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). 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 |
|---|---|
abstract | declares that a class or method is abstract |
boolean | declares a boolean variable or return type |
break | prematurely exits a loop |
byte | declares a byte variable or return type |
case | one case in a switch statement |
catch | handle an exception |
char | declares a character variable or return type |
class | signals the beginning of a class definition |
continue | prematurely return to the beginning of a loop |
default | default action for a switch statement |
do | begins a do while loop |
double | declares a double variable or return type |
else | signals the code to be executed if an if statement is not true |
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 |
finally | declares a block of code guaranteed to be executed |
float | declares a floating point variable or return type |
for | begins a for loop |
if | execute statements if the condition is true |
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 |
int | declares an integer variable or return type |
interface | signals the beginning of an interface definition |
long | declares a long integer variable or return type |
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 |
short | declares a short integer variable or return type |
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 |
switch | tests for the truth of various possible cases |
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 |
try | attempt an operation that may throw an exception |
void | declare that a method does not return a value |
volatile | Warns the compiler that a variable changes asynchronously |
while | begins a while loop |
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.
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.