Lecture 11. Example 1. Introduction. Method definition and invocation. Parameter passing: value vs. reference parameters. Scope and Lifetime.

Transcription

1 Lecture 11 Example 1 Method definition and invocation. Parameter passing: value vs. reference parameters. Scope and Lifetime. Constructors Material from Holmes Chapter 6: sections 1 through to 8, except section Introduction By now you know how to write small programs. A new twist in your understanding is needed to learn how to write bigger chunks of code. In the Object-oriented paradigm a broad programmed activity should be thought of as formed by a number of building blocks known as methods, grouped into classes. (Analogy) A factory that employs a number of worker each with her/his own skills and these skills are combined to perform a major task. Example 2. The production of a car is not thought of as a flat sequence of operations do this, then that, then repeat this several times, then do that, etc but split into large independent parts assemble the engine, build the frame, etc which have no time dependence on one another, then each part is like a program but the single instructions are calls to the execution of a task (method) by a worker. 2 4

2 Advantages Creating a system in terms of classes and methods is more natural, because life is formed by entities that interact by performing actions. Testing single classes (in isolation) is easier than dealing with the whole system. From the programming point of view, the final code layout is neater. Well-defined classes will often be re-usable in several systems. Methods We first concentrate on methods. Later we will talk about classes in more details. A method is a group of self-contained declarations and executable statements that performs a specific task or operation. I will often use the words function or operation as synonyms of method. I may seldom use the word procedure to refer to those methods that do not produce an explicit result, but only trigger a change of state in the system. Examples. Methods in the Math or String classes. 5 7 Disadvantages Hardly ever a system can be defined in terms of truly independent classes. Organising things in classes and methods is not natural for people with some experience in traditional programming. Testing one such a system for correctness is even harder. Theoretical results state that one cannot prove rigorously that a program meets its specifications. Parts of the system can be tested and an intuitive confidence about the program behaviour can be reached. For object-oriented programs it is even harder to reach this confidence. Programmer-defined Methods // chap_6\ex_1.java // program to demonstrate how to call a programmer-defined method class Ex_1 { System.out.println("Program execution starts at the [main] method;"); display(); System.out.println("returned to the [main] method."); // method to display a message static void display() { System.out.println("method [display] has been called;"); How does it work? Let s go through each step slowly 6 7-1

3 Method definition modifiers type identifier ( parameter-list ) declarations statements. In the example the modifier was static (imposing the method to be invoked only with the owner class name). Methods come in two flavours : those which return/produce a value (e.g. Math.abs()), and those that perform some action/change in state without producing a specific value (e.g. System.out.println()). The word type stands for the type of the value (result) that the method will return. Keyword void for methods returning NO value. The parameter-list identifies the list of data needed by the method to perform its task.. The invocation to sum() in the main program creates a new execution environment with its own memory, allocates space in this memory for the variables defined inside the method and starts executing the method s code. If you are doing a trace this is like adding a new sheet of paper... The statement return expression is used to assign a value to the method and return the control to the calling method. If the method type is void there is no need to use a return statement, but one may be used to force the control back to the calling method. It is syntactically legal to have many return statements inside a method... but not a very good practice for maintainance as it may become difficult to trace all of them. 8 9 The return Keyword // chap_6\ex_2.java // program to demonstrate calling a method that returns a value class Ex_2 { static BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in)); System.out.println("Sum of numbers is " + sum()); static int sum() throws IOException { int first, second; // numbers to input System.out.print("First number? "); System.out.print("Second number? "); second = new Integer(keyboard.readLine()).intValue(); return first+second; Parameters Methods can receive data from the calling method in several ways: 1. global variables; 2. (value) parameters; 3. reference parameters. We will look at the same piece of code

4 // chap_6\ex_2_global.java // program to demonstrate calling a method that returns a value // using global variables. class Ex_2_global { static BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in)); static int first, second; // numbers to input System.out.print("First number? "); System.out.print("Second number? "); second = new Integer(keyboard.readLine()).intValue(); System.out.println("Sum of numbers is " + sum()); System.out.println("First number is now " + first); // chap_6\ex_2_value.java // program to demonstrate calling a method that returns a value // using value parameters. class Ex_2_value { static BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in)); static int first, second; // numbers to input System.out.print("First number? "); System.out.print("Second number? "); second = new Integer(keyboard.readLine()).intValue(); System.out.println("Sum of numbers is " + sum( first, second)); System.out.println("First number is now " + first); static int sum() { first = first + second; return first; static int sum( int f, int s) { f = f+s; return f; The static modifier was added to the declarations of first and second otherwise Java would complain as follows: Value parameters are normally used for variables of primitive data type. Ex_2_global.java:17: Can t make a static reference to nonstatic variable first in class Ex_2_global. ˆ The actual parameter list must contain the same number of elements, in the same order and of the same type as the parameter list in the method definition. Question: what happens if you move the definitions of first and second inside main? Question: what happens if you called the formal parameters first and second? 11 12

5 // method to display the contents of an array static void display(int[] array) { int lengthofarray = array.length; int i; Reference Parameters A second way of passing arguments to a called method relates to those items of data that are stored by reference. The actual address of the variable is passed. Any change that happens to the parameters inside the called method will remain after the control is returned to the caller. for ( i = 0; i < lengthofarray; i++) System.out.print(array[i] + " "); System.out.println(); // chap_6\ex_4.java // program to demonstrate reference parameters for an array class Ex_4 { int[] array = {0,0,0,0,0; System.out.print("Array before call "); convert(array); System.out.print("Array after call "); display(array); display(array); // method that changes the value of each cell in the array static void convert(int[] array) { int lengthofarray = array.length; int i; Trace it! The use of display has reduced the amount of code! String objects are immutable. Operations (e.g. tolowercase() or touppercase()) that change a string actually have to define a new, modified, string. Use the class StringBuffer. for ( i = 0; i < lengthofarray; i++) array[i] = 2*i;

6 // chap_6\ex_5.java // program to demonstrate reference parameters class Ex_5 { StringBuffer data = new StringBuffer("abracadabra"); System.out.println("Data before call " + data); convert(data); System.out.println("Data after call " + data); // method to change every character in the array to upper case static void convert(stringbuffer data) { int lengthofstring = data.length(); int i; Give an example of block scope (local variable) or class scope (global variable). Formal parameters can be thought of as having block scope. for ( i = 0; i < lengthofstring; i++) data.setcharat( i, (char)(i + 65)); Scope and Lifetime of Identifiers 1. The scope of an identifier refers to the region of a program in which an identifier can be used. 2. An identifier with class scope is accessible from its point of declaration throughout the entire class and anywhere in the program where an object X of the given class is defined, prepended by the prefix X.. 3. A block is any segment of code beginning with a a. and ending with 4. An identifier with block scope is accessible from its point of declaration throughout the entire block in which it is defined. The lifetime of an identifier is the period during which the value of the identifier exists in the computer memory. static identifiers live throughout the program. identifiers with block scope only exist during the execution of the particular block. When an object goes out of scope garbage collection takes place

7 Constructors The instantiation of an object is the allocation of memory for storing the object s data and the initialisation of this memory space with appropriate values. Object instantiation is performed through the constructor methods. A constructor is given the same name as the class to allow for the data type of objects to be declared. A constructor is normally used in conjunction with the keyword new which allocates memory space from the heap. A constructor provides the storage in memory and the initialisation of the instance variables allocated to the object. For each separate invocation of the constructor a new object become instantiated. 18