# Java 8 - Functional Interfaces

Functional interfaces have a single functionality to exhibit. For example, a Comparable interface with a single method ‘compareTo’ is used for comparison purpose. Java 8 has defined a lot of functional interfaces to be used extensively in lambda expressions. Following is the list of functional interfaces defined in java.util.Function package.

Sr.No. | Interface & Description |
---|---|

1 |
Represents an operation that accepts two input arguments, and returns no result. |

2 |
Represents a function that accepts two arguments and produces a result. |

3 |
Represents an operation upon two operands of the same type, producing a result of the same type as the operands. |

4 |
Represents a predicate (Boolean-valued function) of two arguments. |

5 |
Represents a supplier of Boolean-valued results. |

6 |
Represents an operation that accepts a single input argument and returns no result. |

7 |
Represents an operation upon two double-valued operands and producing a double-valued result. |

8 |
Represents an operation that accepts a single double-valued argument and returns no result. |

9 |
Represents a function that accepts a double-valued argument and produces a result. |

10 |
Represents a predicate (Boolean-valued function) of one double-valued argument. |

11 |
Represents a supplier of double-valued results. |

12 |
Represents a function that accepts a double-valued argument and produces an int-valued result. |

13 |
Represents a function that accepts a double-valued argument and produces a long-valued result. |

14 |
Represents an operation on a single double-valued operand that produces a double-valued result. |

15 |
Represents a function that accepts one argument and produces a result. |

16 |
Represents an operation upon two int-valued operands and produces an int-valued result. |

17 |
Represents an operation that accepts a single int-valued argument and returns no result. |

18 |
Represents a function that accepts an int-valued argument and produces a result. |

19 |
Represents a predicate (Boolean-valued function) of one int-valued argument. |

20 |
Represents a supplier of int-valued results. |

21 |
Represents a function that accepts an int-valued argument and produces a double-valued result. |

22 |
Represents a function that accepts an int-valued argument and produces a long-valued result. |

23 |
Represents an operation on a single int-valued operand that produces an int-valued result. |

24 |
Represents an operation upon two long-valued operands and produces a long-valued result. |

25 |
Represents an operation that accepts a single long-valued argument and returns no result. |

26 |
Represents a function that accepts a long-valued argument and produces a result. |

27 |
Represents a predicate (Boolean-valued function) of one long-valued argument. |

28 |
Represents a supplier of long-valued results. |

29 |
Represents a function that accepts a long-valued argument and produces a double-valued result. |

30 |
Represents a function that accepts a long-valued argument and produces an int-valued result. |

31 |
Represents an operation on a single long-valued operand that produces a long-valued result. |

32 |
Represents an operation that accepts an object-valued and a double-valued argument, and returns no result. |

33 |
Represents an operation that accepts an object-valued and an int-valued argument, and returns no result. |

34 |
Represents an operation that accepts an object-valued and a long-valued argument, and returns no result. |

35 |
Represents a predicate (Boolean-valued function) of one argument. |

36 |
Represents a supplier of results. |

37 |
Represents a function that accepts two arguments and produces a double-valued result. |

38 |
Represents a function that produces a double-valued result. |

39 |
Represents a function that accepts two arguments and produces an int-valued result. |

40 |
Represents a function that produces an int-valued result. |

41 |
Represents a function that accepts two arguments and produces a long-valued result. |

42 |
Represents a function that produces a long-valued result. |

43 |
Represents an operation on a single operand that produces a result of the same type as its operand. |

## Functional Interface Example

Predicate <T> interface is a functional interface with a method test(Object) to return a Boolean value. This interface signifies that an object is tested to be true or false.

Create the following Java program using any editor of your choice in, say, C:\> JAVA.

### Java8Tester.java

Live Demoimport java.util.Arrays; import java.util.List; import java.util.function.Predicate; public class Java8Tester { public static void main(String args[]) { List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9); // Predicate<Integer> predicate = n -> true // n is passed as parameter to test method of Predicate interface // test method will always return true no matter what value n has. System.out.println("Print all numbers:"); //pass n as parameter eval(list, n->true); // Predicate<Integer> predicate1 = n -> n%2 == 0 // n is passed as parameter to test method of Predicate interface // test method will return true if n%2 comes to be zero System.out.println("Print even numbers:"); eval(list, n-> n%2 == 0 ); // Predicate<Integer> predicate2 = n -> n > 3 // n is passed as parameter to test method of Predicate interface // test method will return true if n is greater than 3. System.out.println("Print numbers greater than 3:"); eval(list, n-> n > 3 ); } public static void eval(List<Integer> list, Predicate<Integer> predicate) { for(Integer n: list) { if(predicate.test(n)) { System.out.println(n + " "); } } } }

Here we've passed Predicate interface, which takes a single input and returns Boolean.

### Verify the Result

Compile the class using **javac** compiler as follows −

C:\JAVA>javac Java8Tester.java

Now run the Java8Tester as follows −

C:\JAVA>java Java8Tester

It should produce the following output −

Print all numbers: 1 2 3 4 5 6 7 8 9 Print even numbers: 2 4 6 8 Print numbers greater than 3: 4 5 6 7 8 9