Transcript 09.0.4_Java02_Modules

Modules (Methods)
Functions and Procedures
Parameters...Output
Procedures and Functions
• Some of our pseudocode rules translate into
firm rules in Java:
– Functions return a single value
• Value returned can be any complex data type
(including an Object)
– Procedures can “return” information by changing
an object that is passed in via a parameter.
• Other rules are style issues:
– Functions having no side effects
– Functions not doing I/O
Java Methods
Single construct for both procedures and functions:
•
When a function is called for, specify the appropriate return
type before the method name
public float average (float fNum1, float fNum2)
{
float fReturnVal;
fReturnVal = (fNum1 + fNum2)/ 2;
return fReturnVal;
} // of average
•
to specify a procedure, make the return type void
(More later…)
Writing Methods
A Java requirement:
--All methods belong to an object (or class).
--Name of object (or class) must be
unambiguous when the method called.
--To run a program, there must be a class
(whose name is the name-of-the-program),
containing a special method called main:
visible
to all
for command
line parameters
nothing
returned
public static void main (String[ ] argv)
a class
method,
not an
instance
method
Method name
Method Overloading
• Sometimes more than one method is required to do the
same job.
• In Pseudocode, we frequently used a helper module
Procedure Convert(Lhead iot in Ptr toa LLNode)
Procedure ConvertHelper(Lhead iot Ptr toa LLNode,
NewHead iot Ptr toa LLNode)
• In Java, multiple methods can have the same name as
long as the number, type or order of their
parameters is different
public void Convert(LLNode Lhead)
public void Convert(LLNode Lhead, LLNode NewHead)
Method Signatures
• “The signature of a method consists of the name of the method and the
number and types of formal parameters to the method. A class may not
declare two methods with the same signature, or a compile time error
occurs.”
Java Language Specification s.8.4.2
• Method overloading occurs where identically named methods have
different parameter (number, type or order NOT name!)
public int getCube(int iNum){
return iNum * iNum * iNum;
}
public int getCube(float fNum){
return (int)(fNum * fNum * fNum);
}
public int getCube(double dNum){
return (int) (dNum * dNum *dNum);
}
Methods: Common Mistakes
public float average (float fNum1, float fNum2, float fNum3);
{
float fReturnVal;
fReturnVal =
(fNum1 + fNum2 + fNum3)/ 3;
return (fReturnVal);
} // of average
Note ending semicolon
-- results in unhelpful error message
Parameters
• Java only has in parameters
• Initially this will appear to be a big
change. It is a common feature in many
languages.
• We can get information out of a module via
two techniques
– Return something from a function
– Pass in a reference to some object and inside the
method modify the object [side effect]
Printing to Screen
• Pseudocode:
print (<arguments>)
• Java:
System.out.print(<A String>);
System.out.println(<A String>);
Inserts a newline after the
printing is complete
Printing to Screen
int x = 10;
System.out.println(5);
System.out.println( );
System.out.println(“Hello World”);
System.out.println(“x = “ + x);
System.out.println(“y = “ + 5);
System.out.print(“All on one “);
System.out.println(“line”);
5
Hello World
x = 10
y = 5
All on one line