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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 - FirstInterface.java
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 SecondInterface.java has a slight variation of the abstract method -
Example 2 - SecondInterface.java
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