Java Abstract Class<\/h3>\n
An Abstract class is a class that represents a concept and whose objects can\u2019t be created. A class that contains at least one abstract method (method without any implementation or method body) is called an abstract class.<\/p>\n
An abstract class is declared with the help of an abstract keyword. There is always a default constructor in an abstract class, it can also have a parameterized constructor.<\/p>\n
Note:<\/strong> Using an abstract class, we can achieve 0 to 100% abstraction.<\/em><\/p>\n To declare an abstract class in Java we use the keyword abstract. The syntax is given below:<\/p>\n We can not create objects or instances from the abstract classes, but they can be subclassed. That is, to access the methods inside the abstract classes we have to inherit them. We will discuss the reason behind this concept in the later section of this article.<\/p>\n Revise the full concept of Java Inheritance<\/a> in detail.<\/strong><\/em><\/p>\n Code to illustrate the above concept:<\/strong><\/p>\n Output:<\/strong><\/p>\n In the above example, we have created an abstract class Person. We cannot create objects of this abstract class Person and also cannot access the displayInfo() directly.<\/p>\n To access the displayInfo() method of Person, we have extended a subclass Student from the Person. Then we created the object obj1 of this subclass and used this object to access the method displayInfo().<\/p>\n In Java, it is compulsory to override abstract methods of the parent class in its child class because the derived class extends the abstract methods of the base class.<\/p>\n If we do not override the abstract methods in the subclasses then there will be a compilation error. Therefore, it is necessary for a subclass to override the abstract methods of its base class.<\/p>\n Hold On! It’s the right t<\/strong><\/em>ime to get familiar with the concept of Method Overriding in Java.\u00a0<\/a><\/strong><\/em><\/p>\n Note:<\/strong> If the subclass is also declared as abstract then it is not necessary to override the abstract methods.<\/em><\/p>\n Code to illustrate the above concept:<\/strong><\/p>\n Output:<\/strong><\/p>\n You might be thinking what is the need for abstract classes if there is no implementation in them and also we can\u2019t create an object from them.<\/p>\n To answer this question let\u2019s take a situation where we want to create a class that just declares the structure or general form of a particular concept without providing a complete implementation of every method.<\/p>\n And, we want that this generalized form is shared by all of its child classes, and all the implementation details will be filled by these subclasses.<\/p>\n Let\u2019s take the example of a banking application or software. Suppose we have a class BankAccount<\/strong> that has a method deposit()<\/strong> and withdraw()<\/strong> and the subclasses of it like SavingsAccount, CurrentAccount, FixedDepositAccount, etc.<\/strong><\/p>\n Since the process of deposit and withdrawal differs from one account to another, there is no point to implement these two methods in the parent class BankAccount.<\/strong> This is because every child class must override these methods and provide an implementation of them.<\/p>\n Thus we can declare these methods as abstract in the parent class. That is, we will not provide any implementation of these abstract methods. Making this method abstract will enforce all the subclasses to implement these abstract methods otherwise, you will get a compilation error.<\/p>\n Get to know more about Java Methods<\/a> in detail with Techvidvan.<\/strong><\/em><\/p>\n Syntax of declaring abstract methods:<\/strong><\/p>\n Example of Abstract class and Abstract methods:<\/strong><\/p>\n Output:<\/strong><\/p>\n We can\u2019t instantiate an abstract class because these classes are incomplete classes, with no implementation. Abstract classes have abstract methods that have no method body.<\/p>\n Suppose, if Java allows you to create an object of this class and use this object if someone calls the abstract method then what would happen? There would not be any actual implementation of the invoked method!<\/p>\n Also, an abstract class is like a general structure or a template that has to be extended by the subclasses for the implementation.<\/p>\n Code to illustrate that object creation of an abstract class is not valid:<\/strong><\/p>\n As discussed above, we cannot instantiate an abstract class. This program throws a compilation error.<\/p>\n Output:<\/strong><\/p>\n As the constructors of non-abstract classes can be accessed, we can also access the constructor of an abstract class. We access the constructor from the subclass with the help of the super<\/strong> keyword. For example,<\/p>\n To access the constructor of the parent class, we have used the super() inside the constructor of the Child. Note that the super statement should always be the first statement of the constructor of the subclass.<\/p>\n There are some rules that you should remember while working the abstract classes.<\/p>\nDeclaring a Java Abstract Class<\/h3>\n
abstract class ClassName\n{\n \/\/class body\n}<\/pre>\nInheritance of Java Abstract Class<\/h3>\n
\/\/abstract parent class\nabstract class Person\n{\n public void displayInfo()\n {\n System.out.println(\"I am a person.\");\n }\n}\nInheritance of abstract class\nclass Student extends Person\n{\n\n}\nclass AbstractClassDemo\n{\n public static void main(String[] args)\n {\n \/\/Creating object of the child class\n Student obj1 = new Student();\n\n Accessing member of the abstract class\n obj1.displayInfo();\n }\n}<\/pre>\nOverriding Abstract Classes in Java<\/h3>\n
package com.techvidvan.abstractclass;\nabstract class Parent\n{\n \/\/concrete method\n public void display1()\n {\n System.out.println(\"Concrete method of parent class\");\n }\n \/\/abstract method\n abstract public void display2();\n}\nclass Child extends Parent\n{\n \/\/ Must Override this method while extending Parent class\n public void display2()\n {\n System.out.println(\"Overriding abstract method\");\n }\n\n \/\/Overriding concrete method is not compulsory\n public void display1()\n {\n System.out.println(\"Overriding concrete method\");\n }\n}\npublic class AbstractClassDemo\n{\n public static void main(String[] args)\n {\n Child obj = new Child();\n obj.display2();\n obj.display1();\n }\n}<\/pre>\n
\nOverriding concrete method<\/div>\nWhy do we need Abstract Classes in Java?<\/h3>\n
![\"java](\"https:\/\/techvidvan.com\/tutorials\/wp-content\/uploads\/sites\/2\/2020\/03\/example-of-abstract-class-in-java.jpg\")
<\/a><\/p>\nAbstract Methods in Java<\/h3>\n
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access-specifier abstract return-type method-name();\n<\/pre>\n
package com.techvidvan.abstractclass;\n\n\/\/abstract parent class\nabstract class BankAccount\n{\n \/\/abstract methods\n public abstract void deposit();\n public abstract void withdraw();\n\n}\n\/\/SavingsAccount class extends BankAccount class\nclass SavingsAccount extends BankAccount\n{\n public void deposit()\n {\n System.out.println(\"This is the concrete deposit method of SavingsAccount\");\n }\n public void withdraw()\n {\n System.out.println(\"This is the concrete withdraw method of SavingsAccount\\n\");\n }\n}\n\/\/CurrentAccount class extends BankAccount class\nclass CurrentAccount extends BankAccount\n{\n public void deposit()\n {\n System.out.println(\"This is the concrete deposit method of CurrentAccount\");\n }\n public void withdraw()\n {\n System.out.println(\"This is the concrete withdraw method of CurrentAccount\\n\");\n }\n}\n\/\/FixedDepositAccount class extends BankAccount class\nclass FixedDepositAccount extends BankAccount\n{\n public void deposit()\n {\n System.out.println(\"This is the concrete deposit method of FixedDepositAccount\");\n }\n public void withdraw()\n {\n System.out.println(\"This is the concrete withdraw method of FixedDepositAccount\");\n }\n}\npublic class AbstractClassDemo\n{\n public static void main(String args[])\n {\n BankAccount obj = new SavingsAccount();\n obj.deposit();\n obj.withdraw();\n\n BankAccount obj1 = new CurrentAccount();\n obj1.deposit();\n obj1.withdraw();\n\n BankAccount obj2 = new FixedDepositAccount();\n obj2.deposit();\n obj2.withdraw();\n }\n}<\/pre>\n
\nThis is the concrete withdraw method of SavingsAccount
\nThis is the concrete deposit method of CurrentAccount
\nThis is the concrete withdraw method of CurrentAccount
\nThis is the concrete deposit method of FixedDepositAccount
\nThis is the concrete withdraw method of FixedDepositAccount<\/div>\nWhy can\u2019t we create an Object of an Abstract Class?<\/h3>\n
package com.techvidvan.abstractclass;\nabstract class AbstractClass\n{\n \/\/Abstract method\n abstract public void display();\n}\nclass MyClass extends AbstractionDemo\n{\n public void display()\n {\n System.out.print(\"Abstract method\");\n }\n}\npublic class AbstractClassDemo\n{\n public static void main(String args[])\n {\n \/\/error: You can't create object of an abstract class\n AbstractClass obj = new AbstractClass();\n obj.display();\n }\n}<\/pre>\n
\nCannot instantiate the type AbstractClass<\/div>\nAccessing Constructors of Abstract Class<\/h3>\n
\/\/parent class\nabstract class Parent\n{\n \/\/constructor of the abstract class\n Parentl()\n {\n \u2026.\n }\n}\n\/\/child class\nclass Child extends Parent\n{\n \/\/constructor of the child class\n Child()\n {\n \/\/Accessing the constructor of the abstract class using the super keyword\n super();\n \u2026.\n }\n}<\/pre>\nRules for using Abstract Class in Java<\/h3>\n
![\"Java](\"https:\/\/techvidvan.com\/tutorials\/wp-content\/uploads\/sites\/2\/2020\/03\/rules-for-abstract-class-in-java-1.jpg\")
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