When a method in a subclass has the same name and type signature in its superclass the method in the subclass is said as method co2 level U?
Classes and Objects are basic concepts of Object Oriented Programming which revolve around the real life entities. Class A class is a user defined blueprint or prototype from which objects are created. It represents the set of properties or methods that are common to all objects of one type. In general, class declarations can include these components, in order: 1. Modifiers : A class can be public or has default access (Refer this for details). 2. Class name: The name should begin with a initial letter (capitalized by convention). 3. Superclass(if any): The name of the class's parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent. 4. Interfaces(if any): A comma-separated list of interfaces implemented by the class, if any, preceded by the keyword implements. A class can implement more than one interface. 5. Body: The class body surrounded by braces, { }. Constructors are used for initializing new objects. Fields are variables that provides the state of the class and its objects, and methods are used to implement the behavior of the class and its objects. There are various types of classes that are used in real time applications such as nested classes, anonymous classes, lambda expressions. Object It is a basic unit of Object Oriented Programming and represents the real life entities. A typical Java program creates many objects, which as you know, interact by invoking methods. An object consist of : 1. State : It is represented by attributes of an object. It also reflect the properties of an object. GeeksforGeeks A computer science portal for geeks Practice GATE CS Placements Videos Contribute Welcome jagdishpr ▲ Show
SCJP java Programmer certification Exam And Training. This site is entirely independent of Sun Microsystems Inc, The Sun Certified Java Programmers Exam (SCJP) is the internationally recognized java certification exam. This site offers free mock Exam preparation questions, practice tests, tutorials, certification faq and sample code. This site contains a very detailed tutorial organized by topic. At the end of the tutorial you can learn from your mistakes and apply your concepts on the free mock exams java certification practice tests. Chapter 6 - Objects and Classes (Part 1) Part 2 Part 3 Part 4 Implementing OO relationships � "is a" relationship is implemented by inheritance (extends keyword) � "has a" relationship is implemented by providing the class with member variables. Overloading and Overriding � Overloading is an example of polymorphism. (operational / parametric) � Overriding is an example of runtime polymorphism (inclusive) � A method can have the same name as another method in the same class, provided it forms either a valid overload or override
� Variables can also be overridden, it's known as shadowing or hiding. But, member variable references are resolved at compile-time. So at the runtime, if the class of the object referred by a parent class reference variable, is in fact a sub-class having a shadowing member variable, only the parent class variable is accessed, since it's already resolved at compile time based on the reference variable type. Only methods are resolved at run-time. public class Shadow { public static void main(String s[]) { S1 s1 = new S1(); S2 s2 = new S2(); System.out.println(s1.s); // prints S1 System.out.println(s1.getS()); // prints S1 System.out.println(s2.s); // prints S2 System.out.println(s2.getS()); // prints S2 s1 = s2; System.out.println(s1.s); // prints S1, not S2 - // since variable is resolved at compile time System.out.println(s1.getS()); // prints S2 - // since method is resolved at run time } } class S1 { public String s = "S1"; public String getS() { return s; } } class S2 extends S1{ public String s = "S2"; public String getS() { return s; } } In the above code, if we didn't have the overriding getS() method in the sub-class and if we call the method from sub-class reference variable, the method will return only the super-class member variable value. For explanation, see the following points. � Also, methods access variables only in context of the class of the object they belong to. If a sub-class method calls explicitly a super class method, the super class method always will access the super-class variable. Super class methods will not access the shadowing variables declared in subclasses because they don't know about them. (When an object is created, instances of all its super-classes are also created.) But the method accessed will be again subject to dynamic lookup. It is always decided at runtime which implementation is called. (Only static methods are resolved at compile-time) public class Shadow2 { String s = "main"; public static void main(String s[]) { S2 s2 = new S2(); s2.display(); // Produces an output - S1, S2 S1 s1 = new S1(); System.out.println(s1.getS()); // prints S1 System.out.println(s2.getS()); // prints S1 - since super-class method // always accesses super-class variable } } class S1 { String s = "S1"; public String getS() { return s; } void display() { System.out.println(s); } } class S2 extends S1{ String s = "S2"; void display() { super.display(); // Prints S1 System.out.println(s); // prints S2 } } � With OO languages, the class of the object may not be known at compile-time (by virtue of inheritance). JVM from the start is designed to support OO. So, the JVM insures that the method called will be from the real class of the object (not with the variable type declared). This is accomplished by virtual method invocation (late binding). Compiler will form the argument list and produce one method invocation instruction - its job is over. The job of identifying and calling the proper target code is performed by JVM. � JVM knows about the variable's real type at any time since when it allocates memory for an object, it also marks the type with it. Objects always know 'who they are'. This is the basis of instanceof operator. � Sub-classes can use super keyword to access the shadowed variables in super-classes. This technique allows for accessing only the immediate super-class. super.super is not valid. But casting the 'this' reference to classes up above the hierarchy will do the trick. By this way, variables in super-classes above any level can be accessed from a sub-class, since variables are resolved at compile time, when we cast the 'this' reference to a super-super-class, the compiler binds the super-super-class variable. But this technique is not possible with methods since methods are resolved always at runtime, and the method gets called depends on the type of object, not the type of reference variable. So it is not at all possible to access a method in a super-super-class from a subclass. public class ShadowTest { public static void main(String s[]){ new STChild().demo(); } } class STGrandParent { double wealth = 50000.00; public double getWealth() { System.out.println("GrandParent-" + wealth); return wealth; } } class STParent extends STGrandParent { double wealth = 100000.00; public double getWealth() { System.out.println("Parent-" + wealth); return wealth; } } class STChild extends STParent { double wealth = 200000.00; public double getWealth() { System.out.println("Child-" + wealth); return wealth; } public void demo() { getWealth(); // Calls Child method super.getWealth(); // Calls Parent method // Compiler error, GrandParent method cannot be accessed //super.super.getWealth(); // Calls Child method, due to dynamic method lookup ((STParent)this).getWealth(); // Calls Child method, due to dynamic method lookup ((STGrandParent)this).getWealth(); System.out.println(wealth); // Prints Child wealth System.out.println(super.wealth); // Prints Parent wealth // Prints Parent wealth System.out.println(((STParent)(this)).wealth); // Prints GrandParent wealth System.out.println(((STGrandParent)(this)).wealth); } } � An inherited method, which was not abstract on the super-class, can be declared abstract in a sub-class (thereby making the sub-class abstract). There is no restriction. � In the same token, a subclass can be declared abstract regardless of whether the super-class was abstract or not. � Private members are not inherited, but they do exist in the sub-classes. Since the private methods are not inherited, they cannot be overridden. A method in a subclass with the same signature as a private method in the super-class is essentially a new method, independent from super-class, since the private method in the super-class is not visible in the sub-class. public class PrivateTest { public static void main(String s[]){ new PTSuper().hi(); // Prints always Super new PTSub().hi(); // Prints Super when subclass doesn't have hi method // Prints Sub when subclass has hi method PTSuper sup; sup = new PTSub(); sup.hi(); // Prints Super when subclass doesn't have hi method // Prints Sub when subclass has hi method } } class PTSuper { public void hi() { // Super-class implementation always calls superclass hello hello(); } private void hello() { // This method is not inherited by subclasses, but exists in them. // Commenting out both the methods in the subclass show this. // The test will then print "hello-Super" for all three calls // i.e. Always the super-class implementations are called System.out.println("hello-Super"); } } class PTSub extends PTSuper { public void hi() { // This method overrides super-class hi, calls subclass hello try { hello(); } catch(Exception e) {} } void hello() throws Exception { // This method is independent from super-class hello // Evident from, it's allowed to throw Exception System.out.println("hello-Sub"); } } � Private methods are not overridden, so calls to private methods are resolved at compile time and not subject to dynamic method lookup. See the following example. public class Poly { public static void main(String args[]) { PolyA ref1 = new PolyC(); PolyB ref2 = (PolyB)ref1; System.out.println(ref2.g()); // This prints 1 // If f() is not private in PolyB, then prints 2 } } class PolyA { private int f() { return 0; } public int g() { return 3; } } class PolyB extends PolyA { private int f() { return 1; } public int g() { return f(); } } class PolyC extends PolyB { public int f() { return 2; } } Chapter 6 - Objects and Classes (Part 1) Part 2 Part 3 Part 4 When a method in a subclass has the same name and type signatures as a method in the superclass?Explanation: When a method in a subclass has the same name and type signatures as a method in the superclass, then the method in the subclass overrides the method in the superclass.
When a method in a subclass has the same name and type signature as a method in its superclass then in the method in the subclass is said to ?Static Methods
If a subclass defines a static method with the same signature as a static method in the superclass, then the method in the subclass hides the one in the superclass.
Can a subclass have a method with the same name?A subclass can do more than that; it can define a method that has exactly the same method signature (name and argument types) as a method in its superclass.
What modifier should you use on the members of a class so that they are not accessible to another class but they are accessible to any subclasses of the class?The private modifier specifies that the member can only be accessed in its own class. The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.
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