boolean : Java Glossary

boolean

Java has a logical type that can only have the values true or false called boolean. The boolean type deals with only a single bit at a time. An immutable object wrapper around a boolean is called a Boolean. You can do boolean operations on ints or longs, many bits at a time using &, |, ^ and ~. With boolean, you can only process one bit at a time, typically with !, && and ||. For bit strings longer than 64 bits, use java. util. BitSet. Even experienced programmers sometimes lapse into redundant newbie-like baby talk when using booleans. To avoid that embarrassement see newbie.

boolean expressions

Anything you can put inside an if (…) is a boolean expression e.g.

// example boolean expressions
x > 2
0 <= y && y <= 10
! (0 <= y && y <= 10 )
button.isVisible()

A boolean expression evaluates to true or false. You can store it in a boolean variable e.g.

// storing a boolean expression in a variable
boolean whether = x > 2;

You can print out a boolean variable. You don’t need an if to do it.

// displaying a boolean variable on the console
System.out.println( whether );

You can use a boolean variable in an if later rather than recalculating the expression.

// referencing a boolean variable in an if
if ( whether ) makeDate();
if ( ! whether && done() ) finish();

boolean vs int

In C, booleans and ints are pretty well interchangeable. In Java they are totally separate types. You can’t do the following in Java :

// ILLEGAL IN JAVA!!  int is not the same as boolean
int seen = button.isVisible();
if ( seen ) doSomething();

You must write:

// boolean must be used in Java where in C you could use an int.
boolean visible = button.isVisible();
if ( visible ) doSomething();

// or using an int with int-style syntax.
int seen = button.isVisible() ? 0 : 1;
if ( seen != 0 ) doSomething();

& vs && and | vs ||

&& and || are sometimes called short circuit & and | or McCarthy & and |.

Stuttering

Avoid goony expressions like:

// Goony stuttering code often written by newbies

if ( whether == true )

// or

if ( whether == false )

// or

if ( whether == true && done == false )

Don’t stutter, just write:

// terse code

if ( whether )

// or

if ( !whether )

// or

if ( whether && !done )

Gotchas

You can drive yourself crazy writing Java programs using booleans to represent pairs of states in a program i.e. busy vs active, ok vs error, Comparable vs Comparator, ascending vs descending where there is no blindingly obvious convention about which of the pair should be true and which false. The convention will not be obvious scaning the code. All you will see are trues and falses. It is very easy to accidentally reverse the convention on a method expecting the opposite convention. The compiler will not warn you. If you get a program half one convention and half the other you will have one heck of a time sorting it all out. What to do?
- Use enums instead. You can hide the convention inside the enum, and use boolean getters with names like isAscending and isDescending on enum constants with names like ASCENDING and DESCENDING.
- Using named static final boolean constants helps a little.
- Make up some crazy mnemonic about which is which that will stick in your brain.
- Use consistent variable and parameter names that make the convention obvious.

binary
bit
enum
masking
newbie

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