Campbell Ritchie

How do you know there is any meat on them in the first place? They would appear to be all batter.
Did it say pollock or flying fish?
We sometimes have skate wings, but it hasn't been cold enough for there to be any ice to skate on.

Gerard Gauthier wrote:. . . what areas of Java programming do rookie Java programmers tend to overlook . . .?

We see far too many people not writing object‑oriented code.

. . . the class path. . . .

Most true rookies don't need a CLASSPATH at all. You only need it after you have started using external resources.

What distinguishes a library from a book?

Monica Shiralkar wrote:. . . any subclass in Java has the right to override and change the implementation. . . . .

Any subclass must always obey the principle that a subclass IS‑A superclass. An overriding method must always obey the specification given by the overridden method. Any changes must follow those constraints.abstract class Employee {    // ...   /**    * Calculates gross pay for the current working period. If the employee    * is salaried, simply divides the annual pay by 12 (months) or 52 (weeks).    * If the employee is paid a wage per hour uses the time worked to    * calculate pay with or without overtime. Doesn't calculate taxes.    * Records the pay internally to allow tax calculation later.    *@param hours hours worked Must be ≥0    *@param minutes minutes after the hours have been calculated Must be ≥0 and <60    *@throws IllegalArgumentException if either parameter is negative or    *too large, e.g. hours >168 per week.    */   abstract BigDecimal calculatePay(int hours, int minutes);   // etc }class SalariedEmployee extends Employee {   // ...   /**    * Calculates gross pay for the current working month. Simply divides the annual pay    * by 12 (months). Both parameters are ignored internally.    *@param hours hours worked    *@param minutes minutes after the hours have been calculated    *@throws IllegalArgumentException if either parameter is negative    */   BigDecimal calculatePay(int hours, int minutes);   {      if (hours < 0 || minutes < 0)      {          throw new IllegalArgumentException(...);      }      return salary.divide(new BigDecimal("12". MathContext...);   }   // etc }class WagedEmployee extends Employee {   // ...   /**    * Calculates gross pay for the current working period. Multiplies the time worked by wage to    * calculate pay. Calculates overtime by enhancing anything more than 40 hours    * by ⅓.    *@param hours hours worked Must be ≥0    *@param minutes minutes after the hours have been calculated Must be ≥0 and <60    *@throws IllegalArgumentException if either parameter is negative or    *too large, e.g. hours >168 per week.    */   BigDecimal calculatePay(int hours, int minutes)   {      if (someCondition)      {          throw new IllegalArgumentException(...);      }      return ...;   }   // etc }Both those methods calculate pay (without calculating taxes) and may or may not throw such an exception. It would be wrong to implement that abstract method to calculate taxes. It would be all right to pass 1000000 hours to one overriding version because it ignores that parameter anyway. It is necessary to explain what the method does (=documentation comments) in order to allow it to be overridden. I shall let you guess what the last change I made to the documentation comments was, and how that causes both implementations to break the LSP. Also tell me what I put in the documentation comments that restrains the code in the future.

That became much easier with Java9 when JShell was introduced. You can run one line of code and see it go wrong. I firmly believe that you can learn more from code going wrong than from code working beautifully first time.

Neither class is immutable.

Sam Lewis wrote:. . . it has to be less then 0

You mean the fraction must be less than 1, probably. As Piet says, you will have problems:-System.out.println(new Fraction(3, 4).add(new Fraction(5, 6)));

Junilu Lacar wrote:. . . . If the superclass accepts one kind of input, its subclasses should also accept the same input. . . . .

In OO that would translate as

Junilu Lacar wrote:. . . . If the overridden method accepts one kind of input, itsoverriding methods should also accept the same input. . . . .

. . . or more input. As far as I am concerned, if the overriding method would throw an exception for input which the overridden method would accept, that is incorrect overriding. It is permissible for the overriding method to accept more inputs than the overridden method. There is a description in Object‑Oriented Software Construction by Bertrand Meyer, but I think he doesn't say, ”Liskov.” Can't fid the page in my copy; sorry.
You get questions in cert exams about whether an overriding (or implementing) method will or won't compile:-interface Foo {   /**    * Does something.    *@param text the text it is going to use    */   void foo(String text); } class Bar implements Foo {   public void foo(String text)   {      System.out.println(Objects.requireNonNull(text));   } }That code might compile because the compiler doesn't notice the unchecked exception, but I still think that is incorrectly implemented because the specification doesn't say anything about rejecting nulls.

To go back to what we said earlier:-

1: Have you got any way of determining whether a fraction has a simpler form or not?

2: Once you have that worked out, please consider your loop, so it only counts fractions that can be simplified. Then collect ten fractions or pairs of fractions.

[additional:-]

3: Calculate the denominator first, then use that value as an argument for the Random method, so the numerator will be smaller. Work out whether you need to avoid numerator=0.

Have a cow for finding that strange behaviour. Is that a Java® bug?

That suggests that the compiler goes through the inheritance tree of your types when testing your generics. At least Number, String, and Integer are well‑known types. Do you have a class extending Number and also Comparable to Strings (‍)?

Because Jens Gustedt in Modern C (Manning) says that only unsigned has strictly defined ranges and strictly defined arithmetical rules.

What happens if you passjava -version javac -version which javaas terminal arguments?
I cannot promise that my suggestions will actually work.

Welcome to the Ranch

You should omit “bin” from JAVA_HOME. Don't set a CLASSPATH.

Eric Hehner (A Practical Theory of Programming) page 14 says strings are

unpackaged, indexed

...data structures.