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Each field is described by a field_info structure.
No two fields in one class file may have the same name and descriptor ().
The structure has the following format:
field_info { u2 access_flags; u2 name_index; u2 descriptor_index; u2 attributes_count; attribute_info attributes[attributes_count];} The items of the field_info structure are as follows:
The value of the access_flags item is a mask of flags used to denote access permission to and properties of this field. The interpretation of each flag, when set, is specified in .
Table 4.5-A. Field access and property flags
| Flag Name | Value | Interpretation |
|---|---|---|
ACC_PUBLIC | 0x0001 | Declared public; may be accessed from outside its package. |
ACC_PRIVATE | 0x0002 | Declared private; usable only within the defining class. |
ACC_PROTECTED | 0x0004 | Declared protected; may be accessed within subclasses. |
ACC_STATIC | 0x0008 | Declared static. |
ACC_FINAL | 0x0010 | Declared final; never directly assigned to after object construction (JLS §17.5). |
ACC_VOLATILE | 0x0040 | Declared volatile; cannot be cached. |
ACC_TRANSIENT | 0x0080 | Declared transient; not written or read by a persistent object manager. |
ACC_SYNTHETIC | 0x1000 | Declared synthetic; not present in the source code. |
ACC_ENUM | 0x4000 | Declared as an element of an enum. |
Fields of classes may set any of the flags in . However, each field of a class may have at most one of its ACC_PUBLIC, ACC_PRIVATE, and ACC_PROTECTED flags set (JLS §8.3.1), and must not have both its ACC_FINAL and ACC_VOLATILE flags set (JLS §8.3.1.4).
Fields of interfaces must have their ACC_PUBLIC, ACC_STATIC, and ACC_FINAL flags set; they may have their ACC_SYNTHETIC flag set and must not have any of the other flags in set (JLS §9.3).
The ACC_SYNTHETIC flag indicates that this field was generated by a compiler and does not appear in source code.
The ACC_ENUM flag indicates that this field is used to hold an element of an enumerated type.
All bits of the access_flags item not assigned in are reserved for future use. They should be set to zero in generated class files and should be ignored by Java Virtual Machine implementations.
The value of the name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info structure () which represents a valid unqualified name denoting a field ().
The value of the descriptor_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info structure () which represents a valid field descriptor ().
The value of the attributes_count item indicates the number of additional attributes of this field.
Each value of the attributes table must be an attribute_info structure ().
A field can have any number of optional attributes associated with it.
The attributes defined by this specification as appearing in the attributes table of a field_info structure are listed in .
The rules concerning attributes defined to appear in the attributes table of a field_info structure are given in .
The rules concerning non-predefined attributes in the attributes table of a field_info structure are given in .
Table 4.3-A. Interpretation of field descriptors
| FieldType term | Type | Interpretation |
|---|---|---|
B | byte | signed byte |
C | char | Unicode character code point in the Basic Multilingual Plane, encoded with UTF-16 |
D | double | double-precision floating-point value |
F | float | single-precision floating-point value |
I | int | integer |
J | long | long integer |
L ClassName ; | reference | an instance of class ClassName |
S | short | signed short |
Z | boolean | true or false |
[ | reference | one array dimension |
The field descriptor of an instance variable of type int is simply I.
The field descriptor of an instance variable of type Object is Ljava/lang/Object;. Note that the internal form of the binary name for class Object is used.
The field descriptor of an instance variable of the multidimensional array type double[][][] is [[[D.
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