Develop Reference

Throw Expected After This Token Error

When I run my code, it gives me an error that says, "Syntax error on token "{", throw expected after this token." The error is on line 7's code.
class WhileLoopTest {
public static void main(String[] args){
apple = 0;
while (apple = 0) {
(int)(Math.random( )*(60) + 5);
return;
}
}
}

on the line while (apple = 0) you are setting the variable instead of declaring it. The while loop expects that you pass it a boolean. You are probably trying to use the comparison equals ==. The full line should read while (apple == 0).

First , you need to define a type for your variable apple because Java is statically type
apple = 0;
Read more About Statically typed vs Dynamically typed
change to
int apple = 0;
Second, (int)(Math.random( )*(60) + 5); is not statement so you need to either print the value or return it
Third, while (apple = 0) { is wrong because compiler looking for Boolean expression
while(Boolean_expression)
{
//Statements
}
change to while (apple == 0 ) {

You need to add an extra equals sign to the condition within the while statement (at the moment you are assigning the value of 0 to apple, instead of texting if it is equal), so it looks like this
while(apple == 0){
Pleas note that the while loop has no function at all, since you are returning within the loop. This will stop your program execution as you are returning from the main method. The computation of a random number doesn't serve a purpose here as you aren't assigning a variable to it or printing it.
Also, you are not defining a type for the apple variable. Try making it of type int.
int apple = 0;
I suggest that you look up some tutorials on java as you seem to misunderstand several concepts within the language.

Converting a string to an integer with a ternary operator

I have a string that I would like to convert into an integer before storing it as a property of an object. Although I can use regular if statements, I wanted to use a ternary operation to build my understanding of it. Here is the code I've tried
field_num = (((boolean bool_is_int = is_integer(string)) == true) ? (Integer int = Integer.parseInt(string)) : null);
What I'm trying to do (very basically) is set "field_num" (which is of type int) to the value of "string" if it is equal to an integer (by first converting it). is_integer is a function I have to check if a string is equal to an integer. It returns a boolean value.
Thanks for any help.

I would do something like this:
Integer theint = is_integer(thestr) ? Integer.parseInt(thstr) : null;
You cannot assign NULL to an intrinsic int but you can to an Integer object. Typically, of course, you'd simply rely on the parseInt() call throwing an exception rather than explicitly testing for integerness of the string beforehand.

field_num = is_integer(string) ? Integer.parseInt(string): -1;
In plain english this says if 'string' is an integer then parse string for the int and set it to field_num otherwise, set it to -1. -1 is arbitrary. you should instead use a number that is invalid for field_num.
You do not need is_integer(string) == true because that evaluates to the same thing as is_integer(string). You also don't need to set the boolean bool_is_int because unless you actually want that value later in the program.

You should just use an if/else statement. The Ternary operator is useful when you you want to set a variable to one of two values based on a condition. In your example, you don't want to set the value if the string is not an integer so ternary doesn't fit the situation well.

Keep it simple :)
int field_num = isInt(string) ? Integer.parseInt(string) : Integer.MAX_VALUE;
if (field_num == Integer.MAX_VALUE) {
// error; string is not a valid representation of int
}
To determine Whether a String represents an int value :
public static boolean isInt(String s) {
try {
Integer.parseInt(s);
} catch(NumberFormatException e) {
return false;
}
return true;
}

[Corrected]
Learn one thing at a time.
First, the ?: operator (more often referred to as the conditional operator, or the if/else operator; "ternary" just means it takes three arguments, and it's the only C operator that does so, hence the confusion)...
field_num = is_integer(string) ? Integer.parseInt(string) : null;
Ahhh. So field_num is an Integer. Would have help if you'd said that.
Second: Assignment-in-passing. If you don't know that you need to do this, and you can't make it perfectly obvious what you're doing and why, DON'T. It's hard to read, and it's rarely appropriate.
Also, "int" is not a legal variable name.
But if you insist:
Integer myint;
boolean bool_is_int;
field_num = (bool_is_int = is_integer(string)) ? (myint = Integer.parseInt(string)) : null;
What's myint's value in the false/else case? It's left as whatever it had been set to previously. This might be what you intended, but it's very hard for someone reading your code to understand.
In most cases, unless the ?: is a very simple one that can be read at a glance -- (foo!=null) ? foo.doSomething() : defaultValue -- you're better off using a real if/then/else statement. It's likely to be just as efficient after the compiler and JIT are done with it, and it'll be a lot easier to maintain.

Check for empty JFormattedTextField

I'm new here, and I'd like some help on a small Java project I'm doing. This is the code snippet I need help with:
private void CalculateButtonActionPerformed(java.awt.event.ActionEvent evt)
{
// TODO add your handling code here:
float Principal, Rate, Time, Result, Temp;
Principal = Float.valueOf(PrincipalTextField.getText());
Rate = Float.valueOf(RateTextField.getText());
Time = Float.valueOf(TimeTextField.getText());
Temp = (float) Math.pow((1 + Rate / 100), Time);
Result = Principal * Temp;
ResultTextField.setText(String.valueOf(Result));
}
I'd like to check if PrincipalTextField, OR RateTextField, OR TimeTextField aren't filled by the user, and if so, display a dialog box that asks him/her to recheck them. The text fields are JFormattedTextField variables. I realise that I can do this with a if/else or a while loop, but I'm not sure how to set about doing so. Please help!

You can do something like this:
The getText() returns you a String value. So you can always invoke length() and check whether the length comes to 0 or not. (*I would suggest calling trim() on the String before calling length() to remove any whitespaces)
Next if any of the length comes to be zero, what you want to do is display a Dialog Box. This you can do by calling JOptionPane.showMessageDialog(). You can read more about "How to Make Dialogs" over here.
So, you would do something like this:
String principalText = PrincipalTextField.getText();
String rateText = RateTextField.getText();
String timeText = TimeTextField.getText();
if(principalText.trim().length == 0 || rateText.trim().length == 0 || timeText.trim().length == 0){
JOptionPane.showMessageDialog(null, "YOUR_ERROR_MSG", "ERROR_TITLE", JOptionPane.ERROR_MESSAGE);
}
This might be off-topic, but I would suggest looking at Java Naming Convention. The convention for variables is to compose variable names using mixed case letters starting with a lower case letter

you miss reason for why there is JFormattedTextField
have to set Number Formatter for JFormattedTextField, then
you not need to parsing Float value (better could be to use double)
empty coudl be 0 (zero) value by default
take value in the form ((Number)PrincipalTextField.getValue()).floatValue();
look at code example for tutorial,

Also consider subclassing InputVerifier, as discussed in Validating Input. There's a related example here.

Javabat substring counting

public boolean catDog(String str)
{
int count = 0;
for (int i = 0; i < str.length(); i++)
{
String sub = str.substring(i, i+1);
if (sub.equals("cat") && sub.equals("dog"))
count++;
}
return count == 0;
}
There's my code for catDog, have been working on it for a while and just cannot find out what's wrong. Help would be much appreciated!*/
EDIT- I want to Return true if the string "cat" and "dog" appear the same number of times in the given string.

One problem is that this will never be true:
if (sub.equals("cat") && sub.equals("dog"))
&& means and. || means or.
However, another problem is that your code looks like your are flailing around randomly trying to get it to work. Everyone does this to some extent in their first programming class, but it's a bad habit. Try to come up with a clear mental picture of how to solve the problem before you write any code, then write the code, then verify that the code actually does what you think it should do and that your initial solution was correct.
EDIT: What I said goes double now that you've clarified what your function is supposed to do. Your approach to solving the problem is not correct, so you need to rethink how to solve the problem, not futz with the implementation.

Here's a critique since I don't believe in giving code for homework. But you have at least tried which is better than most of the clowns posting homework here.
you need two variables, one for storing cat occurrences, one for dog, or a way of telling the difference.
your substring isn't getting enough characters.
a string can never be both cat and dog, you need to check them independently and update the right count.
your return statement should return true if catcount is equal to dogcount, although your version would work if you stored the differences between cats and dogs.
Other than those, I'd be using string searches rather than checking every position but that may be your next assignment. The method you've chosen is perfectly adequate for CS101-type homework.
It should be reasonably easy to get yours working if you address the points I gave above. One thing you may want to try is inserting debugging statements at important places in your code such as:
System.out.println(
"i = " + Integer.toString (i) +
", sub = ["+sub+"]" +
", count = " + Integer.toString(count));
immediately before the closing brace of the for loop. This is invaluable in figuring out what your code is doing wrong.
Here's my ROT13 version if you run into too much trouble and want something to compare it to, but please don't use it without getting yours working first. That doesn't help you in the long run. And, it's almost certain that your educators are tracking StackOverflow to detect plagiarism anyway, so it wouldn't even help you in the short term.
Not that I really care, the more dumb coders in the employment pool, the better it is for me :-)
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sbe (vag v = 0; v < fge.yratgu() - 2; v++) {
Fgevat fho = fge.fhofgevat(v, v+3);
vs (fho.rdhnyf("png")) {
qvssrerapr++;
} ryfr {
vs (fho.rdhnyf("qbt")) {
qvssrerapr--;
}
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erghea qvssrerapr == 0;
}

Another thing to note here is that substring in Java's built-in String class is exclusive on the upper bound.
That is, for String str = "abcdefg", str.substring( 0, 2 ) retrieves "ab" rather than "abc." To match 3 characters, you need to get the substring from i to i+3.

My code for do this:
public boolean catDog(String str) {
if ((new StringTokenizer(str, "cat")).countTokens() ==
(new StringTokenizer(str, "dog")).countTokens()) {
return true;
}
return false;
}
Hope this will help you
EDIT: Sorry this code will not work since you can have 2 tokens side by side in your string. Best if you use countMatches from StringUtils Apache commons library.

String sub = str.substring(i, i+1);
The above line is only getting a 2-character substring so instead of getting "cat" you'll get "ca" and it will never match. Fix this by changing 'i+1' to 'i+2'.
Edit: Now that you've clarified your question in the comments: You should have two counter variables, one to count the 'dog's and one to count the 'cat's. Then at the end return true if count_cats == count_dogs.

I can't understand this programming code for psedorandom number generator for hashing

First of all I just begun learning Java and i can say it more challenging then C or python. I'm not very keen on programming to so I have hard time understanding how some codes works. This one in particular
public class Pseudo
{
final int a = 2;
final int c = 3;
int address;
String list[][] = new String [100][6];
public void AddRecord(String ID, String Name, String Course, String Address, String Email, String Contact)
{
address = (a * Integer.parseInt(ID) + c) % list.length;
if((Integer.parseInt(ID)<100000||Integer.parseInt(ID)>999999)||ID.length()==0 || Name.length()==0 || Course.length()==0 || Address.length()==0)
{
showMessageDialog(null,"The ID number should be in six digit and the particular field should not be empty","",ERROR_MESSAGE);
}
else{
if(list[address][0]!=null){
showMessageDialog(null,"Collison is occur, the same address is get. Recalculating...............","",WARNING_MESSAGE);
while(list[address][0]!=null)
{
address = (a * address + c) % list.length;
}
}
list[address][0] = ID;
list[address][1] = Name;
list[address][2] = Course;
list[address][3] = Address;
list[address][4] = Email;
list[address][5] = Contact;
showMessageDialog(null,"Student Information " + ID + " will be saved in address: " + address,"",INFORMATION_MESSAGE);
}
}
The confusion come when
address = (a * Integer.parseInt(ID) + c) % list.length;
if((Integer.parseInt(ID)<100000||Integer.parseInt(ID)>999999)||ID.length()==0 || Name.length()==0 || Course.length()==0 || Address.length()==0)
What does it mean. From what I understand from this code is that inside an IF statement you can have more then 1 condition. I'm no very sure since this is my first time seeing such a code.
The second is this
if(list[address][0]!=null){
showMessageDialog(null,"Collison is occur, the same address is get. Recalculating...............","",WARNING_MESSAGE);
while(list[address][0]!=null)
{
address = (a * address + c) % list.length;
}
}
list[address][0] = ID;
list[address][1] = Name;
list[address][2] = Course;
list[address][3] = Address;
list[address][4] = Email;
list[address][5] = Contact;
showMessageDialog(null,"Student Information " + ID + " will be saved in address: " + address,"",INFORMATION_MESSAGE);
If collision occurs the address of which it is stored should be altered using a psedorandom number generator again but what I can't grasped is
list[address][0]!=null.I am just baffle with this line. I know its job is change the address if collision happens but i don't know the exact mechanics of how this part is executed.

From what I understand from this code is that inside an IF statement you can have more then 1 condition.
Well, yes and no. You can construct complex conditions based on many smaller conditions, but ultimately the whole thing has to resolve to a single boolean true/false result.
Consider the condition in this case:
(Integer.parseInt(ID)<100000||Integer.parseInt(ID)>999999)||ID.length()==0 || Name.length()==0 || Course.length()==0 || Address.length()==0
Let's break that down into its components:
(
Integer.parseInt(ID)<100000 ||
Integer.parseInt(ID)>999999
) ||
ID.length()==0 ||
Name.length()==0 ||
Course.length()==0 ||
Address.length()==0
It's really just chaining together a bunch of comparisons into one big true/false statement. You can essentially read something like this as:
If (something) or (something else) or (another thing) then...
And each something can itself contain small somethings, etc. You can build as complex a logical condition as you want, grouping sub-conditions with parentheses, as long as the whole thing resolves to a single true/false result.
what I can't grasped is list[address][0]!=null
That is just checking if a particular value is null. That value is part of a nested (jagged) array. So you have a variable called list. That variable is an array. Each element in that array is, itself, also an array. So you end up with a kind of 2-dimensional array (but a jagged one, where any given sub-array doesn't have to be the same length as any other).
That specific piece of code looks into the list array, at the address index, and looks at the 0 index of that sub-array, and checks if that value is null.

First of all, understanding any code is much easier if it's properly formatted. All good IDEs have such a function, e.g. for Eclipse the shortcut is Ctrl+Shift+F, for IntelliJ IDEA Ctrl+Alt+L.
The most important part, which might resolve your first confusion: || is the logical OR in Java, meaning the ID must be a number between 100000 and 999999 and the attributes must not be empty. Or literally, if the ID is smaller than 100000 or larger than 999999 or any of the values are empty, there will be an error message and nothing will be done.
For the second part: null means that a variable is not set, so to prevent overwriting an entry you can check if it's already set, i.e. not equal to null. So the code changes the address variable until an address is found for which no data is set yet and then uses it to store the given data.
There are several potential problems in this code, among which:
several calls to the relatively slow Integer.parseInt(String) where it could be called once and stored into a variable
potential NumberFormatException if ID isn't a number (or is empty, or has some excess white spaces)
potential infinite loop if the array is full
But as it looks like some CS homework it shouldn't matter.

Thank You so much Mr David. I understand the first part where if u have a condition u can stack it on each other and from what i can understand it only works with the ||(OR) statement since using this will guarantee either a true or false ending.
while(list[address][0]!=null)
But I'm still a little confuse for part 2 of my problem. Since that line is to check the array is null meaning no value right.This is my understanding of the situation.That particular part of the code is suppose to resolve any collision if the user enters the same ID number right so shouldn't it be checking the value that's causing the collision. But the line seems to be doing is as long as a null value is detected the corresponding procedure would be implemented.