//Direct-mapped "field_id_item".
        struct DexFieldId {
            u2  classIdx;       类的类型, DexTypeId中的索引下标
            u2  typeIdx;        字段类型, DexTypeId中的索引下标
            u4  nameIdx;        字段名称, DexStringId中的索引下标
        };

  //Direct-mapped "method_id_item".
        struct DexMethodId{
            u2  classIdx;           类的类型, DexTypeId中的索引下标
            u2  protoIdx;            声明类型, DexProtoId中的索引下标
            u4  nameIdx;            方法名, DexStringId中的索引下标
        };

 //Direct-mapped "map_list".
        struct DexMapList {
            u4  size;                       //DexMapItem的个数
            DexMapItem list[1];             //变长数组
        };

  struct DexMapItem {
            u2 type;                        //kDexType开头的类型
            u2 unused;                      //未使用, 用于字节对齐
            u4 size;                        //指定类型的个数
            u4 offset;                      //指定类型数据的文件偏移
        };

/* map item type codes */
enum {
    kDexTypeHeaderItem               = 0x0000,
    kDexTypeStringIdItem             = 0x0001,
    kDexTypeTypeIdItem               = 0x0002,
    kDexTypeProtoIdItem              = 0x0003,
    kDexTypeFieldIdItem              = 0x0004,
    kDexTypeMethodIdItem             = 0x0005,
    kDexTypeClassDefItem             = 0x0006,
    kDexTypeMapList                  = 0x1000,
    kDexTypeTypeList                 = 0x1001,
    kDexTypeAnnotationSetRefList     = 0x1002,
    kDexTypeAnnotationSetItem        = 0x1003,
    kDexTypeClassDataItem            = 0x2000,
    kDexTypeCodeItem                 = 0x2001,
    kDexTypeStringDataItem           = 0x2002,
    kDexTypeDebugInfoItem            = 0x2003,
    kDexTypeAnnotationItem           = 0x2004,
    kDexTypeEncodedArrayItem         = 0x2005,
    kDexTypeAnnotationsDirectoryItem = 0x2006,
};