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The concrete object model

• Objects are just one kind of data structures;
• Objects are containers. They have slots to hold primitive types and/or other objects.
• Things an object holds are called "properties"/"attributes".
  • Values are called "attributes"
  • Procedures are called "methods"
• Object hierarchical property retrieval.

Primitives V.S. Objects

int:

address      value
0xabcd1234 | 0x00000001 | # 1

int[] in C V.S. [1,2,3] in Python

0xabcd1234  | 0x00000001 | # [1,2,3]
            | 0x00000002 |
            | 0x00000003 |
            | 0x00000004 | # 4
            | 0xabcd1234 |

Object data structrue:

object 1
| 0x10100001 | # type
| 0xc3ccab10 | # some other meta data
| 0xabcd1230 | # value or address to data

object 2
| 0x10100001 | # type
| 0xc3ccab10 | # some other meta data
| 0xabcd4440 | # value or address to data

Object model in Java:

reference 1
| 0x10101000 | # type
| 0x1203cccc | # meta
| 0xabcd1234 | # address to data

...
in memory pool
0xabcd1234 | 0x00000001 |

int[] in Java:

reference 1
| 0x10101000 | # type
| 0x1203cccc | # meta
| 0xabcd1234 | # address to data

...
in memory pool
0xabcd1234  | 0x00000003 | # meta
            | 0x00000002 | # length
            | 0x00000003 |
            | 0x00000004 | # 4

class Point { static int x; int y } Point p = new Point(1, 2);

reference of Point.class
| 0x10101000 | # type
| 0x1203cccc | # meta
| 0xabcd0000 | # address to data

reference of p
| 0x10101000 | # type
| 0x1203cccc | # meta
| 0xabcd1234 | # address to data

...
in memory pool
0xabcd0000  | 0x00000001 | # x
...
0xabcd1234  | 0x10101000 | # meta
            | 0x00000003 | # y

Object[] in Java

reference 1
| 0x10101000 | # type
| 0x1203cccc | # meta
| 0xabcd1234 | # address to data

...
in memory pool
0xabcd1234  | 0x00000003 | # meta
            | 0x00000002 | # length
            | 0x10101000 | # type
            | 0x1203ccc0 | # meta
            | 0xabcd1234 | # address to data
            | 0x10101000 | # type
            | 0x1203ccc0 | # meta
            | 0xabcd1234 | # address to data

List l = new ArrayList<>(); implementation: use Object[] to store data

how many primitive types are there in Java?

• 4 integral types
  • byte
  • short
  • int
  • long
• 2 floating point types
  • float
  • double
• char
• boolean

UTF-8: backward compatible to ASCII; memory compact UTF-16: easy to randomly locate

Java syntax

• class instantiation
  • class
  • new
  • this: reference to enclosing instance itself
  • static: no implicit this argument
    • variable
    • method
    • inner class
• class hierarchy
  • extends
    • override
    • overload
  • final
    • address of reference can not be changed once initialized
    • value of primitives can not be changed once initialized
    • class cannot be extended
  • super: reference to superclass
    • you only have to invoke a super constructor if there isn't a default constructor in the parent class
    • call super version functions
• interfaces
  • interface
  • implements
    • public InterfaceA extends InterfaceB
  • abstract
• access control
  • package/private/protected/public
• module system
  • import
• multi-thread
  • volatile