Announcements

1
Announcements

• Homework P1 due tomorrow (Thursday)
• Homework P2 has been handed out and is on the web

2
Todays Topics

• Review
• Method calls in (excruciating -) ) detail.

3
Review

• Overview of material so far
• Scope of variables
• fields in classes
• parameters of methods
• local variables, variables in blocks
• The while loop

4
Method Calls

• Today, we will describe how a method call is
  executed
• You should know this material very well
• Many difficulties in programming come from not
  understanding exactly how statements, method
  calls, etc. are being executed.

5
Executing an Assignment

• To execute an assignment x e(1) Evaluate e
  (yielding a value v, say)(2) Store v in the
  variable described by x.
• b.c 2 b.c value is 8, so store 8 in field c
  of b
• b new C(8, 7) value is name of new instance of
  class C, so store that in b

7
d
6
Method Invocation

• 0. Allocate a new set of locations to contain the
  parameters and local variables of the method
  being called, in what we call a frame.
• 1. Assign the arguments of the call to the
  parameters of the method.
• 2. Execute the procedure body.
• 3. Delete the frame produced in step 0 if the
  method is a function, return the value of the
  function to the place of call.
• Terminology parameter formal parameter
  argument actual parameter

7
Example

• // Yield the maximum of x and y
• public int max(int x, int y)
• int z
• z x
• if (y gt x)
• z y
• return z
• The call is in the assignment given below.
• int b
• b max (320, 65) 2

8

• b max (320, b5) 2
• The state before execution, with b 7
• The state after execution of step 0.
• The state after execution of step 1.

9

• Performing step 2, initial state
• After execution of z x
• After execution of the conditional statement

10

• Executing the return, which terminates execution
  of the call (perform step 3).
• 60
• b max (320, b5) 2
• Memorize the steps in executing a procedure
  call, given on slide 3. Practice executing simple
  procedure calls yourself.

11

• // Add b to c
• public void addC(Coordinate b, Coordinate c)
• c.x c.xb.x
• c.y c.yb.y
• Execute the call
• addC(d,b)
• assuming the initial state is

12

• Step 0 allocate a frame for the method
• Note that, below, there are two variables named
  b. There is no ambiguity. Later, when executing
  the method body, always use the variables in the
  frame for the call.

13

• Execution of step 1 yields

14

• Execution of step 2 yields

15

• Execution of step 3 yields

16
Execution of a new Coordinate constructor

• public class Coordinate
• public int x
• public int y
• // Constructor an instance with x b and y0
• public Coordinate(int b)
• x b y 0
• Coordinate d
• d new Coordinate(9)
• To evaluate new Coordinate(9)
• 0. Create a new instance of Coordinate
• 1. Execute the call Coordinate(9)

17
Initial State and Step 0

• Initial state
• Execution of step 0 yields the following. Note
  that we have placed method Coordinate within the
  instance. The instance contains all the fields
  and methods defined within the class. This will
  be important when executing the method body, as
  shown below, to follow the scope rules.

18

• Step 1 Execute the call on the constructor.
• Rule when drawing the frame, place the frame
  within the class instance that contains the
  called method!

19

• After execution of constructor body
• After call is completed d new Coordinate(9)
• So the name of this instance is stored in d.

Coordinate
0
y
Coordinate (constructor)
20
Call by Value

• All of this is to say that all parameters to Java
  methods are call by value
• If you pass a boolean, e.g. to a method, its
  parameter is a copy of whatever value was being
  passed
• The method does not change the original
  variables value

21
Example -- call by value

• Class passByValue
• public static void main(String args)
• double one 1.0
• System.out.println(before one one)
• halveIt(one)
• System.out.println(after one one)
• public static void halveIt(double arg)
• arg arg / 2.0 // divide arg by two
• System.out.println(halved arg arg)

22
Slightly different for objects

• class passRefByValue
• public static void main(String args)
• Body sirius new Body(Sirius, null)
• System.out.println(before sirius)
• commonName(sirius)
• System.out.println(after sirius)
• public static void commonName(Body bodyRef)
• bodyRef.name Dog Star
• bodyRef null

23
Output of previous

• before 0 (Sirius)
• after 0 (Dog Star)
• The contents of the object have been modified,
  but. . .
• . . .the reference bodyRef still refers to the
  Body object, even though commonName changed its
  value to null
• Remember aliases of objects

24
this -- briefly

• Commonly used to pass a reference to the current
  object to other methods
• The this reference always refers the current
  object that is executing the code
• Suppose you want to add the current object to a
  list of objects w/in a method called on that
  object Service.add(this)

25
Increment and Decrement

• and --
• a is equivalent to a a 1
• a is equivalent to a a 1
• Difference is when the value is returned if
  prefix, the operation is applied before the value
  is return, if postfix, after
• Examplea 16System.out.println(a
  a a)

26
Assignment Operators

• Any arithmetic or binary operator can be
  concatenated with to form another assignment
  operator
• a b 1 // same as a a (b 1)
• a 5 // same as a a 5
• var op expr is equivalent tovar ((var) op
  (expr))