Java Glossary : lazy evaluation

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lazy evaluation

In computing, laziness is a virtue. The more ways you can think of to have the computer avoid doing work, the faster your programs will run.

It can often be optimal in computing to procrastinate computation to the last possible microsecond. If you are lucky, the computer never needs to do the work.

One common example of procrastination is to use a singleton object instead of static fields. You don't create the object until you actually need it. Just before every use, you must execute code roughly like this:

if ( singleton == null )
   {
   singleton = new Something();
   // further initialisation code ...
   }

Two other procrastination examples:

postponing opening files until you are sure you need to read records.
Just marking a list as to-be-sorted, rather than sorting it right away. Don't bother sorting it until somebody actually makes a request that requires it to be in sorted order. When you do sort it, mark it as sorted, so you won't waste time resorting it.

The && operator in Java avoids evaluation the right hand operand if the left hand operand is false. This is sometimes called lazy evaluation, short circuit evaluation or McCarthy evaluation. (John McCarthy was the creator of Lisp and is sometimes known as the father of artificial intelligence.)

When a function computes a value and caches the result, to produce it quickly in future without recomputing in case someone asks for the same inputs again, that is also sometimes referred to as lazy evaluation.

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