4.6_Methods.md

4.6. Methods

Each method, including each instance initialization method (§2.9) and the class or interface initialization method (§2.9), is described by a method_info structure.

No two methods in one class file may have the same name and descriptor (§4.3.3).

The structure has the following format:

method_info {
    u2             access_flags;
    u2             name_index;
    u2             descriptor_index;
    u2             attributes_count;
    attribute_info attributes[attributes_count];
}

The items of the method_info structure are as follows:

• access_flags

  The value of the access_flags item is a mask of flags used to denote access permission to and properties of this method. The interpretation of each flag, when set, is specified in Table 4.6-A.Table 4.6-A. Method access and property flagsFlag NameValueInterpretationACC_PUBLIC0x0001Declared public; may be accessed from outside its package.ACC_PRIVATE0x0002Declared private; accessible only within the defining class.ACC_PROTECTED0x0004Declared protected; may be accessed within subclasses.ACC_STATIC0x0008Declared static.ACC_FINAL0x0010Declared final; must not be overridden (§5.4.5).ACC_SYNCHRONIZED0x0020Declared synchronized; invocation is wrapped by a monitor use.ACC_BRIDGE0x0040A bridge method, generated by the compiler.ACC_VARARGS0x0080Declared with variable number of arguments.ACC_NATIVE0x0100Declared native; implemented in a language other than Java.ACC_ABSTRACT0x0400Declared abstract; no implementation is provided.ACC_STRICT0x0800Declared strictfp; floating-point mode is FP-strict.ACC_SYNTHETIC0x1000Declared synthetic; not present in the source code.Methods of classes may have any of the flags in Table 4.6-A set. However, each method of a class may have at most one of its ACC_PUBLIC, ACC_PRIVATE, and ACC_PROTECTEDflags set (JLS §8.4.3).Methods of interfaces may have any of the flags in Table 4.6-A set except ACC_PROTECTED, ACC_FINAL, ACC_SYNCHRONIZED, and ACC_NATIVE (JLS §9.4). In a class file whose version number is less than 52.0, each method of an interface must have its ACC_PUBLIC and ACC_ABSTRACT flags set; in a class file whose version number is 52.0 or above, each method of an interface must have exactly one of its ACC_PUBLIC and ACC_PRIVATE flags set.If a method of a class or interface has its ACC_ABSTRACT flag set, it must not have any of its ACC_PRIVATE, ACC_STATIC, ACC_FINAL, ACC_SYNCHRONIZED, ACC_NATIVE, or ACC_STRICTflags set.Each instance initialization method (§2.9) may have at most one of its ACC_PUBLIC, ACC_PRIVATE, and ACC_PROTECTED flags set, and may also have its ACC_VARARGS, ACC_STRICT, and ACC_SYNTHETIC flags set, but must not have any of the other flags in Table 4.6-A set.Class and interface initialization methods are called implicitly by the Java Virtual Machine. The value of their access_flags item is ignored except for the setting of the ACC_STRICT flag.The ACC_BRIDGE flag is used to indicate a bridge method generated by a compiler for the Java programming language.The ACC_VARARGS flag indicates that this method takes a variable number of arguments at the source code level. A method declared to take a variable number of arguments must be compiled with the ACC_VARARGS flag set to 1. All other methods must be compiled with the ACC_VARARGS flag set to 0.The ACC_SYNTHETIC flag indicates that this method was generated by a compiler and does not appear in source code, unless it is one of the methods named in §4.7.8.All bits of the access_flags item not assigned in Table 4.6-A are reserved for future use. They should be set to zero in generated class files and should be ignored by Java Virtual Machine implementations.

• name_index

  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 (§4.4.7) representing either one of the special method names <init> or <clinit> (§2.9), or a valid unqualified name denoting a method (§4.2.2).

• descriptor_index

  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 representing a valid method descriptor (§4.3.3).A future edition of this specification may require that the last parameter descriptor of the method descriptor is an array type if the ACC_VARARGS flag is set in the access_flagsitem.

• attributes_count

  The value of the attributes_count item indicates the number of additional attributes of this method.

• attributes[]

  Each value of the attributes table must be an attribute_info structure (§4.7).A method can have any number of optional attributes associated with it.The attributes defined by this specification as appearing in the attributes table of a method_info structure are listed in Table 4.7-C.The rules concerning attributes defined to appear in the attributes table of a method_info structure are given in §4.7.The rules concerning non-predefined attributes in the attributes table of a method_info structure are given in §4.7.1.