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Overloading T-Java and Overriding Java Fundamentals 4 Java Java Accessibility Part 2 Method Overloading Method Overriding Overriding (Shadowing) Variables Constructor Sequence T-Java 2 Method Signature Within a class, methods must have different signatures The method signature consists of: 1 1 method name 2 2 type and order of the parameters public class A { } public class A { } public public String method1(int i) { } String method1(int i) { } method1(int j) { } // syntax error, same signature method1(int j) { } // syntax error, same signature Address method2(String id) throws Exception{ } Address method2(String id) throws Exception{ } private static Address method2(String s) { } // syntax error private static Address method2(String s) { } // syntax error private int private int T-Java 3 Overloading Methods in a Class Overloading a method in a class means: The methods have the same name but parameters differ in: type number of parameters order of parameters public class Signal { public class Signal { public public public public public public Signal(int i){ } // overloaded Signal(int i){ } // overloaded Signal(int i, String s){ } // overloaded Signal(int i, String s){ } // overloaded Signal(float i){ } // overloaded Signal(float i){ } // overloaded CTOR CTOR CTOR CTOR CTOR CTOR public Signal(int i, String s){ } public Signal(int i, String s){ } // syntax error // syntax error Navigate to T-Java\Exercises\exp46a T-Java 4 Overloading and Inheritance Overloading does not hide superclass methods public class CellPhoneAntenna { public class CellPhoneAntenna { String String String String signal(String s) { } signal(String s) { } booster(String s) { } booster(String s) { } CellPhoneAntenna Not-Hidden public class CellPhoneAntennaD14 public class CellPhoneAntennaD14 extends CellPhoneAntenna { extends CellPhoneAntenna { float signal(float i){ } float signal(float i){ } float booster(float i) { } float booster(float i) { } extends CellPhoneAntennaD14 Navigate to T-Java\Exercises\exp46b T-Java 5 Overriding a Method Overriding a method means: The method in a class has the same signature and same return type as a method in the superclass The class method hides the superclass method public class CellPhoneAntenna { public class CellPhoneAntenna { float signal(float f){ } float signal(float f){ } String booster(String s) { } String booster(String s) { } CellPhoneAntenna Hidden extends public class CellPhoneAntennaD14 public class CellPhoneAntennaD14 extends CellPhoneAntenna { extends CellPhoneAntenna { float signal(float f){ } float signal(float f){ } String booster(String s) { } String booster(String s) { } CellPhoneAntennaD14 T-Java 6 The Keyword super In a method body, super is a reference to the superclass method public class CellPhoneAntenna { public class CellPhoneAntenna { float signal(float f){ } float signal(float f){ } CellPhoneAntenna public class CellPhoneAntennaD14 public class CellPhoneAntennaD14 extends CellPhoneAntenna { extends CellPhoneAntenna { float signal(float f){ float signal(float f){ super.signal(f); super.signal(f); } } extends CellPhoneAntennaD14 Navigate to T-Java\Exercises\exp46c T-Java 7 Constructor Review A constructor is called when a new class instance is created For all user defined classes, the first statement in all constructors is a call to another constructor The default constructor has an implicit call to super() public class CellPhoneAntennaD14 . . . { public class CellPhoneAntennaD14 . . . { public CellPhoneAntennaD14() {} public CellPhoneAntennaD14() {} . . . . . . public class CellPhoneAntennaD14 . . . { public class CellPhoneAntennaD14 . . . { public CellPhoneAntennaD14() { public CellPhoneAntennaD14() { super(); super(); } } . . . . . . T-Java explicit call to super() 8 Constructors invoke super A class s constructor is called whenever an instance of that class (or any subclass of that class) is created If the first statement of a constructor is not super, Java inserts super() which invokes the superclass s default constructor Object s constructor is run first, then its subclass constructor, etc. Object extends Antenna extends CellPhoneAntenna extends CellPhoneAntennaD14 T-Java 9 Constructors invoke super step 1 = new CellPhoneAntennaD14(); 5 6 4 7 3 8 2 Object Java invokes constructors, steps 2 3 4 5 Antenna name = unitPrice; Java runs constructors, steps 6 7 8 9 CellPhoneAntenna range = unitPrice; default values explicit initializers constructor body T-Java CellPhoneAntennaD14 9 name = Superior unitPrice = 10995 10 Summary A method signature consists of the method name and the parameters Overloading same name, different method signature Overriding same signature as a method in the superclass In a method, super is a reference to the superclass method of the same name Constructors invoke super T-Java Key Terms Key Terms Signature Overriding Overloading Inheritance Super Constructor 11 Java Fundamentals 4 Java Java Accessibility Part 3 Overriding (Shadowing) Variables Scope of Local Variables Scope of block variables T-Java 12 Overriding (Shadowing) Variables A member variable that has the same name as one of its superclass s variables is said to shadow the superclass variable Shadowed variables Shadowed variables are hidden in the local scope Access shadowed variables with super T-Java Object extends Antenna name = unitPrice; extends CellPhoneAntenna name = unitPrice; extends CellPhoneAntennaD14 name = Superior unitPrice = 10995 13 Scope of Block Variables A block is a group of simple statements enclosed in braces The body of a method is a block Blocks are also commonly used in if-else, while, do-while, and for statements public void init() { public void init() { for (int j=0, j<length, i++) { for (int j=0, j<length, i++) { int count = 0; int count = 0; while(count< 100) {. . . while(count< 100) {. . . } } } // end for } // end for } // end init() } // end init() T-Java 14 block 1 block 1 block 2 block 2 block 3 block 3 Scope of Local Variables Local (or method) variables are declared within a method They are visible only within the method, and not visible in other methods method method void init() { void init() { int i; int i; . . . . . . void validate() { void validate() { int j; int j; method method code code No access to init() method variables No access to init() method variables T-Java 15 Field and Block Scope Variables declared in blocks are visible only within the block and its nested blocks void method() { void method() { int i; int i; block 1 block 1 block 2 block 2 for (int j=0, j<length, i++) { for (int j=0, j<length, i++) { } } } } code code OK to access variable j, and method variable i. OK to access variable j, and method variable i. Not OK to redefine these variables in these blocks Not OK to redefine these variables in these blocks Navigate to T-Java\Exercises\exp42e T-Java 16 Summary Shadow when a member variable has the same name as one of its superclass s variables Local variables are declared within a method Local variables are visible only within the method Key Terms Key Terms Variables Shadow Local variable Block variable Variable scope T-Java 17

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