The purpose of inheritance is same in C++ and Java. Inheritance is used in both languages for reusing code and/or creating is-a relationship. There are following differences in the way both languages provide support for inheritance.
1) In Java, all classes inherit from the Object class directly or indirectly. Therefore, there is always a single inheritance tree of classes in Java, and Object class is root of the tree. In Java, if we create a class that doesn’t inherit from any class then it automatically inherits from Object class . In C++, there is forest of classes; when we create a class that doesn’t inherit from anything, we create a new tree in forest.
Following Java example shows that Test class automatically inherits from Object class.
t is instanceof Object: true
2) In Java, members of the grandparent class are not directly accessible. See this G-Fact for more details.
3) The meaning of protected member access specifier is somewhat different in Java. In Java, protected members of a class “A” are accessible in other class “B” of same package, even if B doesn’t inherit from A (they both have to be in the same package). For example, in the following program, protected members of A are accessible in B.
4) Java uses extends keyword for inheritence. Unlike C++, Java doesn’t provide an inheritance specifier like public, protected or private. Therefore, we cannot change the protection level of members of base class in Java, if some data member is public or protected in base class then it remains public or protected in derived class. Like C++, private members of base class are not accessible in derived class.
Unlike C++, in Java, we don’t have to remember those rules of inheritance which are combination of base class access specifier and inheritance specifier.
5) In Java, methods are virtual by default. In C++, we explicitly use virtual keyword. See this G-Fact for more details.
6) Java uses a separte keyword interface for interfaces, and abstract keyword for abstract classes and abstract functions.
Following is a Java abstract class example.
Following is a Java interface example
7) Unlike C++, Java doesn’t support multiple inheritance. A class cannot inherit from more than one class. A class can implement multiple interfaces though.
8 ) In C++, default constructor of parent class is automatically called, but if we want to call parametrized constructor of a parent class, we must use Initializer list. Like C++, default constructor of the parent class is automatically called in Java, but if we want to call parametrized constructor then we must use super to call the parent constructor. See following Java example.
Base constructor called Derived constructor called
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