class file taken apart.

decompiled.txt

first 4 bytes: magic number
0xCAFEBABE

next 2 bytes: minor version
0x00

next 2 bytes: major version
0x34 = 52

next 2 bytes: constant pool count
0x001B = 27

=== cp_info[] ===

cp info 0x01 = methodref
tag: 0x0A = 10 = CONSTANT_Methodref
class_index: 0x0005
name_and_type_index: 0x000E

cp info 0x02
tag: 0x09 = CONSTANT_Fieldref
class_index: 0x000F
name_and_type_index: 0x0010

cp info 0x03
tag: 0x0A = 10 = CONSTANT_Methodref
class_index: 0x0011
name_and_type_index: 0x0012

cp info 0x04
tag: 0x07 = CONSTANT_Class
name_index = 0x0013

cp info 0x05
tag: 07 = CONSTANT_CLASS
name_index = 0x0014

cp info 0x06
tag: 01 = CONSTANT_Utf8
length: 0x0006
bytes = 3C 69 6E 69 74 3E = "<init>"

cp info 0x07
tag: 01 = Utf8
length = 0x0003
bytes = 28 29 56 = "()V"

cp info 0x08
tag: 01 = Utf8
length = 0x0004
bytes = 43 6F 64 65 = "Code"

cp info 0x09
tag: 01 = Utf8
length = 0x000F
bytes = 4C 69 6E 65  4E 75 6D 62 65 72 54 61  62 6C 65 = "LineNumberTable"

cp info 0x0A
tag: 01 = Utf8
length = 0x0004
bytes = 6D 61  69 6E = "main"

cp info 0x0B
tag: 01 = Utf8
length = 0x0016
bytes = 28 5B 4C  6A 61 76 61  2F 6C 61 6E  67 2F 53 74 72 69 6E 67  3B 29 56 = "([Ljava/lang/string;)V"

cp info 0x0C
tag: 01
length = 0x000A
bytes = 53 6F  75 72 63 65  46 69 6C 65 = "SourceFile"

cp info 0x0D
tag: 01
length = 0x0012
bytes = 74  65 73 74 63  61 73 65 30 30 5F 30 31  2E 6A 61 76  61 = "testcase00_01.java"

cp info 0x0E
tag: 0x0C = Constant_NameAndType
name_index = 0x0006
descriptor_index = 0x0007

cp info 0x0F
tag: 0x07 = Constant_Class
name_index: 0x0015

cp info 0x10
tag: 0x0C = NameAndType
name_index = 0x0016
descriptor_index = 0x0017

cp info 0x11
tag: 0x07 = Class
name_index = 0x0018

cp info 0x12
tag: 0x0C = NameAndType
name_index = 0x0019
descriptor_index = 0x001A

cp info 0x13
tag: 0x01 = Utf8
length = 0x0004
bytes = 4D 61 69  6E = "Main"

cp info 0x14
tag: 0x01
length = 0x0010
bytes = 6A 61 76 61  2F 6C 61 6E  67 2F 4F 62  6A 65 63 74 = "java/lang/Object"

cp info 0x15
tag: 0x01
length = 0x0010
bytes = 6A  61 76 61 2F 6C 61 6E 67  2F 53 79 73  74 65 6D

cp info 0x16
tag: 0x01
length = 0x0003
bytes = 6F 75  74 = "out"

cp info 0x17
tag: 0x01
length = 0x0015
bytes = 4C 6A 61 76  61 2F 69 6F  2F 50 72 69 6E 74 53 74  72 65 61 6D  3B = "Ljava/io/PrintStream;"

cp info 0x18
tag: 0x01
length = 0x0013
bytes = 6A 62 76 61  2F 69 6F 2F  50 72 69 6E  74 53 74 72  65 61 6D = "java/io/PrintStream"

cp info 0x19
tag: 0x01
length = 0x0007
bytes = 70 72  69 6E 74 6C  6E = "println"

cp info 0x1A
tag: 0x01
length = 0x0004
bytes = 28 49 29 56 = "(I)V"

=== end constant_pool ===

access_flags = 0x0020 = ACC_SUPER = backwards compat default

this_class = 0x0004

super_class = 0x0005

interfaces_count = 0x0000
interfaces = []
fields_count = 0x0000
fields = []
methods_count = 0x0002

=== methods ===

method 1:
access_flags = 0x0000 = none applied?
name_index = 0x0006
descriptor_index = 0x0007
attributes_count = 0x0001
  === attributes ===
  attribute_name_index = 0x0008  ("Code")
  attribute_length = 0x0000001D
  info = 00 01  00 01 00 00  00 05 2A B7  00 01 B1 00  00 00 01 00  09 00 00 00 06 00 01 00  00 00 01
    === Parsing attribute as a code attribute ===
      attribute_name_index = 0x0008 ("Code")
      attribute_length = 0x0000001D
      max_stack = 0x0001
      max_locals = 0x0001
      code_length = 0x00000005
      code = 2A B7 00 01 B1
        === Code breakdown
          2A = aload_0
          // puts this object on stack
          B7 00 01 = invokespecial 0 1
          // 0x0001 is a reference to java.lang.Object constructor.
          // java.lang.Object constructor is called on the current object.
          B1 = return void
        ===
      exception_table_length = 0x0000
      exception_table = []
      attributes_count = 0x0001
      attributes = [
        { attribute_name_index = 0x0009 ("LineNumberTable")
        , attribute_length = 0x00000006
        , info = 00 01 00 00 00 01
        , === parsing attribute as LineNumberTable ===
          attribute_name_index = 0x0009 ("LineNumberTable")
          attribute_length = 0x00000006
          line_number_table_length = 0x0001
          line-number_table = [
            { start_pc = 0x0000
            , line_number = 0x0001
            }
          ]
          === end LineNumberTable attribute ===
        }
      ]
    === end code attribute ===
  === end attributes ===

method2
access_flags = 0x0009 (???)
name_index = 0x000A ("main")
descriptor_index = 0x000B ("([Ljava/lang/String;)V")
attributes_count = 0x0001
attributes = [
  { === parsed as Code ===
    attribute_name_index = 0x0008 ("Code"),
    attribute_length = 0x00000024,
    max_stack = 0x0002
    max_locals = 0x0001
    code_length = 0x00000008
    code = B2 00 02 07 B6 00 03  B1
      === Code breakdown
        getstatic 0 2
        // 0x0002 is the reference to "System.out". The System.out reference is put on the stack.
        iconst_4 // pushes 4 onto stack
        invokevirtual 0 3
        // 0x0003 is the reference to the println function.
        // This takes the object and the argument 4 off the stack, and calls println with them.
        return
        // returns void
      ===
    exception_table_length = 0x0000
    execption_table = []
    attributes_count = 0x0001
    attributes = [{
      attribute_name_index = 0x0009 ("LineNumberTabl")
      attribute_length = 0x0000000A
      line_number_table_length = 0x0002
      line_number_table = [
        { start_pc = 0x0000
        , line_number = 0x0005
        },
        { start_pc = 0x0007
        , line_number = 0x0006
        }
      ]
    }]
    === end Code ===
  }
]

=== end methods ===

attributes_count = 0x0001
attributes = [
  { attribute_name_index = 0x000C ("SourceFile")
  , attribute_length = 0x00000002
  , sourcefile_index = 0x000D
  }
]

Main.class

 ˛∫æ4
	

<init>
()V
Code
LineNumberTable
main
([Ljava/lang/String;)V

SourceFile
testcase00_01.java
Main
java/lang/Object
java/lang/System
out
Ljava/io/PrintStream;
java/io/PrintStream
println
(I)V 


*∑
±
	

	

$
≤∂±
	



testcase00_01.java

class Main
{
   public static void main( String[] args )
   {
      System.out.println(4);
   }
}

text.md

For educational purposes, I decompiled a simple Java class file by hand. I used https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html as a reference.