A quick tutorial to Method references

A quick Update on Method References

Parth Joshi

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Method References

• We can pass the reference of the method to get invoked as a callback in Java 8 using :: operator

Part of Online video tutorial series: Upgrade to Java 8

https://www.udemy.com/upgrade-to-java-8/

Method References

• We can use this for:

1. Static methods of a class

2. Instance methods

3. Constructor calls



Method References – Static Methods

class Dog {

// static method

public static void barks() {

System.out.println("Bhow bhow");

}

public void doggyThing (Doggy doggy){

doggy.doDoggyThing();

}

}

interface Doggy {

public void doDoggyThing();

}

Suppose:Our objective is to pass some behavior in the doggyThing function




By our Old School Method of anonymous inner class method:

// using old school anonymous inner class

Dog dog1 = new Dog();

dog.doggyThing(new Doggy() {

@Override

public void doDoggyThing() {

System.out.println("Bang Bang");

}

});




class Dog {

// static method



}



}

}

interface Doggy {


}

Using Method Reference:We will pass body of barks() functions as behavior to doDoggyThing() of Doggy Functional Interface.




• By using static Method reference feature of Java 8:

Dog dog = new Dog();

dog.doggyThing(Dog::barks);




• By using static Method reference feature of Java 8:





}

interface Doggy {


}



}Part of Online video tutorial series: Upgrade to Java 8



• Same operation by using Lambda Expression

// using new Lambda Expression

Dog dog2 = new Dog();

dog.doggyThing(()->{System.out.println("New Bhow");});




• Comparison of anonymous inner class, lambda and method reference

// 2. using new Lambda Expression

dog.doggyThing(()->{System.out.println("New Bhow");});

// 3. using method reference


//1. using anonymous inner class


dog.doggyThing(new Doggy() {

@Override

public void doDoggyThing() {

System.out.println("Bang Bang");

}

});



Method References – Instance Method

MyCar myCar = new MyCar(500);

//1. using old school anonymous inner class

myCar.getCarServiceStatus(new Servicing() {

@Override

public String getServiceStatus(int distance) {

return new CompanyGarage().repairStatus(distance);

}

});

//2. using lambda expression...

myCar.getCarServiceStatus((distance)->{return new CompanyGarage().repairStatus(distance);});

//3. using Method Reference expression...

myCar.getCarServiceStatus((new CompanyGarage())::repairStatus);



Method References – Constructor

class Eggs {

int n;

public Eggs (int n){

this.n = n;

}

}

interface WhiteEggs {

public Eggs getEggs (int noOfEggs);

}


class Eggs {

int n;


this.n = n;

}

}



}

Method Reference for Constructor:

WhiteEggs whiteEggs = Eggs::new;

Eggs eggs = whiteEggs.getEggs(5);




class Eggs {

int n;


this.n = n;

}

}



}

Lets understand whats happening



Constructor is called on the call of getEggs(5) with 5 as parameter to the constructor




class Eggs {

int n;


this.n = n;

}

}



}

Lets understand whats happening



Object reference is given to instance variable




• Observe the convention:

class Eggs {

int n;


this.n = n;

}

}



}

Eggs eggs = whiteEggs.getEggs(5);WhiteEggs whiteEggs = Eggs::new;

Here the return type of this Functional Interface is Egg which is the type of instance variable egg. We are calling constructor of Egg class by this method reference.




• Observe the convention:

class Eggs {

int n;


this.n = n;

}

}



}

Eggs eggs = whiteEggs.getEggs(5);WhiteEggs whiteEggs = Eggs::new;

Observe that parameter of Functional interface should be same as the constructor of class



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A quick tutorial to Method references