Introducing Methods

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Introducing Methods

• Corresponds with Chapter 5

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What is a Method?

• Method declaration
• Signature
• Modifier(s)
• e.g. public or private, static
• Return type
• e.g. void, int, boolean, etc. OR a class type. OR
  no type (if a constructor)
• Identifier
• Formal Parameter List
• Enclosed in parentheses
• types and identifiers (like variable declaration)
• Separated by commas
• Method Body
• requires return statement if return type is not
  void.

A method is a collection of statements that are
grouped together to perform an operation.

• Method call
• Specify the identifier
• Place actual parameters (arguments) in
  parentheses
• Actual data (literal, constant, variable, or
  expression)
• Use return value (if not void)

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Introducing Methods(Example 5.1)
calling a method
method
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Anatomy of Method Declaration and Call
method declaraion
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Processing Sequence of a Method Call
method call
3) Function body executes.
method declaration
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Parameter Order and Type Assocation

• void nPrintln (String message, int n)
• for (int i0 iltn i)
• System.out.println(message)

IMPORTANT the order and data type of actual
parameters in a method call MUST match the order
and data type of formal parameters in the method
signature. Otherwise you will get a syntax error.
OK
Not OK
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Frame Stack

• The Java Virtual Machine keeps manages the local
  variables and parameters of method in a Frame
  Stack (also referred to as Call Stack or Program
  Stack).
• Frame a data structure that holds the values of
  all the local variables and formal parameters of
  a method.
• Stack a last-in, first-out data structure.
  Items are pushed onto the top of the stack. Items
  are popped off the top of the stack.
• When a method is called, its frame (local
  variables and parameters) are pushed onto the top
  of the stack.
• When a method terminates, its frame is removed
  from the stack.
• ? the formal parameters and local variables of a
  method exist ONLY AS LONG AS THE METHOD IS
  EXECUTING.

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Memory Changes During Processing(Listing 5.1,
p132)
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Memory Changes During Processing(Listing 5.1)
In main(), before calling max()
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Frame Stack
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Memory Changes During Processing(Listing 5.1)
In max(), just started
2
Frame Stack
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Memory Changes During Processing(Listing 5.1)
In max(), before it terminates
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2
Frame Stack
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Memory Changes During Processing(Listing 5.1)
Back in main(), after max() returns
NOTE the value returned from max() was assigned
into the variable k.
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Frame Stack
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Debugger

• You can view the contents of the frame stack in
  the debugger.
• The Java JDK includes a program for debugging
  applications (called jdb.exe, in the bin
  subdirectory).
• NetBeans provides a GUI interface to the debugger.

NOTE You will be learning how to use the
debugger in future assignments!
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Stopped at this statement (breakpoint)
mains frame on the frame stack
Local data in main method
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Stopped at this statement (breakpoint)
maxs frame pushed on top of mains frame
Local data in max method
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Stopped at this statement (breakpoint)
maxs return value was assigned to variable k
maxs frame was popped off of the frame stack
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Scope

• A variables scope is its visibility.
• Which statements can refer to the variables
  identifier.
• Local variables and formal parameters have method
  scope. They can only be used inside the method
  for which they are declared.
• Variables declared inside blocks have block
  scope. They can be used only inside the block for
  which they are declared.
• Variables declared in the parentheses of a
  control structure can only be used within the
  control structure for which they are declared.
• You can declare multiple variables of the same
  name as long as they are not in the same nesting
  structure.
• You cannot declare variables of the same name
  within the same nesting structure.

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Variables i, j, and k, and parameter args are in
mains scope.
Note max cannot refer to any of mains variables
or parameters...otherwise youll get a syntax
error.
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Variable result, and parameters num1 and num2 are
in maxs scope. main cannot refer to these
identifiers.
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Another Scope Example

• The following example shows variables with
• Method scope (available throughout an entire
  method)
• Block scope (available only within a block of
  code)
• Loop scope (available only within a loop)

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Note the identifiers x and y are not available
to the main method.
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y
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Method Overloading

• Overloading declaring multiple methods of the
  same name, but with different formal parameter
  lists.
• When you call an overloaded method, the compiler
  knows which version you are calling based on the
  types of actual parameters you pass.

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Method Overloading(Listing 5.4)
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Frame Stack
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maxs Frame (int)
Frame Stack
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Frame Stack
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maxs Frame (Double)
Frame Stack
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Frame Stack
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Note here the result returned from a method is
the actual parameter of another method call.
maxs Frame (Double, 3 params)
Frame Stack
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maxs Frame (Double)
maxs Frame (Double, 3 params)
Frame Stack
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maxs Frame (Double, 3 params)
5.4
Frame Stack
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maxs Frame (Double)
maxs Frame (Double, 3 params)
Frame Stack
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maxs Frame (Double, 3 params)
Frame Stack
10.14
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Frame Stack