Java How to Check Expression Again

A snippet of Java lawmaking with keywords highlighted in blue and bold font

In the Java programming language, a keyword is whatever one of 67 reserved words^[1] that have a predefined meaning in the linguistic communication. Because of this, programmers cannot utilize keywords in some contexts, such as names for variables, methods, classes, or as whatever other identifier.^[two] Of these 67 keywords, sixteen of them are only contextually reserved, and can sometimes be used equally an identifier, dissimilar standard reserved words. Due to their special functions in the language, virtually integrated evolution environments for Java utilize syntax highlighting to display keywords in a different colour for easy identification.

Listing [edit]

_

Added in Java 9, the underscore has get a keyword and cannot exist used as a variable name anymore.^[iii]

abstruse

A method with no definition must be alleged every bit abstruse and the class containing it must be declared as abstract. Abstract classes cannot be instantiated. Abstract methods must exist implemented in the sub classes. The abstract keyword cannot exist used with variables or constructors. Note that an abstract class isn't required to have an abstract method at all.

assert (added in J2SE 1.4)^[four]

Affirm describes a predicate (a true–false argument) placed in a Java program to indicate that the programmer thinks that the predicate is always true at that place. If an assertion evaluates to imitation at run-time, an assertion failure results, which typically causes execution to arrest. Assertions are disabled at runtime by default, but can be enabled through a command-line choice or programmatically through a method on the class loader.

boolean

Defines a boolean variable for the values "true" or "false" only. Past default, the value of boolean primitive type is false. This keyword is also used to declare that a method returns a value of the primitive type boolean.

break

Used to end the execution in the current loop torso.

byte

The byte keyword is used to declare a field that can agree an 8-bit signed 2'south complement integer.^{[5] [6]} This keyword is likewise used to declare that a method returns a value of the primitive type byte.^{[seven] [8]}

case

A statement in the switch cake tin be labeled with i or more than instance or default labels. The switch statement evaluates its expression, then executes all statements that follow the matching case label; run into switch.^{[9] [10]}

catch

Used in conjunction with a effort block and an optional finally block. The statements in the catch block specify what to exercise if a specific blazon of exception is thrown by the try block.

char

Defines a character variable capable of holding any character of the java source file'southward grapheme set.

class

A blazon that defines the implementation of a item kind of object. A class definition defines instance and grade fields, methods, and inner classes likewise 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 tin also be used in the form Class.class to get a Course object without needing an instance of that grade. For case, String.class can be used instead of doing new String().getClass().

const

Unused just reserved.

continue

Used to resume program execution at the terminate of the electric current loop body. If followed by a label, proceed resumes execution at the end of the enclosing labeled loop body.

default

The default keyword tin optionally exist used in a switch statement to label a block of statements to be executed if no case matches the specified value; see switch .^{[nine] [10]} Alternatively, the default keyword tin likewise be used to declare default values in a Java annotation. From Coffee 8 onwards, the default keyword tin can be used to permit an interface to provide an implementation of a method.

do

The practise 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 truthful, the block is executed once again; this continues until the expression evaluates to false.^{[11] [12]}

double

The double keyword is used to declare a variable that tin concur a 64-flake double precision IEEE 754 floating-signal number.^{[5] [6]} This keyword is likewise used to declare that a method returns a value of the primitive type double.^{[7] [viii]}

else

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 cake of statements associated with the else are evaluated.^{[13] [14]}

enum (added in J2SE v.0)^[4]

A Java keyword used to declare an enumerated blazon. Enumerations extend the base course Enum.

extends

Used in a course declaration to specify the superclass; used in an interface declaration to specify i 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 ane or more interfaces by calculation methods. Class X is said to be a subclass of class Y; Interface Z is said to exist a subinterface of the interfaces it extends.

Also used to specify an upper bound on a type parameter in Generics.

final

Ascertain an entity once that cannot be changed nor derived from after. More specifically: a concluding grade cannot be subclassed, a last method cannot be overridden, and a terminal variable tin occur at most once as a left-paw expression on an executed command. All methods in a final form are implicitly final.

finally

Used to define a block of statements for a cake defined previously by the try keyword. The finally block is executed after execution exits the try block and any associated take hold of clauses regardless of whether an exception was thrown or caught, or execution left method in the middle of the try or catch blocks using the render keyword.

float

The float keyword is used to declare a variable that tin can hold a 32-bit single precision IEEE 754 floating-bespeak number.^{[5] [6]} This keyword is as well used to declare that a method returns a value of the archaic type float.^{[vii] [8]}

for

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 cake of statements associated with the loop are executed, and and then the incrementation is performed. The boolean expression is and so evaluated again; this continues until the expression evaluates to false.^[15]

As of J2SE five.0, the for keyword can also exist used to create a so-chosen "enhanced for loop",^[16] which specifies an array or Iterable object; each iteration of the loop executes the associated block of statements using a different chemical element in the array or Iterable.^[15]

goto

Unused just reserved.

if

The if keyword is used to create an if argument, which tests a boolean expression; if the expression evaluates to true, the cake of statements associated with the if argument is executed. This keyword can besides be used to create an if-else statement; see else .^{[xiii] [14]}

implements

Included in a class declaration to specify ane or more interfaces that are implemented by the current grade. A grade inherits the types and abstruse methods declared by the interfaces.

import

Used at the offset of a source file to specify classes or entire Java packages to be referred to later without including their bundle 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 every bit its first operand and a form or interface as its second operand and produces a boolean effect. The instanceof operator evaluates to truthful if and only if the runtime blazon of the object is consignment uniform with the course or interface.

int

The int keyword is used to declare a variable that tin hold a 32-bit signed 2'south complement integer.^{[5] [6]} This keyword is also used to declare that a method returns a value of the archaic type int.^{[vii] [viii]}

interface

Used to declare a special type of grade that only contains abstract or default methods, constant (static final) fields and static interfaces. Information technology can later be implemented past classes that declare the interface with the implements keyword. As multiple inheritance is not immune in Java, interfaces are used to circumvent it. An interface can exist divers inside another interface.

long

The long keyword is used to declare a variable that tin can hold a 64-flake signed two'due south complement integer.^{[5] [6]} This keyword is also used to declare that a method returns a value of the primitive type long.^{[seven] [viii]}

native

Used in method declarations to specify that the method is non implemented in the aforementioned Java source file, merely rather in another language.^[8]

new

Used to create an case of a class or array object. Using keyword for this terminate is non completely necessary (as exemplified by Scala), though it serves two purposes: it enables the existence of different namespace for methods and class names, information technology defines statically and locally that a fresh object is indeed created, and of what runtime type it is (arguably introducing dependency into the lawmaking).

package

Java parcel is a group of similar classes and interfaces. Packages are alleged with the bundle keyword.

private

The private keyword is used in the announcement of a method, field, or inner class; private members tin simply exist accessed by other members of their own class.^[17]

protected

The protected keyword is used in the declaration of a method, field, or inner class; protected members tin can just exist accessed by members of their own course, that class's subclasses or classes from the same package.^[17]

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 grade.^[17]

return

Used to finish the execution of a method. Information technology can be followed past a value required past the method definition that is returned to the caller

curt

The short keyword is used to declare a field that tin can concur a sixteen-scrap signed two's complement integer.^{[5] [6]} This keyword is besides used to declare that a method returns a value of the archaic blazon short.^{[vii] [8]}

static

Used to declare a field, method, or inner class equally a class field. Classes maintain one copy of class fields regardless of how many instances exist of that course. static also is used to ascertain a method equally a class method. Class methods are jump to the class instead of to a specific instance, and can only operate on form fields. (Classes and interfaces declared as static members of another course or interface are really peak-level classes and are not inner classes.

super

Inheritance basically used to achieve dynamic binding or run-time polymorphism in java. Used to access members of a class inherited by the form in which information technology appears. Allows a subclass to admission overridden methods and subconscious members of its superclass. The super keyword is also used to frontwards a phone call from a constructor to a constructor in the superclass.

Also used to specify a lower bound on a type parameter in Generics.

switch

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 specific instance, and executes the block of statements associated with that case. If no case matches the value, the optional cake labelled past default is executed, if included.^{[9] [x]}

synchronized

Used in the proclamation of a method or code block to learn the mutex lock for an object while the electric current thread executes the lawmaking.^[8] For static methods, the object locked is the class's Course. Guarantees that at nigh ane 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.

this

Used to represent an instance of the course in which it appears. this can be used to access form members and as a reference to the electric current case. The this keyword is besides used to forrad a phone call from one constructor in a grade to some other constructor in the same course.

throw

Causes the declared exception instance to exist thrown. This causes execution to continue with the first enclosing exception handler declared by the catch keyword to handle an consignment uniform exception type. If no such exception handler is establish in the electric current method, then the method returns and the procedure is repeated in the calling method. If no exception handler is establish in whatever method call on the stack, then the exception is passed to the thread'southward uncaught exception handler.

throws

Used in method declarations to specify which exceptions are non handled within the method simply rather passed to the next higher level of the programme. All uncaught exceptions in a method that are not instances of RuntimeException must exist declared using the throws keyword.

transient

Declares that an instance field is not part of the default serialized course 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 just to their default value. If the default course is not used, e.1000. when a serialPersistentFields tabular array is declared in the class hierarchy, all transient keywords are ignored.^{[eighteen] [19]}

try

Defines a block of statements that take exception handling. If an exception is thrown inside the effort block, an optional grab block tin can handle declared exception types. Also, an optional finally block can be alleged that will be executed when execution exits the try block and catch clauses, regardless of whether an exception is thrown or not. A endeavor cake must take at least one catch clause or a finally block.

void

The void keyword is used to declare that a method does non return any value.^[7]

volatile

Used in field declarations to guarantee visibility of changes to variables across threads. Every read of a volatile variable will exist read from principal memory, and non from the CPU cache, and that every write to a volatile variable will be written to main retention, and not just to the CPU cache.^[twenty] Methods, classes and interfaces thus cannot be declared volatile, nor tin 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; this continues until the expression evaluates to false. This keyword can also be used to create a do-while loop; see do .^{[xi] [12]}

Reserved Identifiers [edit]

The following identifiers are contextual keywords, and are simply restricted in some contexts:

exports

module

non-sealed

Used to declare that a course or interface which extends a sealed class can be extended by unknown classes.^[21]

open

opens

permits

The permits clause specifies the classes that are permitted to extend a sealed class.^[21]

provides

record

requires

sealed

A sealed grade or interface can but be extended or implemented only by classes and interfaces permitted to do so.^[21]

to

transitive

uses

var

A special identifier that cannot be used equally a type name (since Java 10).^[22]

with

yield

Used to set a value for a switch expression, when using labelled statement groups (for example, instance L:).^[23]

Reserved words for literal values [edit]

truthful

A boolean literal value.

fake

A boolean literal value.

zero

A reference literal value.

Unused [edit]

const

Although reserved as a keyword in Java, const is not used and has no function.^{[ii] [24]} For defining constants in Java, see the final keyword.

goto

Although reserved as a keyword in Coffee, goto is not used and has no role.^{[2] [24]}

strictfp (added in J2SE 1.ii)^[4]

Although reserved every bit a keyword in Coffee, strictfp is obsolete, and no longer has any office.^[25] Previously this keyword was used used to restrict the precision and rounding of floating point calculations to ensure portability.^[eight]

Come across also [edit]

• Coffee annotation

References [edit]

1. ^ "Java Platform, Standard Edition Java API Reference". {{cite spider web}}: CS1 maint: url-status (link)
2. ^ ^{a b c} "Java Linguistic communication Specification - Section 3.9: Keywords". The Java Language Specification. Oracle. 2018-08-21. Retrieved 2018-12-25 .
3. ^ Goetz, Brian. "Warning about unmarried underscore identifier". OpenJDK Lambda Development.
4. ^ ^{a b c} "Java Language Keywords". The Java Tutorials. Sun Microsystems, Inc. Retrieved 2017-07-24 .
5. ^ ^{a b c d e f} "Primitive Information Types". The Java Tutorials. Dominicus Microsystems, Inc. Feb 14, 2008. Retrieved 2008-12-03 .
6. ^ ^{a b c d e f} Flanagan 2005, p. 22.
7. ^ ^{a b c d e f m} "Returning a Value from a Method". The Java Tutorials. Sun Microsystems, Inc. Feb 14, 2008. Retrieved 2008-12-03 .
8. ^ ^{a b c d e f k h i} Flanagan 2005, pp. 66–67.
9. ^ ^{a b c} "The switch Argument". The Java Tutorials. Sun Microsystems, Inc. February fourteen, 2008. Retrieved 2014-12-18 .
10. ^ ^{a b c} Flanagan 2005, pp. 46–48.
11. ^ ^{a b} "The while and do-while Statements". The Java Tutorials. Lord's day Microsystems, Inc. February 14, 2008. Retrieved 2008-12-03 .
12. ^ ^{a b} Flanagan 2005, pp. 48–49.
13. ^ ^{a b} "The if-then and if-so-else Statements". The Java Tutorials. Sun Microsystems, Inc. February 14, 2008. Retrieved 2008-12-03 .
14. ^ ^{a b} Flanagan 2005, pp. 44–46.
15. ^ ^{a b} "The for Statement". The Java Tutorials. Sun Microsystems, Inc. Feb 14, 2008. Retrieved 2008-12-03 .
16. ^ Flanagan 2005, pp. l–54.
17. ^ ^{a b c} "Controlling Admission to Members of a Class". The Java Tutorials. Sun Microsystems, Inc. February fourteen, 2008. Retrieved 2008-12-03 .
18. ^ "Java Object Serialization Specification version i.v.0". Dominicus/Oracle. 2004. 1.5 Defining Serializable Fields for a Course. Retrieved 2010-09-16 . ^{[ permanent dead link ]}
19. ^ Grosso, William (Nov 21, 2001). "Java RMI: Serialization". ONJava. O'Reilly Media. Declaring serialPersistentFields. Retrieved 2010-09-xvi .
20. ^ "Java Volatile Keyword".
21. ^ ^{a b c} "Sealed Classes". docs.oracle.com. Oracle Corporation. Retrieved 2021-08-07 .
22. ^ "Chapter 3. Lexical Structure". docs.oracle.com . Retrieved 2018-12-25 .
23. ^ "Switch Expressions". docs.oracle.com. Oracle Corporation. Retrieved 2020-12-27 .
24. ^ ^{a b} Flanagan 2005, p. xx.
25. ^ "JEP 306: Restore Always-Strict Floating-Point Semantics". {{cite web}}: CS1 maint: url-condition (link)

External links [edit]

• Gosling, James; Joy, Neb; Steele, Guy; Bracha, Gilad (June 2005). Java Language Specification (Tertiary ed.). Addison-Wesley Professional. ISBN978-0-321-24678-3 . Retrieved 2008-12-03 .
• Flanagan, David (March 2005). Coffee in a Nutshell (Fifth ed.). O'Reilly Media. ISBN978-0-596-00773-7 . Retrieved 2010-03-03 .

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Source: https://en.wikipedia.org/wiki/List_of_Java_keywords

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