Keywords are identifiers like public,
static and class that have a special meaning
inside Java source code and outside of comments and Strings. Four
keywords are used in Hello World, public,
static, void and class.
Keywords are reserved for their intended use and cannot be used by the programmer for variable or method names.
There are fifty reserved keywords in Java 1.1, 51 in Java 1.2, 52 in Java 1.4, and 54 in Java 5. The forty-eight that are actually used in are listed below. Don't worry if the purposes of the keywords seem a little opaque at this point. They will all be explained in much greater detail later.
| Keyword | Purpose |
|---|---|
boolean |
declares a boolean variable or return type |
byte |
declares a byte variable or return type |
char |
declares a character variable or return type |
double |
declares a double variable or return type |
float |
declares a floating point variable or return type |
short |
declares a short integer variable or return type |
void |
declare that a method does not return a value |
int |
declares an integer variable or return type |
long |
declares a long integer variable or return type |
while |
begins a while loop |
for |
begins a for loop |
do |
begins a do while loop |
switch |
tests for the truth of various possible cases |
break |
prematurely exits a loop |
continue |
prematurely return to the beginning of a loop |
case |
one case in a switch statement |
default |
default action for a switch statement |
if |
execute statements if the condition is true |
else |
signals the code to be executed if an if statement is not true |
try |
attempt an operation that may throw an exception |
catch |
handle an exception |
finally |
declares a block of code guaranteed to be executed |
class |
signals the beginning of a class definition |
abstract |
declares that a class or method is abstract |
extends |
specifies the class which this class is a subclass of |
final |
declares that a class may not be subclassed or that a field or method may not be overridden |
implements |
declares that this class implements the given interface |
import |
permit access to a class or group of classes in a package |
instanceof |
tests whether an object is an instanceof a class |
interface |
signals the beginning of an interface definition |
native |
declares that a method is implemented in native code |
new |
allocates a new object |
package |
defines the package in which this source code file belongs |
private |
declares a method or member variable to be private |
protected |
declares a class, method or member variable to be protected |
public |
declares a class, method or member variable to be public |
return |
returns a value from a method |
static |
declares that a field or a method belongs to a class rather than an object |
super |
a reference to the parent of the current object |
synchronized |
Indicates that a section of code is not thread-safe |
this |
a reference to the current object |
throw |
throw an exception |
throws |
declares the exceptions thrown by a method |
transient |
This field should not be serialized |
volatile |
Warns the compiler that a variable changes asynchronously |
Two other keywords, const and goto,
are reserved by Java but are not actually implemented. This allows
compilers to produce better error messages if these common C++
keywords are improperly used in a Java program.
Java 1.2 adds the strictfp keyword to declare that
a method or class must be run with exact IEEE 754 semantics.
Java 1.4 adds the assert keyword to specify assertions.
Java 5 adds assert and enum.
true and false appear to be missing
from this list. In fact, they are not keywords but rather boolean
literals. You still can't use them as a variable name though.