Java 8 Function Descriptors Explained

Definition – In Java 8 a Function Descriptor is a term used to describe the signature of the abstract method of a Functional Interface
Click to Read tutorial on Java 8 Functional Interfaces
. The signature of the abstract method of a Functional Interface is syntactically the same as the signature of the Lambda Expression
Click to Read Java 8 Lambda Expressions tutorial
. Hence, a Function Descriptor also describes the signature of a lambda.

Examples of function descriptors
To illustrate let us see a few examples of function descriptors –
Example 1: Lets say we have a functional interface named FirstInterface. It’s described as below –

Example 1 -
package com.javabrahman.java8;
public interface FirstInterface {
  //Single abstract method
  public void singleMethod(String param);
For the above interface, named FirstInterface, the signature of the abstract method OR the function descriptor is (String) -> void

Example 2: Functional Interface has a slight variation of the abstract method –

Example 2 -
package com.javabrahman.java8;
public interface SecondInterface {
  //Single abstract method
  public long computeSum(int num1, int num2);
For SecondInterface the function descriptor is (int,int) -> long

Example 3: Java’s in-built java.lang.Runnable interface has a single public void run() method. The function descriptor for Runnable interface will be () -> void

Example 4: Lets see an example of a generic type based in-built functional interface named Function<T, R> introduced in Java 8 –

Example 4 - In-built java.util.function.Function<T, R>
public interface Function<T, R> {
     * Applies this function to the given argument.
     * @param t the function argument
     * @return the function result of type R
    R apply(T t);
The function descriptor for Function<T, R> will be T -> R



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