13.2

What Binary Compatibility Is and Is Not

BINARY COMPATIBILITY

A specification of each field that is not

private

 declared in the class or inter 

face, given as the simple name of the field and a symbolic reference to the

type of the field

If it is a class, then the signature of each constructor, as described above

For each method that is not

private

 declared in the class or interface, its sig 

nature and return type, as described above

The code needed to implement the class or interface:

N

For an interface, code for the field initializers

N

For a class, code for the field initializers, the static initializers, and the

implementation of each method or constructor that is not declared

private

If a Java system defines a binary format that represents a group of classes and

interfaces comprised by an entire package, then this binary format need not

expose information about fields, methods, or constructors that are declared with

default (package) access.

The following sections specify the changes that may be made to class and

interface type declarations without breaking compatibility with pre existing bina 

ries. The Java Virtual Machine and its standard

class

 file format support these

changes; other Java binary formats are required to support these changes as well.

13.2   What Binary Compatibility Is and Is Not

A change to a type is

binary compatible with

 (equivalently, does not

 break binary

compatibility

with) preexisting binaries if preexisting binaries that previously

linked without error will continue to link without error.

As described in  13.1, symbolic references to methods and fields name the

exact class or interface in which the method or field is declared. This means that

binaries are compiled to rely on the accessible members and constructors of other

classes and interfaces. To preserve binary compatibility, a class or interface should

treat these accessible members and constructors, their existence and behavior, as a

contract

 with users of the class or interface.

Java is designed to prevent additions to contracts and accidental name colli 

sions from breaking binary compatibility; specifically:

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