startMPHJLabel = new JLabel(); startMPHJLabel.setBounds( 40, 24, 80, 21 ); startMPHJLabel.setText( "Starting MPH:" ); contentPane.add( startMPHJLabel ); // set up startMPHJTextField startMPHJTextField = new JTextField(); startMPHJTextField.setBounds( 184, 24, 56, 21 ); startMPHJTextField.setHorizontalAlignment( JTextField.RIGHT ); contentPane.add( startMPHJTextField ); // set up stopJLabel stopMPHJLabel = new JLabel(); stopMPHJLabel.setBounds( 40, 56, 96, 21 ); stopMPHJLabel.setText( "Ending MPH:" ); contentPane.add( stopMPHJLabel ); // set up stopJTextField stopMPHJTextField = new JTextField(); stopMPHJTextField.setBounds( 184, 56, 56, 21 ); stopMPHJTextField.setHorizontalAlignment(JTextField.RIGHT ); contentPane.add( stopMPHJTextField ); // set up calculateJButton and register its event handler calculateJButton = new JButton(); calculateJButton.setBounds( 92, 100, 94, 24 ); calculateJButton.setText( "Calculate" ); contentPane.add( calculateJButton ); calculateJButton.addActionListener( new ActionListener() // anonymous inner class { // event handler called when user clicks calculateJButton public void actionPerformed( ActionEvent event ) { calculateJButtonActionPerformed( event ); } } // end anonymous inner class ); // end call to addActionListener // set up chartJTextArea chartJTextArea = new JTextArea(); chartJTextArea.setEditable( false ); JScrollPane scrollJScrollPane = new JScrollPane( chartJTextArea ); scrollJScrollPane.setBounds( 28,130,232,290 ); contentPane.add( scrollJScrollPane ); // set properties of application's window setTitle( "Stopping Distance Calculator" ); // set title bar text setSize( 288, 502 ); // set window's size setVisible( true ); // display window } // end method createUserInterface // method called when user clicks calculateJButton private void calculateJButtonActionPerformed( ActionEvent event ) { int MPH = 0; int counter = 1; chartJTextArea.setText("MPH" + "\t" + "Feet to Stop"); double x = calcSpeed(Integer.parseInt(startMPHJTextField.getText()), Integer.parseInt(stopMPHJTextField.getText())); chartJTextArea.append("\n" + counter +"\t" + x); MPH = MPH + 1; // return feet; } // end method calculateJButtonActionPerformed public double calcSpeed(int startSpeed, int stopSpeed) { double feet = 0; int MPH = startSpeed; int miles = stopSpeed; //for loop to calculate int number = MPH; for (int counter = number; counter <= miles; counter++ ) { feet = (MPH * 2.25) + (MPH * MPH / 21); //return feet; } return feet; [size=12] [/size]} // main method public static void main( String[] args ) { Test4 application = new Test4(); application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE ); } // end method main }


I keep thinking I can loop those returns in a for loop but I'm just not getting it right. I've been looking at it for over 20 hours in the last week so my eyes are going batty about now. I'm going to look over my for loop more closely.

Thanks,
Von

This code is partially from the book Simply Programming An Application Driven Tutorial Approach. Some of the code I have inserted myself to complete the assignment, which is:

The user enters a starting and stopping speed range and the program produces a chart of speeds and stopping distances.
The module should receive 1 data (argument=parameter), an integer representing the speed of travel. The module should convert that speed into the number of feet needed to stop using the formula given above. The module should return the number of feet.

I don't want any answers but just look at my code if you would be so kind and drop some hints as to what I am doing wrong. I keep getting an error message that says:



C:\Test4.java:147: class, interface, or enum expected
}
^

And:

--------------------Configuration: <Default>--------------------
java.lang.NoClassDefFoundError: Test4
Caused by: java.lang.ClassNotFoundException: Test4
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Could not find the main class: Test4. Program will exit.
Exception in thread "main"
Process completed.

import java.awt.*; import java.awt.event.*; import javax.swing.*; import java.text.DecimalFormat; public class Test4 extends JFrame { // JLabel and JTextField for price private JLabel startMPHJLabel; private JTextField startMPHJTextField; // JLabel and JTextField for down payment private JLabel stopMPHJLabel; private JTextField stopMPHJTextField; // JButton to initiate calculation private JButton calculateJButton; // JTextArea to display results private JTextArea chartJTextArea; // no-argument constructor public Test4() { createUserInterface(); } // create and position GUI components; register event handlers private void createUserInterface() { // get content pane and set layout to null Container contentPane = getContentPane(); contentPane.setLayout( null ); // set up startMPHJLabel startMPHJLabel = new JLabel(); startMPHJLabel.setBounds( 40, 24, 80, 21 ); startMPHJLabel.setText( "Starting MPH:" ); contentPane.add( startMPHJLabel ); // set up startMPHJTextField startMPHJTextField = new JTextField(); startMPHJTextField.setBounds( 184, 24, 56, 21 ); startMPHJTextField.setHorizontalAlignment( JTextField.RIGHT ); contentPane.add( startMPHJTextField ); // set up stopJLabel stopMPHJLabel = new JLabel(); stopMPHJLabel.setBounds( 40, 56, 96, 21 ); stopMPHJLabel.setText( "Ending MPH:" ); contentPane.add( stopMPHJLabel ); // set up stopJTextField stopMPHJTextField = new JTextField(); stopMPHJTextField.setBounds( 184, 56, 56, 21 ); stopMPHJTextField.setHorizontalAlignment(JTextField.RIGHT ); contentPane.add( stopMPHJTextField ); // set up calculateJButton and register its event handler calculateJButton = new JButton(); calculateJButton.setBounds( 92, 100, 94, 24 ); calculateJButton.setText( "Calculate" ); contentPane.add( calculateJButton ); calculateJButton.addActionListener( new ActionListener() // anonymous inner class { // event handler called when user clicks calculateJButton public void actionPerformed( ActionEvent event ) { calculateJButtonActionPerformed( event ); } } // end anonymous inner class ); // end call to addActionListener // set up chartJTextArea chartJTextArea = new JTextArea(); chartJTextArea.setEditable( false ); JScrollPane scrollJScrollPane = new JScrollPane( chartJTextArea ); scrollJScrollPane.setBounds( 28,130,232,290 ); contentPane.add( scrollJScrollPane ); // set properties of application's window setTitle( "Stopping Distance Calculator" ); // set title bar text setSize( 288, 502 ); // set window's size setVisible( true ); // display window } // end method createUserInterface // method called when user clicks calculateJButton private void calculateJButtonActionPerformed( ActionEvent event ) { chartJTextArea.setText("MPH" + "\t" + "Feet to Stop"); //for loop to calculate int number = MPH; for (int counter = number; counter <= miles; counter++ ) { double feet = (MPH * 2.25) + (MPH * MPH / 21); double x = calcSpeed(Integer.parseInt(startMPHJTextField.getText()), //method to keep Integer.parseInt(stopMPHJTextField.getText())); return feet; } chartJTextArea.append("\n" + counter +"\t" + x); MPH = MPH + 1; return feet; } // end method calculateJButtonActionPerformed public double calcSpeed(int startSpeed, int stopSpeed) { MPH = startSpeed; miles = stopSpeed; } // main method public static void main( String[] args ) { Test4 application = new Test4(); application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE ); } // end method main } }

I am having trouble figuring out how to return more than one value from this method. From the picture I attached it appears I would have to return several values to have calculated all of those stopping distances between the first and the last one. The only thing I could think of was to put the method in a for loop and call it as many times as needed to keep delivering the return values but I can't seem to get it right.

Any help would be appreciated.

Thanks,
Von

Well, that is simple enough. Thanks so much for the explanation!

Vonique

"Let's say that we want to create a regex pattern to search for hexadecimal literals. As
a first step, let's solve the problem for one-digit hexadecimal numbers:
0[xX][0-9a-fA-F]"


source: "12 0x 0x12 0Xf 0xg" index: 012345678901234567

This is from Kathy Sierra and Bates SCJP book page 492 regarding regex. I don't quite understand why the index goes from 0 to 9, then from 0 to 7 rather than from 0 to 18. And the explanation says that the regex returns position 6 and 11, but I don't see any 11. What am I missing?

I've been studying from the SCJP book and notice that even though Date methods are deprecated, they still use it in almost all of their Calendar examples. Is this for backward compatibility? Can one manipulate dates and times using just the Calendar object? Can it do everything the Date class can do?

Thanks,
Von

Hi Campbell,

Well, it was just a case of not studying it adequately because there were a few labels and buttons missing. Those simple things were revealed upon closer inspection.

Never mind. Figured it out already. Thanks anyway!