Introduction to Methods

1
Introduction to Methods
2
Previously discussed

• There are similarities in make up of
• that can help you remember the construct of a
  class

• a class in the Java programming language     and
  
• a book chapter in the English language

3
Previously discussed (cont.)

• Here is a graphical depiction of the resemblance

4
Previously discussed (cont.)

• They are similar in the following manner

Class (in Java) Book Chapter (English)
A class contains a number of related methods A chapter contains a number of related paragraphs
A method contains a number of statements A paragraph contains a number of sentences
A statement is the smallest unit of execution in the Java language A sentence is the smallest unit of readable text in the English language
5
The 2 types (kinds) of methods in Java

• Older (non-object-oriented) programming languages
  have only one type (kind) of method
• Newer programming languages (so called
  object-oriented languages, explained later) have
  2 types (kinds) of methods

1. Class methods
2. Instance methods

6
The 2 types (kinds) of methods in Java (cont.)

• We will now study the class methods
• We will delay the discussion of the instance
  methods for later (when we discuss abstract data
  types).

7
Why use methods in a computer program

• Convenience of methods

• A statement can only accomplish a very small
  amount of work
• A method contains multiple statements.
  Therefore, a method is a more useful unit of
  program execution

8
Steps in using methods in a Java program

• How to use methods in a Java program

• First, you must define the method.
• How to define a method

• Write down the steps ( statements) contained in
  the method.
• Attach a name to the steps ( statements)

9
Steps in using methods in a Java program (cont.)

• Notes

• You only need to define a method once
• (Remember that in Java, you must define the
  method inside some class.)

10
Steps in using methods in a Java program (cont.)

• 2. After defining the method, you can then call
  a method using the name of the method
• This mechanism is called method invocation (an
  older term is procedure call)

• When a method is called, the statements inside
  the corresponding method are executed
• When all statements in the method has been
  executed, the execution will resume at the
  program location of the method call

11
Steps in using methods in a Java program (cont.)

• Note

• You can invoke a method as many times as you
  wish       

12
Example on how to use method find the minimum of
2 floating point numbers

• We have previously seen an algorithm to find the
  smaller of 2 values x and y (see
  http//mathcs.emory.edu/cheung/Courses/170/Syllab
  us/07/while2.html)

if ( x lt y ) min x // x is
the smaller value else
min y // y is the smaller value
13
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• We can create a method to perform the task of
  "finding the minimum of 2 value as follows
• Remember a method is always contained inside
  some class in Java

• Write down the statements in the algorithm
• Attach a name (e.g. min) to the statements

14
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• Example

public class ToolBox // Methods must
be contained inside a class /
----------------------------------------
A method called "min" that contains
the statements to find the smaller of
2 values a and b -----------------------
---------------- / public static double
min ( double a, double b )
double min 0
15
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• if ( a lt b )
• min a // a is the smaller value
  
• else
• min b // b is the smaller value
• return(min) // Output of the method
• ...
• (You can define more methods inside a
  class)

16
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• Explanation

• The name of the method is min
• The method is contained inside the class named
  ToolBox
• The complete name of the "min" method is

ToolBox.min
17
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• How to use the method ToolBox.min

public class MyProgram public
static void main(String args)
double r r ToolBox.min( 1.0,
4.0 ) System.out.println(r)
r ToolBox.min( 3.7, -2.9 )
System.out.println(r) r ToolBox.min(
-9.9, 3.8 ) System.out.println(r)

18
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• Notice the advantage of using methods

• A non-savvy user that wants to use the
  ToolBox.min method does not need to know the
  statements contained inside the method
  ToolBox.min !!!
• A non-savvy user will only need to know the
  following in order to use the method

1. The (complete) name of the method (i.e.
   ToolBox.min)
2. What information the method needs to do the task
   (i.e. 2 numbers)
3. What information the method returns to the user
   (i.e. 1 number)

19
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• Notice the advantage of using methods

• In fact, you were a non-savvy user of the
  methods in Java's Scanner class
• E.g. You have used nextDouble() without knowing
  exactly what statements this method contains !

20
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• Example Program (Demo above code)
• Prog file http//mathcs.emory.edu/cheung/Courses
  /170/Syllabus/08/Progs/1/MyProgram.java
• File containing the min method
• http//mathcs.emory.edu/cheung/Courses/170/Sylla
  bus/08/Progs/1/ToolBox.java
• How to run the program            

• Right click on both links and save in a scratch
  directory
• To compile   javac MyProgram.java
• To run          java MyProgram

21
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• Output

1.0 -2.9 -9.9
22
Example on how to use method find the minimum of
2 floating point numbers (cont.)

• A note on the Java compiler

• The Java compiler can detect that the program
  MyProgram.java uses a method ToolBox.min from the
  class ToolBox
• The Java compiler will try locate the file
  ToolBox.java and compile it automatically
• So you do not need to compile the Java program
  ToolBox.java explicitly --- the Java compiler
  will do that for you.

23
Method call (invocation) and return

• 64,000 Question

• What happens in a method invocation ???

24
Method call (invocation) and return (cont.)

• We must first explain the effect of a return
  statement

public class ToolBox // Methods must be
contained inside a class /
----------------------------------------
A method called "min" that contains
the statements to find the smaller of
2 values a and b -----------------------
---------------- / public static double
min ( double a, double b )
double min 0 if ( a lt b )

25
Method call (invocation) and return (cont.)

• min a // a is the smaller value
• else
• min b // b is the smaller value
• return(min) // A "return"
  statement
• ...
• (You can define more methods inside a
  class)

26
Method call (invocation) and return (cont.)

• Syntax of a return statement

form 1 return form 2 return EXPRESSION
27
Method call (invocation) and return (cont.)

• Effect of a return statement

• The return statement is used to terminate the
  execution of a method.
• If a return statement returns an EXPRESSION,
  then the value ( result) of the EXPRESSION will
  be used to replace the method call at the point
  of call.

• When the program executes a return statement,
  the method terminates (or exits)
• The execution will continue at the location
  where the method was called

28
Method call (invocation) and return (cont.)

• What happens in a method invocation

• Java program

public class MyProgram public
static void main(String args)
double r r ToolBox.min( 1.0,
4.0 ) // Invokes the "ToolBox.min" method !
System.out.println(r) r
ToolBox.min( 3.7, -2.9 )
System.out.println(r) r ToolBox.min(
-9.9, 3.8 ) System.out.println(r)

29
Method call (invocation) and return (cont.)

• The ToolBox.min method

public class ToolBox // Methods must
be contained inside a class /
----------------------------------------
A method called "min" that contains
the statements to find the smaller of
2 values a and b -----------------------
---------------- / public static double
min ( double a, double b )
double min 0 if ( a lt b )
min a // a is the smaller
value else
min b // b is the smaller
value return(min) //
Return statement
30
Method call (invocation) and return (cont.)

• Execution of the program

• Program starts executing in the main method

31
Method call (invocation) and return (cont.)

• The method invocation ToolBox.min(1.0, 4.0)
  transfers the program execution to the method
  ToolBox.min

32
Method call (invocation) and return (cont.)

• The min method executes and reaches the return
  statement

33
Method call (invocation) and return (cont.)

Notice that the variable min 1.0 when the
return statement is executed.
34
Method call (invocation) and return (cont.)

• The return statement transfers program execution
  back to the point of invocation

35
Method call (invocation) and return (cont.)

• After the method returns, the returned value
  (1.0) will effectively replace the method
  invocation
• In other words, the assignment statement that
  will be executed is

r 1.0
36
Method call (invocation) and return (cont.)

• Bottom line

• The variable r will be assigned with the minimum
  of the 2 values passed to the ToolBox.min method
  !!!

37
Defining a class method

• Syntax used to define a method

38
Defining a class method (cont.)

• Note the construct must appear inside some
  class, so it will look like this

39
Defining a class method (cont.)
Explanation

• The keyword  public  tells the Java compiler that
  we are defining something that had no access
  restriction
• The "something" can be one of two things
• In other words, you can only define 2 kinds of
  things inside a class methods or variables

• A method,    or  
• A variable

40
Defining a class method (cont.)

• The keyword  static  tells the Java compiler that
  we are defining a class typed method (or
  variable).
• Note

• If you omit the keyword static, you will define
  an instance method (or variable) which will be
  discussed later.

41
Defining a class method (cont.)

• The RETURN-TYPE is a Java data type (e.g., int,
  double, etc.)

• The RETURN-TYPE specifies the type of the data
  that is returned by the method.

42
Defining a class method (cont.)

• The method-Name is an identifier that is used to
  identify this method.

• You use the method-Name to identify the method
  in the method invocation

43
Defining a class method (cont.)

• The pair of brackets ( .... ) tells the Java
  compiler that you want to define a method.

• You can in fact define 2 different things inside
  a class
• We will discuss variables defined inside a class
  later in the course.

• methods
• variables    

44
Defining a class method (cont.)

• A definition without the brackets ( ... ) will
  define a variable !!!
• Notice how the Java compiler can tell the
  difference

45
Defining a class method (cont.)

• The FORMAL-PARAMETER-LIST is a comma-separated
  list of parameter variables that is passed (
  given to) the method as additional information

• The items in the FORMAL-PARAMETER-LIST has the
  following form

TYPE variable-name
46
Defining a class method (cont.)

• A formal parameter variable is initialized with
  the value given in the method call
• It is a way to pass information to the method so
  it can use the information to perform its
  designated task

47
Defining a class method (cont.)

• The method body completes the method definition
  The method body is a block (enclosed between
  ... )
• Within the method body, you can write any number
  of the following things

• statements
• variable definitions --- variables defined
  inside a method body are called local variables
      

48
Defining a class method (cont.)

• The method header

• The method header is the first part of the
  method definition without the method body

49
Defining a class method (cont.)

• Example

50
Alternative example on defining the min method

• In the previous example, we put the definition of
  the min method inside a new class named ToolBox
• Alternatively, we can define the min method
  inside the same class as the main method - like
  this

public class MyProgram public
static double min ( double a, double b )
double min 0 if ( a lt b )
min a // a is the
smaller value
51
Alternative example on defining the min method
(cont.)

• else
• min b // b is the smaller value
• return(min) // Return statement
  
• public static void main(String args)
  
• double r
• r MyProgran.min( 1.0, 4.0 ) // The
  name is now "MyProgram.min" !
• System.out.println(r)
• r MyProgram.min( 3.7, -2.9 )
• System.out.println(r)

52
Alternative example on defining the min method
(cont.)

• Note

• Because the definition of min method is contained
  inside the class MyProgram, the name the method
  is now
• We must use this new name to invoke the min
  method.

• MyProgram.min

53
Alternative example on defining the min method
(cont.)

• Example Program (Demo above code)
• Prog file http//mathcs.emory.edu/cheung/Courses
  /170/Syllabus/08/Progs/2/MyProgram.java
• How to run the program            

• Right click on both links and save in a scratch
  directory
• To compile   javac MyProgram.java
• To run          java MyProgram

54
A short hand for invoking a method defined inside
the same class

• Notice that in this example

public class MyProgram public
static double min ( double a, double b )
double m 0 if ( a lt b )
m a // a is the smaller
value else
m b // b is the smaller
value
55
A short hand for invoking a method defined inside
the same class (cont.)

• return(m) // Return statement
  
• public static void main(String args)
  
• double r
• r MyProgram.min( 1.0, 4.0 ) // The
  name is now "MyProgram.min" !
• System.out.println(r)
• r MyProgram.min( 3.7, -2.9 )
• System.out.println(r)
• r MyProgram.min( -9.9, 3.8 )
• System.out.println(r)

56
A short hand for invoking a method defined inside
the same class (cont.)

• contains 2 methods
• The method main invokes (calls) the method min.

• min
• main   

57
A short hand for invoking a method defined inside
the same class (cont.)

• Short hand method invocation

• We can reference a method defined inside the
  same class without using the class name      

58
A short hand for invoking a method defined inside
the same class (cont.)

• Example

public class MyProgram public
static double min ( double a, double b )
double m 0 if ( a lt b )
m a // a is the smaller
value else
m b // b is the smaller
value
59
A short hand for invoking a method defined inside
the same class (cont.)

• return(m) // Return statement
• public static void main(String args)
  
• double r
• r min( 1.0, 4.0 ) //
  Shorthand name "min" used
• System.out.println(r)
• r min( 3.7, -2.9 )
• System.out.println(r)
• r min( -9.9, 3.8 )
• System.out.println(r)

60
A short hand for invoking a method defined inside
the same class (cont.)

• Example Program (Demo above code)
• Prog file Scope1.java http//mathcs.emory.edu/ch
  eung/Courses/170/Syllabus/08/Progs/3/MyProgram.jav
  a
• How to run the program            

• Right click on link and save in a scratch
  directory
• To compile   javac MyProgram.java
• To run          java MyProgram

61
Organizing methods in classes

• Fact
• But

• A method can be placed in any class

• Imagine placing books on a number of shelves

• You can place any book on any shelf
• Then it very difficult to find a desired book
  !!!

62
Organizing methods in classes (cont.)

• Advice to allow easily location of methods

• Place related methods inside the same class
• Give the class a meaningful name
• Give each method a meaningful name also

63
Organizing methods in classes (cont.)

• Example from Java

• The sin, cos, tan, log, exp, sqrt, etc methods
  are all defined inside the same class
• The name of this class is Math
• That's why we use
• to invoke the sqrt method !!!

Math.sqrt( ... )
64
Remaining topics in Methods

• We still need to discuss some important topics
  related to methods

1. The scope and lifetime of local variables
2. How parameters are passed to methods.