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The Java Language Specification Java SE 14 Edition PDF 下载
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1.1 Organization of the Specification
Chapter 2 describes grammars and the notation used to present the lexical and
syntactic grammars for the language.
Chapter 3 describes the lexical structure of the Java programming language, which
is based on C and C++. The language is written in the Unicode character set. It
supports the writing of Unicode characters on systems that support only ASCII.
Chapter 4 describes types, values, and variables. Types are subdivided into
primitive types and reference types.
The primitive types are defined to be the same on all machines and in all
implementations, and are various sizes of two's-complement integers, single- and
double-precision IEEE 754 standard floating-point numbers, a boolean type, and
a Unicode character char type. Values of the primitive types do not share state.
Reference types are the class types, the interface types, and the array types. The
reference types are implemented by dynamically created objects that are either
instances of classes or arrays. Many references to each object can exist. All objects
(including arrays) support the methods of the class Object, which is the (single)
root of the class hierarchy. A predefined String class supports Unicode character
strings. Classes exist for wrapping primitive values inside of objects. In many
cases, wrapping and unwrapping is performed automatically by the compiler (in
which case, wrapping is called boxing, and unwrapping is called unboxing). Class
and interface declarations may be generic, that is, they may be parameterized by
other reference types. Such declarations may then be invoked with specific type
arguments.
Variables are typed storage locations. A variable of a primitive type holds a value
of that exact primitive type. A variable of a class type can hold a null reference or
a reference to an object whose type is that class type or any subclass of that class
type. A variable of an interface type can hold a null reference or a reference to an
instance of any class that implements the interface. A variable of an array type can
hold a null reference or a reference to an array. A variable of class type Object can
hold a null reference or a reference to any object, whether class instance or array.
Chapter 5 describes conversions and numeric promotions. Conversions change the
compile-time type and, sometimes, the value of an expression. These conversions
include the boxing and unboxing conversions between primitive types and
reference types. Numeric promotions are used to convert the operands of a numeric
operator to a common type where an operation can be performed. There are no
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INTRODUCTION Organization of the Specification 1.1
loopholes in the language; casts on reference types are checked at run time to ensure
type safety.
Chapter 6 describes declarations and names, and how to determine what names
mean (that is, which declaration a name denotes). The Java programming language
does not require classes and interfaces, or their members, to be declared before
they are used. Declaration order is significant only for local variables, local classes,
and the order of field initializers in a class or interface. Recommended naming
conventions that make for more readable programs are described here.
Chapter 7 describes the structure of a program, which is organized into packages.
The members of a package are classes, interfaces, and subpackages. Packages,
and consequently their members, have names in a hierarchical name space; the
Internet domain name system can usually be used to form unique package names.
Compilation units contain declarations of the classes and interfaces that are
members of a given package, and may import classes and interfaces from other
packages to give them short names.
Packages may be grouped into modules that serve as building blocks in the
construction of very large programs. The declaration of a module specifies which
other modules (and thus packages, and thus classes and interfaces) are required in
order to compile and run code in its own packages.
The Java programming language supports limitations on external access to the
members of packages, classes, and interfaces. The members of a package may be
accessible solely by other members in the same package, or by members in other
packages of the same module, or by members of packages in different modules.
Similar constraints apply to the members of classes and interfaces.
Chapter 8 describes classes. The members of classes are classes, interfaces, fields
(variables) and methods. Class variables exist once per class. Class methods operate
without reference to a specific object. Instance variables are dynamically created
in objects that are instances of classes. Instance methods are invoked on instances
of classes; such instances become the current object this during their execution,
supporting the object-oriented programming style.
Classes support single inheritance, in which each class has a single superclass. Each
class inherits members from its superclass, and ultimately from the class Object.
Variables of a class type can reference an instance of that class or of any subclass of
that class, allowing new types to be used with existing methods, polymorphically.
Classes support concurrent programming with synchronized methods. Methods
declare the checked exceptions that can arise from their execution, which allows
compile-time checking to ensure that exceptional conditions are handled. Objects
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1.1 Organization of the Specification INTRODUCTION
can declare a finalize method that will be invoked before the objects are discarded
by the garbage collector, allowing the objects to clean up their state.
For simplicity, the language has neither declaration "headers" separate from the
implementation of a class nor separate type and class hierarchies.
A special form of classes, enums, support the definition of small sets of values and
their manipulation in a type safe manner. Unlike enumerations in other languages,
enums are objects and may have their own methods.
Chapter 9 describes interfaces. The members of interfaces are classes, interfaces,
constant fields, and methods. Classes that are otherwise unrelated can implement
the same interface. A variable of an interface type can contain a reference to any
object that implements the interface.
Classes and interfaces support multiple inheritance from interfaces. A class that
implements one or more interfaces may inherit instance methods from both its
superclass and its superinterfaces.
Annotation types are specialized interfaces used to annotate declarations. Such
annotations are not permitted to affect the semantics of programs in the Java
programming language in any way. However, they provide useful input to various
tools.
Chapter 10 describes arrays. Array accesses include bounds checking. Arrays are
dynamically created objects and may be assigned to variables of type Object. The
language supports arrays of arrays, rather than multidimensional arrays.
Chapter 11 describes exceptions, which are nonresuming and fully integrated with
the language semantics and concurrency mechanisms. There are three kinds of
exceptions: checked exceptions, run-time exceptions, and errors. The compiler
ensures that checked exceptions are properly handled by requiring that a method
or constructor can result in a checked exception only if the method or constructor
declares it. This provides compile-time checking that exception handlers exist, and
aids programming in the large. Most user-defined exceptions should be checked
exceptions. Invalid operations in the program detected by the Java Virtual Machine
result in run-time exceptions, such as NullPointerException. Errors result from
failures detected by the Java Virtual Machine, such as OutOfMemoryError. Most
simple programs do not try to handle errors.
Chapter 12 describes activities that occur during execution of a program. A
program is normally stored as binary files representing compiled classes and
interfaces. These binary files can be loaded into a Java Virtual Machine, linked to
other classes and interfaces, and initialized.
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