Java Keywords
13:22:00
Java key
words are reserved that are reserved by Java functions or other uses that cannot be used as identifiers
(e.g., variable names,function names, class names). If a reserved word was used as a variable you would get an error
or unexpected result. The list of reserved words in Java is provided below.
In front of a `class`
keyword, prevents this class from being directly instantiated. In front of a
method signature, allows the implementation of this method to be deferred to an
inheriting class.
Assert describes a
predicate (a true–false statement) placed in a java-program to indicate that
the developer thinks that the predicate is always true at that place. If an
assertion evaluates to false at run-time, an assertion failure results, which
typically causes execution to abort. Optionally enable by ClassLoader method.
Defines a boolean
variable for the values "true" or "false" only (NB:
"null" as in class Boolean is not allowed).
Used to end the
execution in the current loop body.
Defines a byte
variable representing a sequence of 8 bits. (NB: Only 1-byte-characters can be
used, f.i. '€' would produce an error).
A statement in the
switch block can be labeled with one or more case or default labels. The switch statement evaluates its expression, then executes all statements
that follow the matching case label;
Used in conjunction
with a
try block and an optional finally block. The statements in the catch block specify what to do if a specific type of exception is
thrown by the try block.
Defines a character
variable capable of holding any character of the java source file's character
set (NB: Physical storage may exceed one byte).
A type that defines
the implementation of a particular kind of object. A class definition defines instance and
class fields, methods, and inner classes as well as specifying the interfaces the
class implements and the immediate superclass of
the class. If the superclass is not explicitly specified, the superclass is
implicitly
Object. The class keyword can also be used in the form Class.class to get a Class object without needing
an instance of that class. For example,String.class can be used instead of doing new String().getClass().
Although reserved as
a keyword in Java,
const is not used and has no function.[2][1] For defining constants in java, see
the 'final' reserved word.
Used to resume
program execution at the end of the current loop body. If followed by a label,
continue resumes execution at the end of the enclosing labeled loop body.
The
default keyword can optionally be used in a switch statement to label a block of statements to be
executed if no case matches the specified value; seeswitch.[3][4] Alternatively, the default keyword can
also be used to declare default values in a Java annotation.
From Java 8 onwards, the default keyword is also used to specify that a method
in an interface provides the default implementation a method.
The
do keyword is used in conjunction with while to create a do-while loop,
which executes a block of statements associated with the loop and then tests a
boolean expression associated with the while. If the expression evaluates to true,
the block is executed again; this continues until the expression evaluates to false.[5][6]
The
double keyword is used to declare a variable that can hold a 64-bit double precision IEEE 754 floating-point
number.[7][8] This keyword is also used to declare
that a method returns a value of the primitive type double.[9][10]
The
else keyword is used in conjunction with if to create an if-else statement, which tests a boolean
expression; if the expression evaluates to true,
the block of statements associated with the if are evaluated; if it evaluates to false,
the block of statements associated with the else are evaluated.[11][12]
A Java keyword used
to declare an enumerated type.
Enumerations extend the base class
Used in a class
declaration to specify the superclass; used in an interface declaration to
specify one or more superinterfaces. Class X extends class Y to add
functionality, either by adding fields or methods to class Y, or by overriding
methods of class Y. An interface Z extends one or more interfaces by adding
methods. Class X is said to be a subclass of class Y; Interface Z is said to be
a subinterface of the interfaces it extends.
Also used to specify
an upper bound on a type parameter in Generics.
Define an entity once
that cannot be changed nor derived from later. More specifically: a final class
cannot be subclassed, a final method cannot be overridden, and a final variable
can occur at most once as a left-hand expression on an executed command. All
methods in a final class are implicitly
final.
Used to define a
block of statements for a block defined previously by the
try keyword. The finally block is executed after execution exits the try block and any associated catch clauses regardless of whether an exception was thrown or caught,
or execution left method in the middle of the try or catchblocks using the return keyword.
The
float keyword is used to declare a variable that can hold a 32-bit single precision IEEE 754 floating-point number.[7][8] This keyword is also used to declare
that a method returns a value of the primitive type float.[9][10]
The
for keyword is used to create a for loop, which
specifies a variable initialization, a boolean
expression, and an incrementation. The variable initialization is
performed first, and then the boolean expression is evaluated. If the
expression evaluates to true, the block of statements associated with the loop are executed,
and then the incrementation is performed.
The
if keyword is used to create an if statement,
which tests a boolean
expression; if the expression evaluates to true,
the block of statements associated with the if statement is executed. This
keyword can also be used to create an if-else statement; see else.[11][12]Implements
Included in a class declaration to specify one or more interfaces that are implemented by the current
class. A class inherits the types and abstract methods declared by the
interfaces.
import
Used at the beginning
of a source file to specify classes or entire Java packages to be referred to later without
including their package names in the reference. Since J2SE 5.0,
import statements can import static members of a class.instanceof
A binary
operator that takes an
object reference as its first operand and a class or interface as its second
operand and produces a boolean result. The
instanceof operator evaluates to true if and only if the runtime type of
the object is assignment compatible with the class or interface.int
The
int keyword is used to declare a variable that can hold a 32-bit
signed two's complement integer.[7][8] This keyword is also used to declare
that a method returns a value of the primitive type int.[9][10]
Used to declare a
special type of class that only contains abstract or default methods, constant (
static
final) fields and static interfaces. It can later be implemented by classes that declare
the interface with the implements keyword.
The
long keyword is used to declare a variable that can hold a 64-bit
signed two's complement integer.[7][8] This keyword is also used to declare
that a method returns a value of the primitive type long.[9][10]
Used in method
declarations to specify that the method is not implemented in the same Java
source file, but rather in another language.[10]
new
Used to create an
instance of a class or array object. Using keyword for this end is not
completely necessary (as exemplified by Scala), though it serves two purposes: it
enables the existence of different namespace for methods and class names, it
defines statically and locally that a fresh object is indeed created, and of
what runtime type it is (arguably introducing dependency into the code).
A group of types.
Packages are declared with the
package keyword.
The
private keyword is used in the declaration of a method, field, or inner
class; private members can only be accessed by other members of their own class.[15]
The
protected keyword is used in the declaration of a method, field, or inner
class; protected members can only be accessed by members of their own class,
that class's subclasses or
classes from the same package.[15]
The
public keyword is used in the declaration of a class, method, or field;
public classes, methods, and fields can be accessed by the members of any class.[15]
Used to finish the
execution of a method. It can be followed by a value required by the method
definition that is returned to the caller.
The
short keyword is used to declare a field that can hold a 16-bit signed
two's complement integer.[7][8] This keyword is also used to declare
that a method returns a value of the primitive type short.[9][10]
Used to declare a
field, method, or inner class as a class field. Classes maintain one copy of
class fields regardless of how many instances exist of that class.
static also is used to define a method as a class method. Class methods
are bound to the class instead of to a specific
instance, and can only operate on class fields. (Classes and interfaces
declared as static members of another class or interface are actually top-level
classes and are not inner classes.)
A Java keyword used
to restrict the precision and rounding of floating point calculations to ensure
portability.[10]
Used to access
members of a class inherited by the class in which it appears. Allows a
subclass to access overridden methods
and hidden members of its superclass. The
super keyword is also used to forward a call from a constructor to a
constructor in the superclass.
Also used to specify
a lower bound on a type parameter in Generics.
The
switch keyword is used in conjunction with case and default to create a switch statement,
which evaluates a variable, matches its value to a specificcase, and executes the block of statements associated with that case.
If no case matches the value, the optional block labelled by default is executed, if included.[3][4]
Used in the
declaration of a method or code block to acquire the mutex lock
for an object while the current thread executes
the code.[10] For static methods, the object locked
is the class's
Class. Guarantees that at most one thread at a time operating on the
same object executes that code. The mutex lock is automatically released when
execution exits the synchronized code. Fields, classes and interfaces cannot be
declared as synchronized.
Used to represent an
instance of the class in which it appears.
this can be used to access class members and as a reference to the
current instance. Thethis keyword is also used to forward a call from one constructor in a
class to another constructor in the same class.
Causes the declared
exception instance to be thrown. This causes execution to continue with the
first enclosing exception handler declared by the
catchkeyword
to handle an assignment compatible exception type. If no such exception handler
is found in the current method, then the method returns and the process is
repeated in the calling method. If no exception handler is found in any method
call on the stack, then the exception is passed to the thread's uncaught
exception handler.
Used in method
declarations to specify which exceptions are not handled within the method but
rather passed to the next higher level of the program. All uncaught exceptions
in a method that are not instances of
RuntimeException must be declared using the throws keyword.
Declares that an
instance field is not part of the default serialized form of an object. When an object is
serialized, only the values of its non-transient instance fields are included
in the default serial representation. When an object is deserialized, transient
fields are initialized only to their default value. If the default form is not
used, e.g. when a serialPersistentFields table is declared in the class
hierarchy, all
transient keywords are ignored.[16][17]
Defines a block of
statements that have exception handling. If an exception is thrown inside the
try block, an optional catch block can handle declared exception types. Also, an optional finally block can be declared that will be executed when execution exits
the try block and catch clauses, regardless of whether an exception is thrown or not. A try block must have at least one catch clause or a finally block.
The
void keyword is used to declare that a method does not return any
value.[9]
Used in field
declarations to specify that the variable is modified asynchronously by concurrently running threads.
Methods, classes and interfaces thus cannot be declared volatile, nor can local
variables or parameters.
The
while keyword is used to create a while loop, which
tests a boolean
expression and
executes the block of statements associated with the loop if the expression
evaluates to true; this continues until the expression evaluates to false.
This keyword can also be used to create a do-while loop
0 comments