References 7 January 2019 OSU CSE 1 Primitive vs. Reference Types - PowerPoint PPT Presentation

References 7 January 2019 OSU CSE 1

Primitive vs. Reference Types • Java types are divided into two different categories: – The built-in types are called primitive types • Includes boolean , char , int , double – All other types are called reference types (or class types ) • Includes String , XMLTree , SimpleReader , SimpleWriter , NaturalNumber , ... 7 January 2019 OSU CSE 2

Primitive vs. Reference Types There is no limit on • Java types are divided into two different the number of other categories: user-defined types that can be – The built-in types are called primitive types developed. • Includes boolean , char , int , double – All other types are called reference types (or class types ) • Includes String , XMLTree , SimpleReader , SimpleWriter , NaturalNumber , ... 7 January 2019 OSU CSE 3

Categories of Types, v. 1 Primitive Reference Types Types boolean String SimpleReader char XMLTree SimpleWriter int ... NaturalNumber double ... (plus 4 others) 7 January 2019 OSU CSE 4

Primitive vs. Reference Variables • A primitive variable is a variable of a primitive type – This term is used sparingly in practice, and is introduced here for parsimony to distinguish a variable of a primitive type from… • A reference variable is a variable of a reference type – A reference variable is fundamentally different from a primitive variable in ways that can dramatically impact how you reason about program behavior; beware! 7 January 2019 OSU CSE 5

Examples true 'y' 13 3.14 "Go" 7 January 2019 OSU CSE 6

Examples true 'y' 13 3.14 "Go" 7 January 2019 OSU CSE 7

Recall We Said Earlier... "Go" This is a String variable s whose value is "Go" , i.e., s = "Go" 7 January 2019 OSU CSE 8

... But Here’s the “Real Picture”! "Go" 7 January 2019 OSU CSE 9

... But Here’s the “Real Picture”! "Go" There is a String variable s , whose value is a reference to an object whose value is "Go" . 7 January 2019 OSU CSE 10

References and Objects "Go" This is the reference s . 7 January 2019 OSU CSE 11

References and Objects "Go" This is the object s “points to” or “refers to”. 7 January 2019 OSU CSE 12

Reference and Object Values • A reference variable like s may be considered to have either of two values: – The reference value of s in these pictures is the memory address at which the object is stored – The object value of s in these pictures is the mathematical model value of the object the reference s points to, in this case "Go" 7 January 2019 OSU CSE 13

Reference and Object Values Think of the reference value as simply an “id” or “serial number” of some place in memory. • A reference variable like s may be considered to have either of two values: – The reference value of s in these pictures is the memory address at which the object is stored – The object value of s in these pictures is the mathematical model value of the object the reference s points to, in this case "Go" 7 January 2019 OSU CSE 14

Getting to the “Real Picture” Variables Objects … … b true 19 c 'y' 20 i 13 21 d 3.14 "Go" 22 s 22 23 24 … … 7 January 2019 OSU CSE 15

Getting to the “Real Picture” Variables Objects … … b true 19 This part of memory holds: c 'y' 20 i 13 21 • values of primitive variables • reference values of reference d 3.14 "Go" 22 variables s 22 23 24 … … 7 January 2019 OSU CSE 16

Getting to the “Real Picture” Variables Objects … … b true 19 c 'y' 20 This part of memory holds: i 13 21 • object values of reference variables d 3.14 "Go" 22 s 22 23 24 … … 7 January 2019 OSU CSE 17

Getting to the “Real Picture” Variables Objects Each object in memory has a … … unique memory address , or b true 19 “id”; e.g., this one has id = 22. c 'y' 20 i 13 21 d 3.14 "Go" 22 s 22 23 24 … … 7 January 2019 OSU CSE 18

Getting to the “Real Picture” Variables Objects These numbers are the … … “id”s of the objects. b true 19 c 'y' 20 i 13 21 d 3.14 "Go" 22 s 22 23 24 … … 7 January 2019 OSU CSE 19

Getting to the “Real Picture” Variables Objects The reference value of a … … reference variable is the memory address, or “id”, of the object to b true 19 which it refers. c 'y' 20 i 13 21 d 3.14 "Go" 22 s 22 23 24 … … 7 January 2019 OSU CSE 20

Getting to the “Real Picture” The reference value of a Variables Objects reference variable as a number , … … i.e., the address or “id” of the b true 19 object it refers to, is immaterial; so, this situation ... c 'y' 20 i 13 21 d 3.14 "Go" 22 s 22 23 24 … … 7 January 2019 OSU CSE 21

Getting to the “Real Picture” ... is indistinguishable in a Variables Objects program from this one. (Yet in C or C++, you may do … … calculations with these b true 19 addresses/“id”s as if they were numbers! How crazy is that?) c 'y' 20 i 13 21 d 3.14 22 s "Go" 23 23 24 … … 7 January 2019 OSU CSE 22

Getting to the “Real Picture” Variables Objects Similarly, it is immaterial to the … … program exactly where each b true 19 variable is located in memory. 20 i 13 21 s 23 22 c 'y' "Go" 23 d 3.14 24 … … 7 January 2019 OSU CSE 23

So We Can Simplify... Variables Objects … true 19 'y' 20 13 21 3.14 22 23 "Go" 23 24 … 7 January 2019 OSU CSE 24

So We Can Simplify... In our pictures, the types of Variables variables are abstracted as Objects different shapes; all … true reference variables use an 19 'y' equilateral triangle. 20 13 21 3.14 22 23 "Go" 23 24 … 7 January 2019 OSU CSE 25

So We Can Simplify... Variables Variables in the program Objects are somewhere in this part … true of memory. 19 'y' 20 13 21 3.14 22 23 "Go" 23 24 … 7 January 2019 OSU CSE 26

So We Can Simplify... Objects in the program are Variables Objects somewhere in this part of true memory. 'y' 13 3.14 "Go" 23 23 7 January 2019 OSU CSE 27

So We Can Simplify... In our pictures, the connection between the Variables Objects reference value of a reference variable and its true object value is abstracted 'y' as an arrow. 13 3.14 "Go" 7 January 2019 OSU CSE 28

Finally... Our pictures allow us to abstract away even the fact that there are two parts of true memory. 'y' 13 3.14 "Go" 7 January 2019 OSU CSE 29

Notation • We never care about writing down the reference value of a reference variable as a particular numerical value (though we draw a picture of it: an arrow out of a triangle) – So, if you see something like s = "Go" in a contract or a tracing table, it must mean that the object value of s is the mathematical model value "Go" 7 January 2019 OSU CSE 30

Notation • In a tracing table, however, we might want to remind ourselves there is a reference involved, so we might record the value of variable s using a right arrow instead of an equals sign, e.g., s ➞ "Go" – This means that s is a reference variable whose object value is "Go" – Or: s refers to an object with value "Go" – Why would we do this? Coming up... 7 January 2019 OSU CSE 31

The Assignment Operator • The assignment operator = copies the value of the expression on the right-hand side into the variable on the left-hand side • For primitive types, “the value of” can mean only one thing • For reference types, it could mean “the reference value of” or “the object value of” – Which is it? 7 January 2019 OSU CSE 32

Assignment for Primitive Types • Consider: int i = k + 7; – First, the expression k + 7 is evaluated; say k = 3 , so the expression evaluates to 10 – Next, the value 10 is copied into i , so after the above statement has finished executing, we have i = 10 • How does this happen? 7 January 2019 OSU CSE 33

Step by Step: int i = k + 7; 3 We already have k , a primitive variable whose value is 3 . 7 January 2019 OSU CSE 34

Step by Step: int i = k + 7; 3 7 The int literal is an anonymous primitive variable whose value is 7 . 7 January 2019 OSU CSE 35

Step by Step: int i = k + 7; 3 7 10 The int addition operator + results in another anonymous primitive variable whose value is 10 . 7 January 2019 OSU CSE 36

Step by Step: int i = k + 7; 3 7 10 The declaration of the int variable i results ? in an uninitialized primitive variable . 7 January 2019 OSU CSE 37

Step by Step: int i = k + 7; 3 7 10 The assignment operator copies the 10 value of the right- hand side into i . 7 January 2019 OSU CSE 38

Step by Step: int i = k + 7; 3 The temporary anonymous primitive variables disappear 10 now that the statement has completed executing. 7 January 2019 OSU CSE 39