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Introduction

INTRODUCTION

This Java Language Specification is organized as follows:

Chapter 2 describes grammars and the notation used to present the lexical and

syntactic grammars for Java.

Chapter 3 describes the lexical structure of Java, which is based on C and

C++. Java is written in the Unicode character set. Java supports the writing of

Unicode characters on systems that support only ASCII.

Chapter 4 describes Java's types, values, and variables. Java's types are the

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.

Java's 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 standard class

Object

,

which is the (single) root of the class hierarchy. A predefined

String

 class sup 

ports Unicode character strings. Standard classes exist for wrapping primitive val 

ues inside of objects.

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 refer 

ence 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 refer 

ence 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 Java's conversions and numeric promotions. Conversions

change the compile time type and, sometimes, the value of an expression.

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 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 (denote). Java does not require types or their members to be declared

before they are used. Declaration order is significant only for local variables and

the order of initializers of fields in a class or interface.

Java provides control over the scope of names and supports limitations on

external access to members of packages, classes, and interfaces. This helps in

writing large programs by distinguishing the implementation of a type from its

users and those who extend it. Standard naming conventions that make for more

readable programs are described here.

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