methods lecture

Methods and Parameters

Other languages use: subroutine, sub, function, procedure...

A method is a section of code with a name, eg:


private void myTask(){
    x=4;
    y=5;
}

To call it up (invoke it) - we put:

myTask();

private - the method can only be used within its class
void - the method does not use 'return' to pass data back to the caller

Passing parameters to the method

All programming languages have parameter-passing - v.important!

Example: A method which adds any two integers together, and leaves the result in sum. We declare it:

private void addUp(int a, int b) {
   sum=a+b;
}

a and b are termed the 'formal parameters'

Here are some invocations/calls:

int  d=7, e=3;
addUp(5,6);
addUp(d,4);
addUp(d+e, 1);

Note:

when we invoke a method, we provide 'actual parameters'
they must be the right type (int here)
they must be the same number of them (2 here)
they match up left to right with the formal parameters
the current values are copied into the formal names (e.g. 5 into a, 6 into b)
the values of the actual parameters are used inside the method

what values does sum take, above?

The layout of a class


class Fred.... {
    private int sum, n = 0;  // 'instance variables' -global to all methods of class

    public aMethod(){
        int n=3;
        addUp(n,6);
    }

    private void addUp(int a, int b) {
       sum=a+b;
    }

 }

private - can only be used from within the class
public - can be used from other classes

Scopes

Where names clash, local variables and formal parameters take precedence, e.g:

in aMethod, n is 3 (the global n still exists, as 0)
formal parameters (e.g a, b) - treated as locals

Analogy of parameters - hi-fi amp


          --------------------
          |                   |
      ----|CD-IN              |
 input    |                   |
          |       AMP         |----- to speaker....
 sockets  |                   |
          |                   |
      ----|TAPE-IN            |
          |                   |
           -------------------

The socket labels are the formal parameters
what type of cd, tape can we connect? - any models... (as long as the are the right sort of thing (e.g not a guitar)
the actual parameters might be Sony x123 cd, phillips 221 tape

Using 'return'

What if we want the caller of the method to choose where to put the result? here is addUp done this way:

private int addUp(int a, int b){
    int temp;
    temp=a+b;
    return temp;
}

here are some calls:

int e,f;
e=addUp(3,4);
f=5 + addUp(e, 2);
label1.setText("answer is " +  addUp(8,9));

addUp(5,6);  //ERROR- nowhere for returned value to go!

Imagine rubbing out the call and the actual parameters, replacing them by the result.
instead of void, we put the type of the returned item