Announcements

1
Announcements Review

• Last Time
• methods
• logical
• reuseable
• parameterizable

• Announcements
• Lab 2 Due
• Thursday 10pm
• 1 file per pair

2
Today

• More on methods in a class
• Terminology
• method song calls method chorus
• song is the caller, chorus is the callee
• Formal parameters
• Actual parameters
• Call-by-value
• Call-by-reference

3
Formal Parametersin the callee method

• // formal parameters width, length, s
• // the variable parameter names in the
  callee
• public void printBox(int width, int length,
  String s)
• Shapes myshape new Shapes()
• for (int i 0 i lt 3 i)
• myshape.printBox(10, i, )

4
Actual Parametersin the caller method

• public void printBox(int width, int length,
  String s)
• Shapes myshape new Shapes()
• for (int i 0 i lt 3 i)
• // actual parameters 10, i,
• // the parameters in the caller
• myshape.printBox(10, i, )

5
Does this make sense?

• public void printBox(int width, int length,
  String s)
• width 9
• ...
• Shapes myshape new Shapes()
• for (int i 0 i lt 3 i)
• myshape.printBox(10, i, )

6
BlueJ Examples

• Try out changing a parameter

7
Call-by-ValueJava Semantics

• Method cannot make externally visible changes to
  parameters
• Can change it internally, but generally this is a
  bad bad idea. -)

8
Call-by-ReferenceSemantics

• Method can make externally visible changes to
  parameters!
• Algol 60 - declare a parameter as
  call-by-reference
• What does this feature mean for actual parameters?

9
Call-by-ReferenceSemantics

• Method can make externally visible changes to
  parameters!
• Algol 60 - declare a parameter as
  call-by-reference
• What does this feature mean for actual
  parameters?
• method calls may change them

10
Getting aroundCall-by-Value

• Method cannot make externally visible changes to
  parameters
• How do we change state with methods then?

11
Getting aroundCall-by-Value

• Method cannot make externally visible changes to
  parameters
• for primitive types objects
• How do we change state with methods then?
• return values, but Java only has one!
• methods can make persistent changes to the fields
  of a parameter object
• object.color pink

12
BlueJ Examples

• Lets try a return value, and passing in an
  object as a parameter

13
More Questions?