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Java Keywords

Review of most, if not all, java keywords.

KeywordMeaning/Usage
assert Assert describes a true–false statement placed in a java-program to indicate that the developer thinks that the statement is always true at that place. If not, then it will throw an exception. Optionally enable by ClassLoader method.
boolean Defines a boolean variable for the values "true" or "false" only (NB: "null" as in class Boolean is not allowed).
break Used to end the execution in the current loop body.
byte Defines a byte variable representing a sequence of 8 bits. (NB: Only 1-byte-characters can be used, f.i. '€' would produce an error).
case 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.
catch 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.
char Defines a character variable capable of holding any character of the java source file's character set (NB: Physical storage may exceed one byte).
class 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.
const Although reserved as a keyword in Java, "const" is not used and has no function. For defining constants in java, see the final keyword.
continue 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.
default 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 or to specify that a method in an interface provides the default implementation of a method.
do The do keyword is used in a do-while loop, which executes a block of statements associated with the loop and then tests the boolean expression associated with the while. It continues until the expression evaluates to false.
double The double keyword is used to declare a variable that can hold a 64-bit double precision IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of the primitive type double.
else The else keyword is used in an if-else statement, which tests a boolean expression. If the expression is true, the block of statements associated with the if are evaluated; if it is false, the block of statements associated with the else are evaluated.
enum A Java keyword used to declare an enumerated type. Enumerations extend the base class Enum.
extends Used in a class declaration to specify the superclass or superinterfaces. Also used to specify an upper bound on a type parameter in Generics.
final Define an entity once that cannot be changed nor derived from later. All methods in a final class are implicitly final.
finally 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.
float The float keyword is used to declare a variable that can hold a 32-bit single precision IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of the primitive type float.
for The for keyword is used to create a for loop or a for-each loop.
goto Although reserved as a keyword in Java, goto is not used and has no function.
if 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.
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/interface as its second operand and produces a boolean result. It evaluates to true iff 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. This keyword is also used to declare that a method returns a value of the primitive type int.
interface 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.
long The long keyword is used to declare a variable that can hold a 64-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of the primitive type long.
native Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language.
new Used to create an instance of a class or array object.
package A group of types. Packages are declared with the package keyword.
private 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.
protected 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.
public 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.
return 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.
short The short keyword is used to declare a field that can hold a 16-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of the primitive type short.
static Used to declare a field, method, or inner class as a class field. A class field or method stays bound to the class, not a specific instance of that class.
strictfp A Java keyword used to restrict the precision and rounding of floating point calculations to ensure portability.
super 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 or to forward a call from a constructor to a constructor in the superclass.
switch The switch keyword is used in switch statements, which evaluates a variable, matches its value to a specific case, and executes the block of statements associated with that case. Otherwise, the statements under default will execute.
synchronized Used in the declaration of a method or code block to guarantee that at most one thread at a time operating on the same object executes that code. Fields, classes and interfaces cannot be declared as synchronized.
this 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 or to forward a call from one constructor in a class to another constructor in the same class.
throw Causes the declared exception instance to be thrown.
throws Used in method declarations to specify which exceptions (which are not RuntimeExceptions) are not handled within the method but rather passed to the next higher level of the program.
transient Declares that an instance field is not part of the default serialized form of an object.
try Defines a block of statements that have exception handling. If an exception is caught in the try block, then the block associated with catch will be executed. Afterwards, the finally block will be executed.
void The void keyword is used to declare that a method does not return any value.
volatile 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.
while 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 and continues until the expression evaluates to false.
false A boolean literal value.
null A reference literal value.
true A boolean literal value.
Created by: GuiGuy