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Published byRoberta Roberts Modified over 8 years ago
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Abstract methods and classes
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Abstract method A placeholder for a method that will be fully defined in a subclass.
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Abstract method An abstract method has a complete method heading with the addition of the keyword abstract. Cannot be private or final. Ex. abstract public double getPay ( ); abstract public void doSomething ( int count );
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Abstract class A class that has at least one abstract method is called an abstract class. The class definition must have the keyword abstract. Ex. abstract public class Feet { … abstract void doSomething ( int count ); …}
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Abstract vs. concrete class A class without any abstract methods is called a concrete class.
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Points to remember You cannot create an instance of an abstract class. –Ex. Employee joe = new Employee(); //the above is illegal if Employee is abstract
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Points to remember An abstract class is a type. Therefore you can use abstract classes as parameters to functions. Ex. (perfectly “legal”) public abstract class Employee { … public boolean samePay ( Employee other ) { …}…}
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Points to remember An abstract class is a type. Therefore you can use abstract classes as variable types for concrete classes derived from the abstract class. Example follows…
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Points to remember Ex. (perfectly “legal”) public abstract class Employee { …} public class HourlyEmployee extends Employee { …} public class SalaryEmployee extends Employee { …}… Employee e = new HourlyEmployee();
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