11 Introduction to Methods

Size: px

Start display at page:

Download "11 Introduction to Methods"

Spencer Barrett
6 years ago
Views:

1 11 Introduction to Methods The final compulsory section. The idea of methods are essential to good programming and you will make use of them in the next checkpoint Introduction You have been making extensive use of Objects and Methods in your programs, for example Display input/output, manipulating strings and calculating mathematical operations and creating graphs with SimpleGraph. So far all the Objects and their associated methods have been supplied and you have just invoked them. The real power of programming is when you create your own Objects and Methods where the Methods perform certain tasks on the Objects. This short section concentrates on static methods associated with the main program and leaves the creation and manipulation on new Objects to the next optional section of the course or future computing courses 1. A static method is self contained piece of JAVA code that takes parameters, does a calculation and returns a value. You have been using such methods for example Math.sin or Math.sinh. Such methods are frequently used in scientific and numerical calculation and is the first step towards modularisation of your program and structured programming An Example static Method We want to write a method to evaluate a quadratic of the form so we need a method that 1. Takes the value 4 doubles as arguments. ax 2 + bx + c 2. Returns the value of the equation, as a double. We can write this as the method quadratic() which can then be accessed from the main() program as follows: public class QuadCalc { // (1) static double quadratic(double a, double b, double c, double x){ // (2) double value = a*x*x + b*x + c; // (3) return value; // (4) public static void main(string args[]){ // (5) double x = 10; // (6) double calculatedvalue; // (7) calculatedvalue = quadratic( 2.3, 4.6, 3.1, x); // (8) 1 Computer Simulation or Computational Methods in Physics Junior Honours.

2 System.out.println("Value is : " + calculatedvalue); // (9) Lets go through this line by line: (1) Declares the class QuadCalc which contains methods quadratic and main. (2) Declares a method quadratic that take 4 doubles as arguments and returns a double. (Note the static 2 keyword, but no public, so it is only visible from within class QuadCalc). (3) Declares a local variable value inside quadratic, and sets it value to a arithmetic combination of the value of the parameters. (4) return this value and end the method. (5-7) Usual start to a main, and declare two variables. (8) Invoke the method quadratic with 4 doubles are arguments and and use it returned value to set calculatedvalue. (9) Print out value (in this case 279.1). The arguments can be any combination of objects, at this stage in your programming they will usually be int, double or Strings Arrays and Methods The use of arrays and methods takes a little more care. At is simplest a method can have an array as a parameter or can return an array as show in the example below. public class ArrayTest { static double[] makeandfillarray(int length) { double array[] = new double[length]; array[i] = (double)i; return array; static double arraysum(double array[]) { double sum = 0; sum += array[i]; 2 Static means that the method is not associated with an Object, more on this in the next optional section.

3 return sum; public static void main(string args[]){ double arrayofdoubles[] = makeandfillarray(10); double sum = arraysum(arrayofdoubles); System.out.println("Vector sum is : " + sum); The makeandfillarray method takes int as its parameters and being declared as double[] return a double array, in this case filled with 1.0,2,0,... The arraysum takes a double[] as parameter and return the sum of the elements as a double. Note that.length holds the length of the array which can be used inside the method. The two method can then be used by the main program exactly as expected. The conceptual problem comes when an array is passed to a method as a parameter and the method changes the values of the elements. In this case the elements of the array also change in the main program. Lets modify the above method makeandfillarray to a fillarray as detailed below. public class ArrayTest { static void fillarray(double array[]) { array[i] = (double)i; static double arraysum(double array[]) { double sum = 0; sum += array[i]; return sum; public static void main(string args[]){ double arrayofdoubles[] = new double[10]; fillarray(arrayofdoubles); double sum = arraysum(arrayofdoubles); System.out.println("Sum is : " + sum); // Declare array // Set it value // Check its value The method fillarray takes a double[] as a parameter, but, unlike single object, the elements of the array passed become common with the main program so that changes to the

4 elements inside the method will change the element values in the main program. Type in the above program, it prints Sum is : 45.0 exactly as the one above. Use this as a feature at this stage of your programming career, it is actually perfectly logical but you need to understand a bit more about the language before we can cover this. Overload of Methods This is now a most convenient time to introduce the idea of method overload where two, or more, methods have the same name but different parameter lists. The reason that this works is because in JAVA unlike other languages, the method signature in a combination of its name, parameter list and return type. Again this is best illustrated by an example or a better fillarray method, begin: static double[] fillarray(double array[]) { array[i] = (double)i; return array; static double[] fillarray(int length) { double array[] = new double[length]; return fillarray(array); where we have now two methods 1. takes a double[] as argument, fills its elements, and returns the same array. 2. takes an int, allocates a double[] of the correct length, calls the first method to fill its elements, and then returns the double[]. These two methods of the same name can now be used for the same task, that of filling an array with numbers as follows double firstarray[] = new double[20]; // Declare and allocate firstarray = fillarray(firstarray); // Fill using method 1 double secondarray[] = fillarray(30); // Declare, allocate/fill // using method 2 This is an extremely powerful and useful technique where methods that do the same task can be given the same name so making the final code much easier to read. If you did not understand this last section, don t worry, you will come back to it later main() as a Method The observant will notice that the

5 public static void main(string args[]) that is required at the start of your main program looks just like a method, correct, it is a method executed when the program is run. The declaration is now clearer with: 1. public meaning that it can be accessed from outside the class that contains it. 2. static means it is not associated with actions on an object but is a stand-alone method. 3. void means that is does not return anything. 4. main the name of the method automatically invoked. 5. String args[] the parameter list, which is an array of Strings which are passed from the terminal command line when the program in executed. This is an alternative method of inputting data into to program. The command line options are simply accessible as an array of Strings. Considering the simple program below. public class ArgumentTest { public static void main (String args[]) { for(int i = 0; i < args.length; i++) { System.out.println("Argument " + i + " is : " + args[i]); If you execute this with. java ArgumentTest Hello World will result in output: Argument 0 is : Hello Argument 1 is : World showing that args[0] has value "Hello" and args[1] has value "World", which have been passed in from the command line. This is the standard scheme by which command line options are passed to programs on execution. You will see this again in future courses. Examples Source code for the following on-line examples are available, Program with static method quadratic QuadCalc. Examples of using arrays with static methods ArrayMethods. Reading arguments from the command line, (simple test) ArgumentTest. Reading arguments from the command line, used to size Display PanelSizer.

6 What Next? You have now completed all the basic JAVA you require for the final compulsory checkpoint, which you should now attempt. There is an (optional) section on OBJECT which is a very basic introduction to Object Orientated Programming.

CS171:Introduction to Computer Science II

CS171:Introduction to Computer Science II Department of Mathematics and Computer Science Li Xiong 9/7/2012 1 Announcement Introductory/Eclipse Lab, Friday, Sep 7, 2-3pm (today) Hw1 to be assigned Monday,