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Difference Between Abstract Class and Abstract Method in Java

  • Last Updated : 17 Jun, 2021

Abstract is the modifier applicable only for methods and classes but not for variables. Even though we don’t have implementation still we can declare a method with an abstract modifier. That is abstract methods have only declaration but not implementation. Hence, abstract method declaration should compulsory ends with semicolons. 

Illustration: 

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public abstract void methodOne(); ------>valid
public abstract void methodOne(){} ------->Invalid

Example:  

Java




// Java Program to illustrate Abstract class
  
// Abstract Class
// Main class
abstract class GFG {
  
    // Main driver method
    public static void main(String args[])
    {
  
        // Creating object of class inside main() method
        GFG gfg = new GFG();
    }
}

Output:



Output explanation:

If a class contains at least one abstract method then compulsory the corresponding class should be declared with an abstract modifier. Because implementation is not complete and hence we can’t create objects of that class. 

Even though the class doesn’t contain any abstract methods still we can declare the class as abstract which is an abstract class that can contain zero no of abstract methods also.

Illustration 1: 

class Parent 
{  // Method of this class 
   public void methodOne();
}

Output: 

Compile time error.
missing method body, or declared abstract
public void methodOne();

Illustration 2:

class parent {
   // Method of this class
   public abstract void methodOne() {}
}

Output: 



Compile time error.
abstract method cannot have a body.
public abstract void methodOne(){}

 Illustration 3:

class parent {

   // Method of this class
   public abstract void methodOne();
}

Output: 

Compile time error.
Parent is not abstract and does not override abstract method methodOne() in Parent class
Parent

If a class extends any abstract class then compulsory we should provide implementation for every abstract method of the parent class otherwise we have to declare child class as abstract. 

Example: 

Java




// Java Program to Illustrate Abstract Method
  
// Main class
// Abstract class
abstract class Parent {
  
    // Methods of abstract parent class
    public abstract void methodOne();
    public abstract void methodTwo();
}
  
// Class 2
// Child class
class child extends Parent {
  
    // Method of abstract child class
    public void methodOne() {}
}

Output: 

Note: If we declare the child as abstract then the code compiles fine, but the child of a child is responsible to provide an implementation for methodTwo().
 

Now let us finally conclude out the differences between them after having an adequate understanding of both of them.

Abstract classesAbstract methods
Abstract classes can’t be instantiated.Abstract method bodies must be empty.
Other classes extend abstract classes.Sub-classes must implement the abstract class’s abstract methods.
Can have both abstract and concrete methods.Has no definition in the class.

Similar to interfaces, but can 

  • Implement methods
  • Fields can have various access modifiers
  • Subclasses can only extend one abstract class
Has to be implemented in a derived class.



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