Viv.java uses unchecked or unsafe operations

Are you getting Java compiler messages like this?

Dorothy-2:java grahamellis$ javac Viv.java

Note: Viv.java uses unchecked or unsafe operations.

Note: Recompile with -Xlint:unchecked for details.

Dorothy-2:java grahamellis$ javac -Xlint Viv.java

Which when you expand the warning with -Xlint gives:

Dorothy-2:java grahamellis$ javac -Xlint Viv.java

Viv.java:20: warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList

	trowbridge.add(new Rolf("First Great Western",75,3,60));

                      ^

If so, let me explain.

Java's collection classes - such as ArrayLists and Vectors - can hold any type of object, and the mechanisms of the base Java language allow yo to put in any object - including ones that may not be appropriate to a particular use. Alas - Java is NOT a language that trusts the programmer and the compiler wails that there may be a problem (if you're a good programmer, there probably isn't a problem!).

The problem is caused by a declaration such as this:

java.util.ArrayList trowbridge = 

  new java.util.ArrayList();

and can be overcome by telling java what sort of objects you expect to be in the ArrayList so that it can satisfy itself by making the checks on you. Here's how you tell it:

java.util.ArrayList<Peter> trowbridge =

  new java.util.ArrayList<Peter>();

(I'm my example, I have objects of class "Peter" ... Peter is actually an abstract class, and the objects are really "Rolf"s and "Steve"s.

From Java Bootcamp. See source code of this and other early examples on this course (as run this week) here

See also Java Generics
(written 2009-09-24)

Associated topics are indexed under

J714 - Java - Fundamental classes
  [3048] String handling - from first steps to practical examples - (2010-11-13)
  [2920] Sorting - naturally, or into a different order - (2010-08-14)
  [2734] for and foreach in Java - (2010-04-22)
  [2649] Length, size or capacity in Java? - (2010-02-24)
  [2421] Sorting Collections of Objects in Java - (2009-09-25)
  [2323] Java Collection Objects in the java.util package - (2009-08-05)
  [1910] Java - Generics - (2008-11-27)
  [1502] Java, sorting, ArrayList example, generics - (2008-01-11)
  [1062] Java sorting - comparable v comparator - (2007-02-02)
  [42] Do languages change? - (2004-09-08)

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