Java syntax



The syntax of Java is the set of rules defining how a Java program is written and interpreted.

The syntax is mostly derived from C and C++. Unlike in C++, in Java there are no global functions or variables, but there are data members which are also regarded as global variables. All code belongs to classes and all values are objects. The only exception is the primitive types, which are not considered to be objects for performance reasons (though can be automatically converted to objects and vice versa via autoboxing). Some features like operator overloading or unsigned integer types are omitted to simplify the language and to avoid possible programming mistakes.

The Java syntax has been gradually extended in the course of numerous major JDK releases, and now supports capabilities such as generic programming and function literals (called lambda expressions in Java). Since 2017, a new JDK version is released twice a year, with each release bringing incremental improvements to the language.

Identifier
An identifier is the name of an element in the code. There are certain standard naming conventions to follow when selecting names for elements. Identifiers in Java are case-sensitive.

An identifier can contain:
 * Any Unicode character that is a letter (including numeric letters like Roman numerals) or digit.
 * Currency sign (such as ¥).
 * Connecting punctuation character (such as _).

An identifier cannot:
 * Start with a digit.
 * Be equal to a reserved keyword, null literal or boolean literal.

Literals
Integer literals are of  type by default unless   type is specified by appending   or   suffix to the literal, e.g.  . Since Java SE 7, it is possible to include underscores between the digits of a number to increase readability; for example, a number 145608987 can be written as 145_608_987.

Variables
Variables are identifiers associated with values. They are declared by writing the variable's type and name, and are optionally initialized in the same statement by assigning a value.

Multiple variables of the same type can be declared and initialized in one statement using comma as a delimiter.

Type inference
Since Java 10, it has become possible to infer types for the variables automatically by using.

Code blocks
The separators $($ and $)$ signify a code block and a new scope. Class members and the body of a method are examples of what can live inside these braces in various contexts.

Inside of method bodies, braces may be used to create new scopes, as follows:

Comments
Java has three kinds of comments: traditional comments, end-of-line comments and documentation comments.

Traditional comments, also known as block comments, start with  and end with , they may span across multiple lines. This type of comment was derived from C and C++.

End-of-line comments start with  and extend to the end of the current line. This comment type is also present in C++ and in modern C.

Documentation comments in the source files are processed by the Javadoc tool to generate documentation. This type of comment is identical to traditional comments, except it starts with  and follows conventions defined by the Javadoc tool. Technically, these comments are a special kind of traditional comment and they are not specifically defined in the language specification.

Universal types
Classes in the package java.lang are implicitly imported into every program, as long as no explicitly-imported types have the same names. Important ones include:

java.lang.Object
java.lang.Object is Java's top type. Superclass of all classes that do not declare a parent class. All values can be converted to this type, although for primitive values this involves autoboxing.

java.lang.String
java.lang.String is Java's basic string type. Immutable. Some methods treat each UTF-16 code unit as a "character", but methods to convert to an  that is effectively UTF-32 are also available.

java.lang.Throwable
java.lang.Throwable is supertype of everything that can be thrown or caught with Java's  and   statements.

Program structure
Java applications consist of collections of classes. Classes exist in packages but can also be nested inside other classes.

method
Every Java application must have an entry point. This is true of both graphical interface applications and console applications. The entry point is the  method. There can be more than one class with a  method, but the main class is always defined externally (for example, in a manifest file). The  method along with the main class must be declared. The method must be  and is passed command-line arguments as an array of strings. Unlike C++ or C#, it never returns a value and must return.

Packages
Packages are a part of a class name and they are used to group and/or distinguish named entities from other ones. Another purpose of packages is to govern code access together with access modifiers. For example,  is a fully qualified class name for the class   which is located in the package.

A package is declared at the start of the file with the  declaration:

Classes with the  modifier must be placed in the files with the same name and java extension and put into nested folders corresponding to the package name. The above class  will have the following path:.

Type import declaration
A type import declaration allows a named type to be referred to by a simple name rather than the full name that includes the package. Import declarations can be single type import declarations or import-on-demand declarations. Import declarations must be placed at the top of a code file after the package declaration.

Import-on-demand declarations are mentioned in the code. A "type import" imports all the types of the package. A "static import" imports members of the package.

Static import declaration
This type of declaration has been available since J2SE 5.0. Static import declarations allow access to static members defined in another class, interface, annotation, or enum; without specifying the class name:

Import-on-demand declarations allow to import all the fields of the type:

Enum constants may also be used with static import. For example, this enum is in the package called :

It is possible to use static import declarations in another class to retrieve the enum constants:

Operators
Operators in Java are similar to those in C++. However, there is no  operator due to garbage collection mechanisms in Java, and there are no operations on pointers since Java does not support them. Another difference is that Java has an unsigned right shift operator, while C's right shift operator's signedness is type-dependent. Operators in Java cannot be overloaded.

statement
if statements in Java are similar to those in C and use the same syntax:

statement may include optional  block, in which case it becomes an if-then-else statement:

Like C, else-if construction does not involve any special keywords, it is formed as a sequence of separate if-then-else statements:

Also, note that the ?: operator can be used in place of simple if statement, for example

statement
Switch statements in Java can use,  ,  , and   (note: not  ) primitive data types or their corresponding wrapper types. Starting with J2SE 5.0, it is possible to use enum types. Starting with Java SE 7, it is possible to use Strings. Other reference types cannot be used in  statements.

Possible values are listed using  labels. These labels in Java may contain only constants (including enum constants and string constants). Execution will start after the label corresponding to the expression inside the brackets. An optional  label may be present to declare that the code following it will be executed if none of the case labels correspond to the expression.

Code for each label ends with the  keyword. It is possible to omit it causing the execution to proceed to the next label, however, a warning will usually be reported during compilation.

expressions
Since Java 14 it has become possible to use switch expressions, which use the new arrow syntax:

Alternatively, there is a possibility to express the same with the  statement, although it is recommended to prefer the arrow syntax because it avoids the problem of accidental fall throughs.

Iteration statements
Iteration statements are statements that are repeatedly executed when a given condition is evaluated as true. Since J2SE 5.0, Java has four forms of such statements. Note that the condition must have type boolean or Boolean, meaning C's results in a compilation error.

loop
In the  loop, the test is done before each iteration.

loop
In the  loop, the test is done after each iteration. Consequently, the code is always executed at least once.

loop
loops in Java include an initializer, a condition and a counter expression. It is possible to include several expressions of the same kind using comma as delimiter (except in the condition). However, unlike C, the comma is just a delimiter and not an operator.

Like C, all three expressions are optional. The following loop is infinite:

Enhanced loop
Enhanced  loops have been available since J2SE 5.0. This type of loop uses built-in iterators over arrays and collections to return each item in the given collection. Every element is returned and reachable in the context of the code block. When the block is executed, the next item is returned until there are no items remaining. Unlike C#, this kind of loop does not involve a special keyword, but instead uses a different notation style.

Labels
Labels are given points in code used by  and   statements. Note that the Java  keyword cannot be used to jump to specific points in the code.

statement
The  statement breaks out of the closest loop or   statement. Execution continues in the statement after the terminated statement, if any.

It is possible to break out of the outer loop using labels:

statement
The  statement discontinues the current iteration of the current control statement and begins the next iteration. The following  loop in the code below reads characters by calling , skipping the statements in the body of the loop if the characters are spaces:

Labels can be specified in  statements and   statements:

statement
The  statement is used to end method execution and to return a value. A value returned by the method is written after the  keyword. If the method returns anything but, it must use the   statement to return some value.

statement ends execution immediately, except for one case: if the statement is encountered within a  block and it is complemented by a , control is passed to the   block.

statements
Exceptions are managed within  ...   blocks.

The statements within the  block are executed, and if any of them throws an exception, execution of the block is discontinued and the exception is handled by the   block. There may be multiple  blocks, in which case the first block with an exception variable whose type matches the type of the thrown exception is executed.

Java SE 7 also introduced multi-catch clauses besides uni-catch clauses. This type of catch clauses allows Java to handle different types of exceptions in a single block provided they are not subclasses of each other.

If no  block matches the type of the thrown exception, the execution of the outer block (or method) containing the   ...   statement is discontinued, and the exception is passed up and outside the containing block (or method). The exception is propagated upwards through the call stack until a matching  block is found within one of the currently active methods. If the exception propagates all the way up to the top-most  method without a matching   block being found, a textual description of the exception is written to the standard output stream.

The statements within the  block are always executed after the   and   blocks, whether or not an exception was thrown and even if a   statement was reached. Such blocks are useful for providing clean-up code that is guaranteed to always be executed.

The  and   blocks are optional, but at least one or the other must be present following the   block.

-with-resources statements
-with-resources statements are a special type of  statements introduced as an implementation of the dispose pattern in Java SE 7. In a -with-resources statement the   keyword is followed by initialization of one or more resources that are released automatically when the   block execution is finished. Resources must implement. -with-resources statements are not required to have a  or   block unlike normal   statements.

Since Java 9 it is possible to use already declared variables:

statement
The  statement is used to throw an exception and end the execution of the block or method. The thrown exception instance is written after the  statement.

Thread concurrency control
Java has built-in tools for multi-thread programming. For the purposes of thread synchronization the  statement is included in Java language.

To make a code block synchronized, it is preceded by the  keyword followed by the lock object inside the brackets. When the executing thread reaches the synchronized block, it acquires a mutual exclusion lock, executes the block, then releases the lock. No threads may enter this block until the lock is released. Any non-null reference type may be used as the lock.

statement
statements have been available since J2SE 1.4. These types of statements are used to make assertions in the source code, which can be turned on and off during execution for specific classes or packages. To declare an assertion the  keyword is used followed by a conditional expression. If it evaluates to  when the statement is executed, an exception is thrown. This statement can include a colon followed by another expression, which will act as the exception's detail message.

Primitive types
Primitive types in Java include integer types, floating-point numbers, UTF-16 code units and a boolean type. There are no unsigned types in Java except  type, which is used to represent UTF-16 code units. The lack of unsigned types is offset by introducing unsigned right shift operation, which is not present in C++. Nevertheless, criticisms have been leveled about the lack of compatibility with C and C++ this causes.

does not necessarily correspond to a single character. It may represent a part of a surrogate pair, in which case Unicode code point is represented by a sequence of two  values.

Boxing and unboxing
This language feature was introduced in J2SE 5.0. Boxing is the operation of converting a value of a primitive type into a value of a corresponding reference type, which serves as a wrapper for this particular primitive type. Unboxing is the reverse operation of converting a value of a reference type (previously boxed) into a value of a corresponding primitive type. Neither operation requires an explicit conversion.

Example:

Reference types
Reference types include class types, interface types, and array types. When the constructor is called, an object is created on the heap and a reference is assigned to the variable. When a variable of an object gets out of scope, the reference is broken and when there are no references left, the object gets marked as garbage. The garbage collector then collects and destroys it some time afterwards.

A reference variable is  when it does not reference any object.

Arrays
Arrays in Java are created at runtime, just like class instances. Array length is defined at creation and cannot be changed.

Multi-dimensional arrays
In Java, multi-dimensional arrays are represented as arrays of arrays. Technically, they are represented by arrays of references to other arrays.

Due to the nature of the multi-dimensional arrays, sub-arrays can vary in length, so multi-dimensional arrays are not bound to be rectangular unlike C:

Classes
Classes are fundamentals of an object-oriented language such as Java. They contain members that store and manipulate data. Classes are divided into top-level and nested. Nested classes are classes placed inside another class that may access the private members of the enclosing class. Nested classes include member classes (which may be defined with the static modifier for simple nesting or without it for inner classes), local classes and anonymous classes.

Instantiation
Non-static members of a class define the types of the instance variables and methods, which are related to the objects created from that class. To create these objects, the class must be instantiated by using the  operator and calling the class constructor.

Accessing members
Members of both instances and static classes are accessed with the  (dot) operator.

Accessing an instance member Instance members can be accessed through the name of a variable.

Accessing a static class member Static members are accessed by using the name of the class or any other type. This does not require the creation of a class instance. Static members are declared using the  modifier.

Modifiers
Modifiers are keywords used to modify declarations of types and type members. Most notably there is a sub-group containing the access modifiers.


 *   - Specifies that a class only serves as a base class and cannot be instantiated.
 *   - Used only for member classes, specifies that the member class does not belong to a specific instance of the containing class.
 *   - Classes marked as  cannot be extended from and cannot have any subclasses.
 *   - Specifies that all floating-point operations must be carried out conforming to IEEE 754 and forbids using enhanced precision to store intermediate results.

Access modifiers
The access modifiers, or inheritance modifiers, set the accessibility of classes, methods, and other members. Members marked as  can be reached from anywhere. If a class or its member does not have any modifiers, default access is assumed.

The following table shows whether code within a class has access to the class or method depending on the accessing class location and the modifier for the accessed class or class member:



Constructors and initializers
A constructor is a special method called when an object is initialized. Its purpose is to initialize the members of the object. The main differences between constructors and ordinary methods are that constructors are called only when an instance of the class is created and never return anything. Constructors are declared as common methods, but they are named after the class and no return type is specified:

Initializers are blocks of code that are executed when a class or an instance of a class is created. There are two kinds of initializers, static initializers and instance initializers.

Static initializers initialize static fields when the class is created. They are declared using the  keyword:

A class is created only once. Therefore, static initializers are not called more than once. On the contrary, instance initializers are automatically called before the call to a constructor every time an instance of the class is created. Unlike constructors instance initializers cannot take any arguments and generally they cannot throw any checked exceptions (except in several special cases). Instance initializers are declared in a block without any keywords:

Since Java has a garbage collection mechanism, there are no destructors. However, every object has a  method called prior to garbage collection, which can be overridden to implement finalization.

Methods
All the statements in Java must reside within methods. Methods are similar to functions except they belong to classes. A method has a return value, a name and usually some parameters initialized when it is called with some arguments. Similar to C++, methods returning nothing have return type declared as. Unlike in C++, methods in Java are not allowed to have default argument values and methods are usually overloaded instead.

A method is called using  notation on an object, or in the case of a static method, also on the name of a class.

The  keyword indicates that a method throws an exception. All checked exceptions must be listed in a comma-separated list.

Modifiers

 *   - Abstract methods can be present only in abstract classes, such methods have no body and must be overridden in a subclass unless it is abstract itself.
 *   - Makes the method static and accessible without creation of a class instance. However static methods cannot access non-static members in the same class.
 *   - Declares that the method cannot be overridden in a subclass.
 *   - Indicates that this method is implemented through JNI in platform-dependent code. Actual implementation happens outside Java code, and such methods have no body.
 *   - Declares strict conformance to IEEE 754 in carrying out floating-point operations.
 *   - Declares that a thread executing this method must acquire monitor. For  methods the monitor is the class instance or   if the method is static.
 * Access modifiers - Identical to those used with classes.

Varargs
This language feature was introduced in J2SE 5.0. The last argument of the method may be declared as a variable arity parameter, in which case the method becomes a variable arity method (as opposed to fixed arity methods) or simply varargs method. This allows one to pass a variable number of values, of the declared type, to the method as parameters - including no parameters. These values will be available inside the method as an array.

Fields
Fields, or class variables, can be declared inside the class body to store data.

Fields can be initialized directly when declared.

Modifiers

 *   - Makes the field a static member.
 *   - Allows the field to be initialized only once in a constructor or inside initialization block or during its declaration, whichever is earlier.
 *   - Indicates that this field will not be stored during serialization.
 *   - If a field is declared, it is ensured that all threads see a consistent value for the variable.

Inheritance
Classes in Java can only inherit from one class. A class can be derived from any class that is not marked as. Inheritance is declared using the  keyword. A class can reference itself using the  keyword and its direct superclass using the   keyword.

If a class does not specify its superclass, it implicitly inherits from  class. Thus all classes in Java are subclasses of  class.

If the superclass does not have a constructor without parameters the subclass must specify in its constructors what constructor of the superclass to use. For example:

Overriding methods
Unlike C++, all non- methods in Java are virtual and can be overridden by the inheriting classes.

Abstract classes
An Abstract Class is a class that is incomplete, or is to be considered incomplete, so cannot be instantiated.

A class C has abstract methods if any of the following is true:


 * C explicitly contains a declaration of an abstract method.
 * Any of C's superclasses has an abstract method and C neither declares nor inherits a method that implements it.
 * A direct superinterface of C declares or inherits a method (which is therefore necessarily abstract) and C neither declares nor inherits a method that implements it.
 * A subclass of an abstract class that is not itself abstract may be instantiated, resulting in the execution of a constructor for the abstract class and, therefore, the execution of the field initializers for instance variables of that class.

Output:

Enumerations
This language feature was introduced in J2SE 5.0. Technically enumerations are a kind of class containing enum constants in its body. Each enum constant defines an instance of the enum type. Enumeration classes cannot be instantiated anywhere except in the enumeration class itself.

Enum constants are allowed to have constructors, which are called when the class is loaded:

Enumerations can have class bodies, in which case they are treated like anonymous classes extending the enum class:

Interfaces
Interfaces are types which contain no fields and usually define a number of methods without an actual implementation. They are useful to define a contract with any number of different implementations. Every interface is implicitly abstract. Interface methods are allowed to have a subset of access modifiers depending on the language version,, which has the same effect as for classes, and also   since Java SE 8.

Implementing an interface
An interface is implemented by a class using the  keyword. It is allowed to implement more than one interface, in which case they are written after  keyword in a comma-separated list. A class implementing an interface must override all its methods, otherwise it must be declared as abstract.

Functional interfaces and lambda expressions
These features were introduced with the release of Java SE 8. An interface automatically becomes a functional interface if it defines only one method. In this case an implementation can be represented as a lambda expression instead of implementing it in a new class, thus greatly simplifying writing code in the functional style. Functional interfaces can optionally be annotated with the  annotation, which will tell the compiler to check whether the interface actually conforms to a definition of a functional interface.

Lambda's parameters types don't have to be fully specified and can be inferred from the interface it implements. Lambda's body can be written without a body block and a  statement if it is only an expression. Also, for those interfaces which only have a single parameter in the method, round brackets can be omitted.

Method references
It is not necessary to use lambdas when there already is a named method compatible with the interface. This method can be passed instead of a lambda using a method reference. There are several types of method references:

The code above which calls  could be replaced with the following using the method references:

Inheritance
Interfaces can inherit from other interfaces just like classes. Unlike classes it is allowed to inherit from multiple interfaces. However, it is possible that several interfaces have a field with the same name, in which case it becomes a single ambiguous member, which cannot be accessed.

Default methods
Java SE 8 introduced default methods to interfaces which allows developers to add new methods to existing interfaces without breaking compatibility with the classes already implementing the interface. Unlike regular interface methods, default methods have a body which will get called in the case if the implementing class doesn't override it.

Static methods
Static methods is another language feature introduced in Java SE 8. They behave in exactly the same way as in the classes.

Private methods
Private methods were added in the Java 9 release. An interface can have a method with a body marked as private, in which case it will not be visible to inheriting classes. It can be called from default methods for the purposes of code reuse.

Annotations
Annotations in Java are a way to embed metadata into code. This language feature was introduced in J2SE 5.0.

Annotation types
Java has a set of predefined annotation types, but it is allowed to define new ones. An annotation type declaration is a special type of an interface declaration. They are declared in the same way as the interfaces, except the  keyword is preceded by the   sign. All annotations are implicitly extended from  and cannot be extended from anything else.

Annotations may have the same declarations in the body as the common interfaces, in addition they are allowed to include enums and annotations. The main difference is that abstract method declarations must not have any parameters or throw any exceptions. Also they may have a default value, which is declared using the  keyword after the method name:

Usage of annotations
Annotations may be used in any kind of declaration, whether it is package, class (including enums), interface (including annotations), field, method, parameter, constructor, or local variable. Also they can be used with enum constants. Annotations are declared using the  sign preceding annotation type name, after which element-value pairs are written inside brackets. All elements with no default value must be assigned a value.

Besides the generic form, there are two other forms to declare an annotation, which are shorthands. Marker annotation is a short form, it is used when no values are assigned to elements:

The other short form is called single element annotation. It is used with annotations types containing only one element or in the case when multiple elements are present, but only one elements lacks a default value. In single element annotation form the element name is omitted and only value is written instead:

Generics
Generics, or parameterized types, or parametric polymorphism is one of the major features introduced in J2SE 5.0. Before generics were introduced, it was required to declare all the types explicitly. With generics it became possible to work in a similar manner with different types without declaring the exact types. The main purpose of generics is to ensure type safety and to detect runtime errors during compilation. Unlike C#, information on the used parameters is not available at runtime due to type erasure.

Generic classes
Classes can be parameterized by adding a type variable inside angle brackets ( and  ) following the class name. It makes possible the use of this type variable in class members instead of actual types. There can be more than one type variable, in which case they are declared in a comma-separated list.

It is possible to limit a type variable to a subtype of some specific class or declare an interface that must be implemented by the type. In this case the type variable is appended by the  keyword followed by a name of the class or the interface. If the variable is constrained by both class and interface or if there are several interfaces, the class name is written first, followed by interface names with  sign used as the delimiter.

When a variable of a parameterized type is declared or an instance is created, its type is written exactly in the same format as in the class header, except the actual type is written in the place of the type variable declaration.

Since Java SE 7, it is possible to use a diamond in place of type arguments, in which case the latter will be inferred. The following code in Java SE 7 is equivalent to the code in the previous example:

When declaring a variable for a parameterized type, it is possible to use wildcards instead of explicit type names. Wildcards are expressed by writing  sign instead of the actual type. It is possible to limit possible types to the subclasses or superclasses of some specific class by writing the  keyword or the   keyword correspondingly followed by the class name.

Generic methods and constructors
Usage of generics may be limited to some particular methods, this concept applies to constructors as well. To declare a parameterized method, type variables are written before the return type of the method in the same format as for the generic classes. In the case of constructor, type variables are declared before the constructor name.

Generic interfaces
Interfaces can be parameterized in the similar manner as the classes.