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‘ final ‘ to avoid overriding

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Objective :On completion of this period, you would be able to learn

• using final with inheritance

• using final to prevent overriding

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Recap

In the previous class we have learnt• multi-level inheritance hierarchy

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Final Class

Final classes

• Classes that cannot be extended

• To declare final classes, we write,

public final class ClassName{

. . .

}

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• final Methods• Methods that cannot be overridden

• To declare final methods, we write,

public final [return Type] [methodName]([parameters]){

. . .

}

• Static methods are automatically final

final Methods

CM-602.40

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public final String getName(){

return name;

}

Example For final Method

CM-602.40

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Uses of final Keyword

There are three uses of final keyword

• It can be used to create the equivalent of named constant

• To prevent overriding

• To prevent Inheritance

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• Method overriding is one of java’s most powerful features

• There will be times when you want to prevent it from

occurring

• To disallow a method from being overridden use final

modifier at the start of the declaration

• Methods declares as final cannot be overridden

Prevent method overriding

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class A { final void meth() {System.out.println( “ This is a Final method.” ); }} class B extends A { void meth() { // ERROR! Can’t overrideSystem.out.println( “ Illegal! “); }}

Example program to prevent overriding

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• meth() is declared as final hence it cannot be overridden in B

• If you attempt to do so a compile time error will result • Methods declares as final sometimes provide

performance enhancement• Normally java resolves calls to methods dynamically at

run-time• This is call late binding• Since final methods cannot be overridden , a call to one

can be resolved at compile time• This is called early binding

Example Program To Prevent Overriding contd..

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• Sometimes you will want to prevent a class being inherited

• To do this use the final keyword before the class declaration

• Declaring a class as final implicitly declares all of its methods as final, too

Prevent Inheritance

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final class A { // ….

} // The following class is illegal

class B extends A { //ERROR ! Can’t be subclass A

// ….}

Example to prevent inheritance

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Summary

• In this class we have discussed• final keyword• Using final avoiding overriding• Using final avoiding inheritance

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Frequently Asked Questions

1. What is the use of a final key

2. Explain with an example program using final to prevent method overriding

3. Explain with an example program using final to prevent inheritance

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1. To prevent overriding we will use

1. this2. final3.new4.old

Quiz

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2. Final is used to prevent the following 1. Multi level hierarchy2. Abstraction3. Inheritance 4. Polymorphism

Quiz contd..