Abstract Classes

• Recall an interface is merely the specification of expected behavior.
  • The interface only contains method signatures-- no bodies (implementations)
• Can't create an object of the interface type (no implementation)

  interface Set {
  	...
  }
  Set set = new Set();	// Error!! -- can't create interface object
  		

• However, can declare a variable of the interface type, and assign to it a reference to an object of a class that implements the interface

  class HashSet implements Set {
  	...
  }
  Set set = new HashSet();	
  		

• Sometimes, we want something in-between a class and an interface
  • We want to only partially implement behavior.
  • We want to specify the rest of the behavior
    • It will then remain the responsibility of a subclass to implement that behavior

    abstract class Employee {
    	Employee(Name name, String ssNum) {...}
    	...
    	abstract void doPayroll();
     	...
    	Name name;
    	String ssNum;
    	Date hiredOn;
    }
    
    class FactoryWorker extends Employee {
    	FactoryWorker(Name name, String ssNum, Currency hourlyRate) {
    		super(name, ssNUm);
    		this.hourlyRate = hourlyRate;
    	}
    	...
    	void doPayroll() {
    		...
    	}
    	...	
    	...
    	
    	double hourlyRate;
    }
    
    class OfficeWorker extends Employee {
    	OfficeWorker(Name name, String ssNum, Currency annualSalary) {
    		super(name, ssNUm);
    		this.annualSalary = annualSalary;
    	}
    	...
    	void doPayroll() {
    		...
    	}
    	...	
    	Currency annualSalary;
    }
    
    

  • All employeess have a name and a hiredOn date
  • Various subclasses (in this case OfficeWorker and FactoryWorker) will supply the additional state/behavior to perform payroll in the proper manner.

• Any methods whose implementation is deferred for a subclass must be declared abstract
• Note that the superclass must be declared abstract as well
• As with interfaces, you cannot create an object of an abstract class

  Employee employee = new Employee();	// Error!! -- can't create abstract class object
  		

  • Not all methods are implemented
  • What would it mean to create such an object and invoke one of the abstract methods?
• However, again as with interfaces, you can declare a reference variable of the abstract class type and assign it a reference to a (fully specified, i.e., non-abstract) subclass object.

  Employee employee = new OfficeWorker(...);