Exam Objective : Legal return types

Exam Objective : Legal return types PRESENTED BY : SRINIVAS VG

Agenda : • Return types on overloaded methods • Overriding and Return types, and Covariant returns • Returning a value

Return types on Overloaded methods : • To overload a method , you must change the argument list Eg:- publicclass Foo{ void go(){} } publicclass Bar extends Foo{ String go(int x){ returnnull; } }

As long as there is change in argument list , return type doesn’t have to match with that of superclass version • Find the error – publicclass Foo{ void go(){} } publicclass Bar extends Foo{ String go(){ returnnull; } } • Can’t change only the return type

Return types while Overriding : • Only in JAVA 5 you’re allowed to change the return type while overriding only in case of covariant returns Eg : Look at the covariant return class Alpha { Alpha dostuff(char c){ returnnew Alpha(); } } class Beta extends Alpha { Beta doStuff(char c){ // legal override in java 1.5 returnnew Beta(); } }

Covariant returns : • A class could not override the return type of the methods it inherits from a superclass • Applicable only for JAVA 5 version • A method in a subclass may return an object whose type is a subclass of the type returned by the method with the same signature in the superclass Alpha Beta

Till now we know : • Overloaded methods can change the return types • Overridden methods cannot, except in the case of covariant returns

Returning a value : There are 6 rules • You can return null with an object reference return type Eg:public Button dostuff(){ returnnull; } Eg; class Dummy{ public Dummy dum(){ returnnull; } publicstaticvoid main(String args[]){ Dummy n=new Dummy(); Dummy h=n.dum(); System.out.println(h); } }

An array can be returned Eg: public String[] go(){ returnnew String[]{"Fred","Barney","Wilma"}; } Eg: class Dummy{ public String[] dum(){ returnnew String[] {"fred","Barney","wilma"}; } publicstaticvoid main(String args[]){ Dummy n=new Dummy(); String str[]=n.dum(); System.out.println(str[0]+" "+str[1]+" "+str[2]); } }

Valid only for primitive return types : • You can return any value or variable that can be implicitly converted to the declared return type Eg : publicint foo(){ char c='c'; return c; // char is compatible with int } • Eg: class Dummy{ • publicint dum(){ • char c='a'; • return c; • } • publicstaticvoid main(String args[]){ • Dummy n=new Dummy(); • int x=n.dum(); • System.out.println("x value is "+x ); • } • }

You can return any value or variable that can explicitly cast to declared return type Eg : publicint foo(){ float f=32.5f; return (int)f; } Eg: class Dummy{ publicint dum(){ float f=32.5f; return (int)f; } publicstaticvoid main(String args[]){ Dummy n=new Dummy(); int x=n.dum(); System.out.println("x value is "+x ); } }

If the declared return type is void, then you should not return anything Eg : publicvoid bar(){ return" jai only "; // not legal } - However for void return type you can just say as return i.e. in above just write return; // no harm in writing that

With an object reference return type , you can return any object type that can implicitly cast to the declared return type Eg : public Animal getAnimal(){ returnnew Horse(); //Assume Horse extends Animal } Eg: class Animal{ } class Horse extends Animal{ public Animal getAnimal(){ System.out.println("I am inside "); returnnew Horse(); } publicstaticvoid main(String args[]){ Horse h=new Horse(); Animal animals=h.getAnimal(); } }

More Examples : • Eg (1) : public Object getObject() { • int[] nums = {1,2,3}; • return nums; // Return an int array, • // which is still an object • }

Eg (2) : • interface Chewable{ • void hello(); • } • class NotGum { • publicvoid bye(){ • System.out.println(" bye "); • } • } • class Gum implements Chewable{ • publicvoid hello(){ • System.out.println(" hello "); • } • } • publicclass TestChewable{ • public Chewable getChewable(){ • returnnew Gum(); • } • publicstaticvoid main(String args[]){ • TestChewable tt=new TestChewable(); • Chewable ch; • ch=tt.getChewable(); • } • }

Eg (3) : • publicabstractclass Animal { } • publicclass Bear extends Animal { } • publicclass Test { • public Animal go() { • returnnew Bear(); // OK, Bear "is-a" Animal • } • } • - This code will compile, the return value is a subtype

Thank You

Exam Objective : Legal return types