Java – Abstraction

As per dictionary, abstraction is the quality of dealing with ideas rather than events. For example, when you consider the case of e-mail, complex details such as what happens as soon as you send an e-mail, the protocol your e-mail server uses are hidden from the user. Therefore, to send an e-mail you just need to type the content, mention the address of the receiver, and click send.

Java Abstraction

Abstraction is a process of hiding the implementation details from the user, only the functionality will be provided to the user. In other words, the user will have the information on what the object does instead of how it does it. In Java programming, abstraction is achieved using Abstract classes and interfaces.

Java Abstract Classes

A Java class which contains the abstract keyword in its declaration is known as abstract class.

• Java abstract classes may or may not contain abstract methods, i.e., methods without body ( public void get(); )
• But, if a class has at least one abstract method, then the class must be declared abstract.
• If a class is declared abstract, it cannot be instantiated.
• To use an abstract class, you have to inherit it from another class, provide implementations to the abstract methods in it.
• If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.

Example: Java Abstract Class

This section provides you an example of the Java Abstract Class. To create an abstract class in Java, just use the abstract keyword before the class keyword, in the class declaration.

/* File name : Employee.java */publicabstractclassEmployee{privateString name;privateString address;privateint number;publicEmployee(String name,String address,int number){System.out.println("Constructing an Employee");this.name = name;this.address = address;this.number = number;}publicdoublecomputePay(){System.out.println("Inside Employee computePay");return0.0;}publicvoidmailCheck(){System.out.println("Mailing a check to "+this.name +" "+this.address);}publicStringtoString(){return name +" "+ address +" "+ number;}publicStringgetName(){return name;}publicStringgetAddress(){return address;}publicvoidsetAddress(String newAddress){
      address = newAddress;}publicintgetNumber(){return number;}}

You can observe that except abstract methods the Employee class is same as normal class in Java. The class is now abstract, but it still has three fields, seven methods, and one constructor.

Now you can try to instantiate the Employee class in the following way −

/* File name : AbstractDemo.java */publicclassAbstractDemo{publicstaticvoidmain(String[] args){/* Following is not allowed and would raise error */Employee e =newEmployee("George W.","Houston, TX",43);System.out.println("\n Call mailCheck using Employee reference--");
      e.mailCheck();}}abstractclassEmployee{privateString name;privateString address;privateint number;publicEmployee(String name,String address,int number){System.out.println("Constructing an Employee");this.name = name;this.address = address;this.number = number;}publicdoublecomputePay(){System.out.println("Inside Employee computePay");return0.0;}publicvoidmailCheck(){System.out.println("Mailing a check to "+this.name +" "+this.address);}publicStringtoString(){return name +" "+ address +" "+ number;}publicStringgetName(){return name;}publicStringgetAddress(){return address;}publicvoidsetAddress(String newAddress){
      address = newAddress;}publicintgetNumber(){return number;}}

When you compile the above class, it gives you the following error −

Employee.java:46: Employee is abstract; cannot be instantiated
      Employee e = new Employee("George W.", "Houston, TX", 43);
                   ^
1 error

Inheriting the Java Abstract Class

We can inherit the properties of Employee class just like concrete class in the following way −

Example: Inheriting the Abstract Class in Java

/* File name : Salary.java */publicclassSalaryextendsEmployee{privatedouble salary;// Annual salarypublicSalary(String name,String address,int number,double salary){super(name, address, number);setSalary(salary);}publicvoidmailCheck(){System.out.println("Within mailCheck of Salary class ");System.out.println("Mailing check to "+getName()+" with salary "+ salary);}publicdoublegetSalary(){return salary;}publicvoidsetSalary(double newSalary){if(newSalary >=0.0){
         salary = newSalary;}}publicdoublecomputePay(){System.out.println("Computing salary pay for "+getName());return salary/52;}}

Here, you cannot instantiate the Employee class, but you can instantiate the Salary Class, and using this instance you can access all the three fields and seven methods of Employee class as shown below.

/* File name : AbstractDemo.java */publicclassAbstractDemo{publicstaticvoidmain(String[] args){Salary s =newSalary("Mohd Mohtashim","Ambehta, UP",3,3600.00);Employee e =newSalary("John Adams","Boston, MA",2,2400.00);System.out.println("Call mailCheck using Salary reference --");
      s.mailCheck();System.out.println("\n Call mailCheck using Employee reference--");
      e.mailCheck();}}abstractclassEmployee{privateString name;privateString address;privateint number;publicEmployee(String name,String address,int number){System.out.println("Constructing an Employee");this.name = name;this.address = address;this.number = number;}publicdoublecomputePay(){System.out.println("Inside Employee computePay");return0.0;}publicvoidmailCheck(){System.out.println("Mailing a check to "+this.name +" "+this.address);}publicStringtoString(){return name +" "+ address +" "+ number;}publicStringgetName(){return name;}publicStringgetAddress(){return address;}publicvoidsetAddress(String newAddress){
      address = newAddress;}publicintgetNumber(){return number;}}classSalaryextendsEmployee{privatedouble salary;// Annual salarypublicSalary(String name,String address,int number,double salary){super(name, address, number);setSalary(salary);}publicvoidmailCheck(){System.out.println("Within mailCheck of Salary class ");System.out.println("Mailing check to "+getName()+" with salary "+ salary);}publicdoublegetSalary(){return salary;}publicvoidsetSalary(double newSalary){if(newSalary >=0.0){
         salary = newSalary;}}publicdoublecomputePay(){System.out.println("Computing salary pay for "+getName());return salary/52;}}

Output

Constructing an Employee
Constructing an Employee
Call mailCheck using Salary reference --
Within mailCheck of Salary class 
Mailing check to Mohd Mohtashim with salary 3600.0

 Call mailCheck using Employee reference--
Within mailCheck of Salary class 
Mailing check to John Adams with salary 2400.0

Java Abstract Methods

If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as an abstract.

• abstract keyword is used to declare the method as abstract.
• You have to place the abstract keyword before the method name in the method declaration.
• An abstract method contains a method signature, but no method body.
• Instead of curly braces, an abstract method will have a semoi colon (;) at the end.

Example 1: Implementing Abstract Method in Java

Following is an example of the abstract method.

publicabstractclassEmployee{privateString name;privateString address;privateint number;publicabstractdoublecomputePay();// Remainder of class definition}

Declaring a method as abstract has two consequences −

• The class containing it must be declared as abstract.
• Any class inheriting the current class must either override the abstract method or declare itself as abstract.

Note − Eventually, a descendant class has to implement the abstract method; otherwise, you would have a hierarchy of abstract classes that cannot be instantiated.

Suppose Salary class inherits the Employee class, then it should implement the computePay() method as shown below −

/* File name : Salary.java */publicclassSalaryextendsEmployee{privatedouble salary;// Annual salarypublicdoublecomputePay(){System.out.println("Computing salary pay for "+getName());return salary/52;}// Remainder of class definition}

Example 2: Implementing Abstract Method in Java

Following example showcases the concept of abstract method.

/* File name : AbstractDemo.java */publicclassAbstractDemo{publicstaticvoidmain(String[] args){Salary s =newSalary("Mohd Mohtashim","Ambehta, UP",3,3600.00);System.out.println("salary: "+ s.computePay());}}abstractclassEmployee{privateString name;privateString address;privateint number;publicEmployee(String name,String address,int number){System.out.println("Constructing an Employee");this.name = name;this.address = address;this.number = number;}publicabstractdoublecomputePay();// Remainder of class definitionpublicStringgetName(){return name;}publicvoidsetName(String name){this.name = name;}publicStringgetAddress(){return address;}publicvoidsetAddress(String address){this.address = address;}publicintgetNumber(){return number;}publicvoidsetNumber(int number){this.number = number;}}classSalaryextendsEmployee{privatedouble salary;// Annual salarypublicSalary(String name,String address,int number,double salary){super(name, address, number);this.salary = salary;}publicdoublecomputePay(){System.out.println("Computing salary pay for "+getName());return salary/52;}// Remainder of class definition}

Output

Constructing an Employee
Computing salary pay for Mohd Mohtashim
salary: 69.23076923076923