1. Methods Coding in Java
Copyright © Curt Hill

2. What is a Method? A chunk of executable code
It always has four things:
A return type
A name
A parameter list
A compound statement
It may optionally have:
Visibility
Static or other attributes
The main method is an example
Copyright © Curt Hill

3. Main method
Our main method is an example public static void main (String [] a) {…}
public is visibility
void means that no value is returned
main is the method name
(…) contains parameters
{…} contains declarations and executables
Copyright © Curt Hill

4. The Return Type May be any of the primitives May be any of the objects
int, double, char, boolean
May be any of the objects
Turtle, World, Color, even arrays
May also be void
Reserved word indicating no value is returned
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5. The Name
Any Java name
The convention is that it starts with a lower case letter
Each letter that starts a word is then capitalized
Examples:
forward
moveTo
setPenColor
This is just a convention, but a good thing for you to follow
Copyright © Curt Hill

6. The Parameter List
A list of values that needs to be made available to the method
The parameter list is enclosed in parentheses
The parentheses are always present, even if there are no parameters
The parameters are denoted by a type followed by its local name
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7. Parameters A method may have multiple parameters
Each has same form: type localname
These pairs are separated by commas
A Turtle method signature: void forward(int steps) { … }
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8. The Compound Statement
The body of the method is a compound statement
Starts with a brace {
One or more statements
Ends with other brace }
Every opening brace { must be matched by a closing brace }
The parameter is part of the compound statement for scope considerations
Copyright © Curt Hill

9. Method header
That part of the method excluding the compound statement is called the method header
This is the part usually shown in documentation
Such as the doc directory within the bookClasses directory
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10. Calling a Method Multiple parameters are separated by commas
The general form is: variable.method(parm list)
This may be a stand-alone statement
yertle.forward(150);
It may be part of a statement:
x = y + Math.sqrt(3.5);
Multiple parameters are separated by commas
The names do not have to match
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11. The end of a method There are two ends to consider The syntax
The execution
As far as syntax is concerned a method ends when we find the closing brace of its compound statement
In this sense a method is only processed once
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12. Ending Execution A method ends when one of two things happens:
A return is executed
The last line of the method is executed
This only works on void methods
Return is the mechanism for identifying the value
With void functions there is no value to identify
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13. The return statement Has two effects
Ends the function and returns to calling statement
Determines the value that needs to be returned
Form is: return expression;
Expression should match return type
Copyright © Curt Hill

14. Identifying Parameters
The parameter described in the method header is called the formal parameter
The parameter used in the call is the actual parameter
The names of the two do not need to match
Copyright © Curt Hill

15. Formals and Actuals A formal parameter is i void print(int i);
Three actual parameters int j=5,k=2; … System.out.print(j); System.out.print(12); System.out.print(k*-j+1);
Formals and actuals should match in type
Copyright © Curt Hill

16. Example Method static void triangle( Turtle t){ t.forward(50);
t.turn(120); }
Copyright © Curt Hill

17. Called by public static void main( String [] a){
World w = new World(400,240);
Turtle yertle = new Turtle(w);
yertle.setPenWidth(3);
triangle(yertle);
Turtle sam = new Turtle(100, 100, w);
sam.setPenWidth(4);
triangle(sam); }
Copyright © Curt Hill

18. Gives Results
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19. Example commentary
The name of the formal and actual parameters did not have to match
This allows the method to make any turtle draw, not just one
This or any method may be called as many times as desired
With the same or different parameters
Copyright © Curt Hill

20. Why is method static? static is a Java keyword
For methods it means that the method has no access to class instance data
It must have access to local data
Since they have no access to class data they may be called before the constructor
They may only use their parameters and local data
Copyright © Curt Hill

21. Static again The main function is a static function
It may actually instantiate a class of the type to which it belongs
A static method may only call other static methods unless it instantiates an instance of its own class
Thus triangle needed to be static
Copyright © Curt Hill

22. Why use static?
Suppose: public class Demo{ public static void main (String s[]){ … } }
To use a non-static method we would need: Demo t = new Demo();
Copyright © Curt Hill

23. The Non-static equivalent
We can do this without static methods but it is slightly more work
Consider the next three screens
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24. Notice class name public class TurtleMethod2 { /**
* Non-static methods */
void triangle(Turtle t){
t.forward(100);
t.turn(120); }
Copyright © Curt Hill

25. New main method public static void main(String[] args) {
TurtleMethod2 item =
new TurtleMethod2();
World w = new World(850,500);
Turtle yertle = new Turtle(w);
Turtle sam = new Turtle(700,450,w);
yertle.setPenWidth(3);
item.triangle(yertle);
sam.setPenWidth(4);
item.triangle(sam); }
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26. Commentary The result is the same What are the differences?
Leave static off of triangle method
Create a TurtleMethod2 item using new
Prefix triangle with the TurtleMethod2 object, which is named item
Copyright © Curt Hill

27. Parameters Again The method needs some refinement
It always draws the same size triangle
It becomes more helpful if the length of a side can be changed
This is a second parameter
Copyright © Curt Hill

28. Second Example static void triangle( Turtle t, int len){
t.forward(len);
t.turn(120); }
Copyright © Curt Hill

29. Called by public static void main( String [] a){
World w = new World(400,240);
Turtle yertle = new Turtle(w);
yertle.setPenWidth(3);
triangle(yertle,80);
Turtle sam = new Turtle(100,100,w);
sam.setPenWidth(4);
triangle(sam,40); }
Copyright © Curt Hill

30. Gives Results
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31. Method Calls Why does triangle not have an object in front?
What is difference between: yertle.forward(20); triangle(yertle,50);
We do not need the object for triangle because the object is the program
If we were in Turtle we could just say forward without the prefixed Turtle name
Copyright © Curt Hill

32. Method Execution When a method call is encountered what happens?
The actual parameters are evaluated
The values are connected to the formal parameters
The calling method is suspended and the method is started
All local variables are created
The method is executed
The return ends the method choosing the returned value
The caller is activated with the returned value
Copyright © Curt Hill

33. Name Overloading
In many languages such as C, Pascal, VB a function is defined by one thing only: the function name
C++ introduced function name overloading
It determines the function to use by examining the signature
Java does the same
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34. Method Signatures
A method is uniquely determined by two things: the name the parameter list
The contribution of the parameter list does not include the parameter names, only number, type and order
These are collectively called the signature
Two signatures within a class may not be the same
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35. Multiple Triangles
There is no conflict with having both triangle methods in one program
The compiler can tell the difference because one has two parameters and the other one
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36. Why use methods? Two good reasons
Several others besides
Allows one instance of identical or similar code
We may now draw a triangle as easily as we could draw a line before
With any size, any turtle
Allows simplification of main program
Five or more statements replaced by one
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37. Finally There is plenty more to know
This will come in several subsequent presentations
We should do a demo
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