



Function Interface in Java with Examples


The Function Interface is a part of the java.util.function package which has been introduced since Java 8, to implement functional programming in Java. It represents a function which takes in one argument and produces a result.
Hence this functional interface which takes in 2 generics namely:-

T: denotes the type of the input argument
R: denotes the return type of the function

The lambda expression assigned to an object of Function type is used to define its apply() which eventually applies the given function on the argument.
Methods in Function Interface
The Function interface consists of the following 4 methods:
1. apply()
This method applies the given function on its only argument.
Syntax:
R apply(T t)
Parameters: This method takes in only one parameter t which is the function argument
Returns: This method returns the function result which is of type R.
Below is the code to illustrate apply() method:
Program 1:







 


 

 













import java.util.function.Function; 
  
public class Main { 
    public static void main(String args[]) 
    { 
  
        // Function which takes in a number 
        // and returns half of it 
        Function<Integer, Double> half = a -> a / 2.0; 
  
        // apply the function to get the result 
        System.out.println(half.apply(10)); 
    } 
} 



















Output:


5.0



Reference: https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html#apply-T-
2. addThen()
It returns a composed function wherein the parameterized function will be executed after the first one. If evaluation of either function throws an error, it is relayed to the caller of the composed function.
Syntax:
default <V> Function<T, V> 

    andThen(Function<? super R, ? extends V> after)
where V is the type of output of the after function, and of the composed function
Parameters: This method accepts a parameter after which is the function to be applied after the current one.
Return Value: This method returns a composed function that applies the current function first and then the after function
Exception: This method throws NullPointerException if the after function is null.
Below is the code to illustrate addThen() method:
Program 1:







 


 

 













import java.util.function.Function; 
  
public class Main { 
    public static void main(String args[]) 
    { 
        // Function which takes in a number and 
        // returns half of it 
        Function<Integer, Double> half = a -> a / 2.0; 
  
        // Now treble the output of half function 
        half = half.andThen(a -> 3 * a); 
  
        // apply the function to get the result 
        System.out.println(half.apply(10)); 
    } 
} 



















Output:


15.0



Program 2: To demonstrate when NullPointerException is returned.







 


 

 













import java.util.function.Function; 
  
public class Main { 
    public static void main(String args[]) 
    { 
  
        // Function which takes in a number and 
        // returns half of it 
  
        Function<Integer, Double> half = a -> a / 2.0; 
  
        try { 
  
            // try to pass null as parameter 
            half = half.andThen(null); 
        } 
        catch (Exception e) { 
            System.out.println("Exception thrown "
                               + "while passing null: " + e); 
        } 
    } 
} 



















Output:


Exception thrown while passing null: java.lang.NullPointerException



Reference: https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html#andThen-java.util.function.Function-
3. compose()
It returns a composed function wherein the parameterized function will be executed first and then the first one. If evaluation of either function throws an error, it is relayed to the caller of the composed function.
Syntax:
default <V> Function<V, R> 

    compose(Function<? super V, ? extends T> before)
where V is the type of input of the before function, and of the composed function
Parameters: This method accepts a parameter before which is the function to be applied first and then the current one
Return Value: This method returns a composed function that applies the current function after the parameterized function
Exception: This method throws NullPointerException if the before function is null.
Below is the code to illustrate compose() method:
Program 1:







 


 

 













import java.util.function.Function; 
  
public class Main { 
    public static void main(String args[]) 
    { 
        // Function which takes in a number and 
        // returns half of it 
        Function<Integer, Double> half = a -> a / 2.0; 
  
        // However treble the value given to half function 
        half = half.compose(a -> 3 * a); 
  
        // apply the function to get the result 
        System.out.println(half.apply(5)); 
    } 
} 



















Output:


7.5



Program 2: To demonstrate when NullPointerException is returned.







 


 

 













import java.util.function.Function; 
  
public class Main { 
    public static void main(String args[]) 
    { 
        // Function which takes in a number and 
        // returns half of it 
        Function<Integer, Double> half = a -> a / 2.0; 
  
        try { 
  
            // try to pass null as parameter 
            half = half.compose(null); 
        } 
        catch (Exception e) { 
            System.out.println("Exception thrown "
                               + "while passing null: " + e); 
        } 
    } 
} 



















Output:


Exception thrown while passing null: java.lang.NullPointerException



Reference: https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html#compose-java.util.function.Function-
4. identity()
This method returns a function which returns its only argument.
Syntax:
static <T> Function<T, T> identity()
where T denotes the type of the argument and the value to be returned
Returns: This method returns a function which returns its own argument
Below is the code to illustrate identity() method:
Program:







 


 

 













import java.util.function.Function; 
  
public class Main { 
    public static void main(String args[]) 
    { 
  
        // Function which takes in a number and 
        // returns it 
        Function i = Function.identity(); 
  
        System.out.println(i); 
    } 
} 



















Output:


java.util.function.Function$$Lambda$1/250421012@119d7047



Reference: https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html#identity–





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