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THE Java® programming language is a general-purpose, concurrent, classbased, object-oriented language. It is designed to be simple enough that many
programmers can achieve fluency in the language. The Java programming language
is related to C and C++ but is organized rather differently, with a number of aspects
of C and C++ omitted and a few ideas from other languages included. It is intended
to be a production language, not a research language, and so, as C. A. R. Hoare
suggested in his classic paper on language design, the design has avoided including
new and untested features.
The Java programming language is strongly and statically typed. This specification
clearly distinguishes between the compile-time errors that can and must be detected
at compile time, and those that occur at run time. Compile time normally consists
of translating programs into a machine-independent byte code representation.
Run-time activities include loading and linking of the classes needed to execute
a program, optional machine code generation and dynamic optimization of the
program, and actual program execution.
The Java programming language is a relatively high-level language, in that details
of the machine representation are not available through the language. It includes
automatic storage management, typically using a garbage collector, to avoid
the safety problems of explicit deallocation (as in C's free or C++'s delete).
High-performance garbage-collected implementations can have bounded pauses to
support systems programming and real-time applications. The language does not
include any unsafe constructs, such as array accesses without index checking, since
such unsafe constructs would cause a program to behave in an unspecified way.
The Java programming language is normally compiled to the bytecode instruction
set and binary format defined in The Java Virtual Machine Specification, Java SE
14 Edition. 1
CSDN
1.1 Organization of the Specification INTRODUCTION
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
2
CSDN
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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