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Chapter 5

Chapter 5. Feb 08, 2008. Chapter 5. 1. Methods in a class are invoked using objects A a1 = new A(); a1.func1(); Calling object and the dot can be omitted if the method is called within the same class. func1(); //same as this.func1() ‏. Chapter 5. 2. null Constant.

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Chapter 5

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  1. Chapter 5 Feb 08, 2008 Chapter 5 1

  2. Methods in a class are invoked using objects • A a1 = new A(); • a1.func1(); • Calling object and the dot can be omitted if the method is called within the same class. • func1(); //same as this.func1()‏ Chapter 5 2

  3. nullConstant • Indicates that the object reference variable does not reference any object. • Person p1 = null; • vs • Person p1 = new Person(“John”); • A null object can not invoke any methods • Results in a NullPointerException if used to invoke methods. • null corresponds to a reference address and hence use == or != to compare instead of 'equals' method. (“equals” compares the contents of two objects)‏ Chapter 5 3

  4. Static Variables • Also referred to as class variables. Accessed using the class name instead of object. • Used primarily to store information common to all the instances of a class. • Another common use of static variables is to define constants. • class Circle { • private static String • SHAPE = “round”; • public String getShape() { • return SHAPE; • } • } Circle c1 = new Circle(); Circle c2 = new Circle(); System.out.println(c1.getShape()); System.out.println(c2.getShape()); Chapter 5 4

  5. Static Methods • Static methods are members of the class but do not need to be invoked through objects of the class. They can be accessed using the class name. • Cannot access any non-static method or variable. • Example : java.lang.Math.pow(x, y); • Signature: • public static return_type method_name (parameters) ; • Invocation: • Class_name.method_name(parameters); Chapter 5 5

  6. Static Variables & Methods public class Champion { public String studentName; public static int numStudentsInGrp = 0; // to share info public static final String grpNickName = “Champs”; // usage as constant public Champion (String name){ studentName = name; numStudentsInGrp++; // tracks of number of members } public static String getGrpName (){ return grpNickName; } } Chapter 5 6

  7. Many Java library classes work with objects rather than primitives. • Example : Collection Classes (HashMap, ArrayList etc..,)‏ • Wrapper classes are used to convert a primitive data type to the corresponding class type. • Example: • Integer obj = new Integer(10); • int val = obj.intValue(); • Additionally, provides many utility methods • Examples: • Integer.parseInt(“10”, 16); • Integer.reverseBytes(10); Chapter 5 7

  8. Primitive to wrapper class conversion is called boxing. • Integer obj = new Integer(10); • Wrapper class to primitive type conversion is called unboxing • int val = obj.intValue(); • Java 5.0 performs boxing and unboxing automatically. • Integer obj = 10; • int val = obj; Chapter 5 8

  9. Use Wrapper classes to convert ‘string’ type to primitive datatypes. • Examples: • int i = Integer.parseInt(“50”); • float f = Float.parseFloat(“50.5”); • String to primitive conversions have to be done explicitly. • Recall the usage in JOptionPane example in Ch.2. Chapter 5

  10. toString() method can be used to convert primitives to String. Examples: • String s = Integer.toString(20); • String s1 = Float.toString(30.3); • The conversion from primitives to String is done implicitly. • Recall the example: • int numOfEggs = 10; • System.out.println(“The number of eggs is “+ numOfEggs ); • instead of • System.out.println(“The number of eggs is “ + Integer.toString(“numOfEggs”); Chapter 5

  11. Use the top down approach to design your task. • The collection of small tasks work together to accomplish a bigger task. • This approach is also called divide and conquer. • Example: Adding two complex numbers • class Complex { • public int real; • public int img; • public Complex(int real, int img) { • this.real = real; • this.img = img; • } • public Complex add (Complex c1, Complex c2) { • int r = addReal(c1, c2); • int i = addImg(c1, c2); • return new Complex (r, i); • } • public addReal(complex c1, complex c2) { return c1.real + c2.real; } • public addImg(complex c1, complex c2) { return c1.img + c2.img; } • } Chapter 5

  12. Bottom-up Testing : func1() invokes func2() : Test func2() completely before testing func1() Advantage : Suppose func1() testing gives incorrect results. It could mean a wrong piece of code either in func1() or in func2() or both. If func2() was tested earlier , we could have concluded that the wrong piece of code was in func1() alone. Chapter 5

  13. 2) Top-down testing using stubs : Stub is a simplified version of a method. Example: class Complex { public int real; public int img; public Complex(int real, int img) { this.real = real; this.img = img; } public Complex add (Complex c1, Complex c2) { int r = addReal(c1, c2); int I = addImg(c1, c2); return new Complex (r, i); } // Return expected values from the methods instead of having the actual code public addReal(complex c1,complex c2) {return x;} // stub for addReal public addImg(complex c1,complex c2) { return y;} // stub for addImg } Chapter 5

  14. Two or more methods of the same class can have the same name provided they differ in the parameters list. • This provision is known as overloading. • Overloading can be done on static, non-static methods as well as methods which do/do not return a value. • Examples: • 1) Complex.add(c1,c2); • Complex.add(c1.real, c2.real); • 2) System.out.println(“Hello!!”); • System.out.println(24); Chapter 5

  15. Should not differ in return type or variable names alone. • public class MyClass{ • public double add (int a , int b) ; • public double add (int c, int d); // Both cannot co-exist • public double add (int a, int b); • public int add(int a, int b); // Both cannot co-exist • } • double sum = MyClass.add(1,2); // Both the methods are applicable Chapter 5

  16. Automatic type conversion of arguments are done to match with one of the signatures. Example: public void add (Complex a, Complex b); public void add (double c, double d); //public void add (double a, int b) //Can it co-exist ? double d1 = 1.5; int d2 = 2; MyClass.add(d1,d2); // Calls the second method. Chapter 5

  17. When an instance of a class is created, a special method Constructor is called. Class_Name Object_Name = new Class_Name (Parameter(s)); • Constructor returns the reference to the new object created. • Like methods, constructors can have parameters • Used to initialize class variables, set up anything else necessary • Different constructors may initialize object to different values • No return type (do not even specify ‘void’)‏ • Examples: • public Classname ( <parameters_optional>)‏ Chapter 5

  18. Java provides a default constructor if no constructors are defined in the class. • You are not provided with the default constructor if there is a constructor (with parameters) already defined. • Use copy constructors to make a copy of an object • Class A { • int val ; • public A()‏ • { val = 10; } • public A(A a1) { • val = a1.val; • } • } A a1 = new A(); A a2 = a1; A a3 = new A(a1); What is the difference between a2 and a3 ? Chapter 5

  19. Calls to other methods in the constructor is common. • In general, the initializations to be done in the constructor can be done by invoking mutators. • Avoids repetition of code at times. • Class Circle { • int radius; • Public Circle(int r){ • if (r < 0 ) { • System.out. • println(“Invalid input”); • System.exit(0); • } • radius =r ; • } • public void setRadius(int r){ • if (r < 0) { • System.out. • println(“Invalid input”); • System.exit(0); • } • radius = r; • } • } Class Circle { int radius; Public Circle(int r){ setRadius(r); } public void setRadius(int r){ if (r < 0) { System.out.println(“Invalid input”); System.exit(0); } radius = r; } } Chapter 5

  20. A package groups and names a collection of related classes. • Usage : • package <package_name>; // This is the first line in the class. • The package name is a relative path name that assumes you start in a class path base directory and follow the path of subdirectories given by the package name. • example class path base directory: \javastuff\libraries • example package classes \javastuff\libraries\general\utilities Chapter 5

  21. Chapter 5

  22. A program or class definition can use all the classes in a package by placing a suitable import statement at the start of the file containing the program or class definition. import Package_Name; • - This is sufficient even if the program or class definition is not in the same directory as the classes in the package. Chapter 5

  23. Packages can help deal with name clashes which are situations in which two classes have the same name. • Ambiguities can be resolved by using the package name. • examples mystuff.CoolClass object1; yourstuff.CoolClass object2; Chapter 5

  24. Point out the errors in the following if any: Public class Circle { public static final double PI = 3.14; public double radius; public Circle (int r) { radius = r;} public static double getArea () { return PI*radius*radius; } } Public class MyClass { int var1; int var2; public MyClass (int a , int b) { var1 = a; var2 = b; } public MyClass(int v1, int v2){ if (v1 < 0 || v2 < 0) System.exit(0); var1 = v1; var2 = v2; } } Chapter 5

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