1. CS2011 Introduction to Programming I Methods (I)
Chengyu Sun
California State University, Los Angeles

2. Example: Range Sum
Find the sum of integers from 1 to 10, 20 to 37, and 35 to 49
Without method
With method

3. Define A Method Method Header
public static int sum( int begin, int end ) {
int sum = 0;
for( int i=begin; i <= end; ++i )
sum += i;
return sum; }
Method Body

4. Method Header A.K.A. method signature Method name A list of parameters??
public static int sum ( int begin, int end )
Access modifier
(in this course
everything is public)
Type of the returned
value, a.k.a. return
type

5. Parameters Function as variables in the method
A.K.A. formal parameters
Some methods don't have parameters, e.g. length() in String

6. Return Value A method may return a value:
return <expression>;
A return statement immediately terminates
the execution of a method
A method may return no value, in which case the return type in the method header should be void
You can still use an empty return statement to end the method early
Example: void splitName(String name)

7. Common Mistakes Related to Return Value
Return value doesn't match return type
Not returning value in some branches
Try to return more than one value
Unreachable code

8. Call A Method A.K.A. invoke a method For example:
int n = sum( 20, 37 );
Values to assigned to the parameters (in order)
Actual parameters, a.k.a. arguments
Difference between calling a value returning method and a void method

9. Trace A Call
Use Eclipse's debugger to trace the TestMax example

10. Why Use Methods … (a) encapsulate reusable logic Examples Range sum
Math.sin(), Math.cos(), Math.random() …

11. … Why Use Methods …
(b) encapsulate certain concept or operation (and it makes code more readable)
Example:
if( year%4 == 0 && year%100 != 0 || year%400 == 0 )
if( isLeapYear(year) )

12. … Why Use Methods
(c) facilitate top-down programming – break down a big problem into smaller problems
Create a method for solving the big
problem, by calling methods that solve
each smaller problem.

13. Example: Factorial n! = 1 * 2 * 3 … * (n-1) * n
Terminate the method early when n < 0
Parameters and return value
Return an error value
Don't put too much in a method (also see the Range Sum example)
Easier to test
Maximize reusability

14. Example: Longest Palindrome Prefix/Suffix
Method naming conventions
Start with a lower-case letter
Usually starts with a verb
Usually starts with is for methods that return a boolean value