COE318 Lecture Notes Week 5 (Oct 3, 2011)

Transcription

1 COE318 Software Systems Lecture Notes: Week 5 1 of 6 COE318 Lecture Notes Week 5 (Oct 3, 2011) Topics Announcements Strings static and final qualifiers Stack and Heap details Announcements Quiz: Today! Midterm: Monday, October 17, ; You must monitor your Departmental . You have this account in addition to your Ryerson . <UserId>@ee.ryerson.ca.. Marks and announcements for this course are sent to this address. (You can, of course, have this automatically forwarded elsewhere if you wish. I do!) Cheating: Note that the absolute minimum penalty permitted by the University in the case of cheating or plagiarism in a lab is a mark of zero. Of course, depending on the circumstances, the penalty may be much more severe such as F in the course or even explusion from the University. More about Strings String objects are special in Java and are directly supported by the language.simply putting letters in double quotes ("like this") creates a String object. (There is no need for the word new.) Strings are immutable. There are many methods for String objects; the most commonly used include charat(int i): returns the character at position i of the String. (Counting starts at zero.) For example: String s = abcde ; System.out.println(s.charAt(2)); //prints 'c'

2 COE318 Software Systems Lecture Notes: Week 5 2 of 6 touppercase(): Creates a new String with the same letters as the original String but all converted to upper case. For example: String s = abcde ; System.out.println(s.toUpperCase());//prints ABCDE //Note that s is still abcdef tolowercase(): Creates a new String with the same letters but all lowercase. equalsignorecase(string s): Returns true if the 2 strings have the same letters irrespective of case; false otherwise. For example: abcd.equalsignorecase( abcd ) is true. length(): Returns the number of characters in the String. For example: String s = abcde ; System.out.println(s.length());//prints 5 The static qualifier A method or a variable may be declared static which makes it belong to the class rather than an individual object. Some classes, such as Math, consist only of static methods and constants such as Math.sin(angleInRadians)or Math.M_PI for the constant π. Recall that instance variables and non-static methods are meaningless unless they are used with an object of the class type. main(...) is a static method so it exists even if no objects exist. (Can yo see why it has to be this way?) But static variables and methods exist even if no objects of the class exist. For example, you cannot even create a Math object. (Writing new Math() causes a compiler error.) The static methods in Math are much like ordinary functions in C. If objects of the class type have been created, they all share the same static variables. If one object modifies a static variable, all of the objects of the class will see the new value. You can write a static method like this: public static double sindegrees(double degrees) { double π = Math.PI; return Math.sin(degrees/180 * π); (Yes, you can use Greek letters like π or Arabic oض or Hebrew א in Java. The char data type is in unicode, a 16-bit code that supports most of the world's alphabets.) Some common uses of static variables:

3 COE318 Software Systems Lecture Notes: Week 5 3 of 6 Keep track of number of objects created of a class Assign a unique ID for every created object Define constants used by all objects of the class An example is shown below: public class Resistor { public static int getnumrs() { return numrs; private double resistance; private static int numrs; private int id; public double getresistance() { return resistance; public Resistor(double resistance) { this.resistance = resistance; numrs++; id = numrs; public String getname() { return "R" + id; public static void main(string[] args) { Resistor w = new Resistor(10); System.out.println(w.getName() + " " + w.resistance); Resistor x = new Resistor(20); System.out.println(x.getName() + " " + x.resistance); Resistor y = new Resistor(50); System.out.println(y.getName() + " " + y.resistance);

4 COE318 Software Systems Lecture Notes: Week 5 4 of 6 The final qualifier When an instance or class variable is declared final it's value can only be set once; any subsequent attempt to change its value results in a run-time error (exception). Immutable instance variables should normally be declared final. Hence, immutable objects should have all their instance variables declared as final. Class variables (declared as static) used as constants should also be declared final. Here's an example taken from the source code for the Math class: public static final double PI = ; (Note: it is rare for constants not to be static.) Note also that PI is uppercase. It is a common convention to make public static final constants all uppercase. Later in the course (once inheritance and overriding have been covered), we shall see other uses for final : A final class cannot be subclassed. (The String class is final). A final method cannot be overridden in a subclass. More details: Stack and Heap Recall: local variables and parameters are allocated on the stack and exist only during the the time the method they are declared in is running; instance variables exist in the memory allocated on the heap for the object they belong to; they continue to exist until the object is garbage collected. Initial values: Initial value means the value assigned after a variable is declared but before it is explicitly set to a definite value. Local variables have random initial values. (This is why the compiler will complain if you try to use a local variable before you have set its value.) Instance variables have initial values of zero when an object is created. Why? Because the area in the heap allocated for the object they belong to has all of the bytes for the instance variables set to 0x00. This means that the initial values of all reference variables are null and the values for primitive data types are zero for numbers (byte, short, int, long, float and double), '\0' for chars and false for booleans. Consider now the following class:

5 COE318 Software Systems Lecture Notes: Week 5 5 of 6 public class G { private int id; public G(int id) { this.id = id; public void incr(int i) { id += i; public static void main(string[] args) { G g, h; int i = 3; g = new G(4); h = new G(8); g.incr(i - 2); i++; h.incr(i); System.out.println("g.id = " + g.id); System.out.println("h.id = " + h.id); Convince yourself that the output will be: g.id = 5 h.id = 12 The diagram below illustrates what the stack and heap look like when we are in the incr method called with g.incr(i 2). Note that there is a stack frame associated with the main method below the stack frame of the incr method. A method can only access local variables and parameters in its own frame. So incr cannot access the local variables of the main method.

6 COE318 Software Systems Lecture Notes: Week 5 6 of 6 Questions 1. What is the output of the following: System.out.println(("abcd".charAt(2) + "x").touppercase()); Answer: The output is: CX