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
BiConsumer<T,U>
Represents an operation that accepts two input arguments, and returns no result.
2
BiFunction<T,U,R>
Represents a function that accepts two arguments and produces a result.
3
BinaryOperator<T>
Represents an operation upon two operands of the same type, producing a result of the same type as the operands.
4
BiPredicate<T,U>
Represents a predicate (Boolean-valued function) of two arguments.
5
BooleanSupplier
Represents a supplier of Boolean-valued results.
6
Consumer<T>
Represents an operation that accepts a single input argument and returns no result.
7
DoubleBinaryOperator
Represents an operation upon two double-valued operands and producing a double-valued result.
8
DoubleConsumer
Represents an operation that accepts a single double-valued argument and returns no result.
9
DoubleFunction<R>
Represents a function that accepts a double-valued argument and produces a result.
10
DoublePredicate
Represents a predicate (Boolean-valued function) of one double-valued argument.
11
DoubleSupplier
Represents a supplier of double-valued results.
12
DoubleToIntFunction
Represents a function that accepts a double-valued argument and produces an int-valued result.
13
DoubleToLongFunction
Represents a function that accepts a double-valued argument and produces a long-valued result.
14
DoubleUnaryOperator
Represents an operation on a single double-valued operand that produces a double-valued result.
15
Function<T,R>
Represents a function that accepts one argument and produces a result.
16
IntBinaryOperator
Represents an operation upon two int-valued operands and produces an int-valued result.
17
IntConsumer
Represents an operation that accepts a single int-valued argument and returns no result.
18
IntFunction<R>
Represents a function that accepts an int-valued argument and produces a result.
19
IntPredicate
Represents a predicate (Boolean-valued function) of one int-valued argument.
20
IntSupplier
Represents a supplier of int-valued results.
21
IntToDoubleFunction
Represents a function that accepts an int-valued argument and produces a double-valued result.
22
IntToLongFunction
Represents a function that accepts an int-valued argument and produces a long-valued result.
23
IntUnaryOperator
Represents an operation on a single int-valued operand that produces an int-valued result.
24
LongBinaryOperator
Represents an operation upon two long-valued operands and produces a long-valued result.
25
LongConsumer
Represents an operation that accepts a single long-valued argument and returns no result.
26
LongFunction<R>
Represents a function that accepts a long-valued argument and produces a result.
27
LongPredicate
Represents a predicate (Boolean-valued function) of one long-valued argument.
28
LongSupplier
Represents a supplier of long-valued results.
29
LongToDoubleFunction
Represents a function that accepts a long-valued argument and produces a double-valued result.
30
LongToIntFunction
Represents a function that accepts a long-valued argument and produces an int-valued result.
31
LongUnaryOperator
Represents an operation on a single long-valued operand that produces a long-valued result.
32
ObjDoubleConsumer<T>
Represents an operation that accepts an object-valued and a double-valued argument, and returns no result.
33
ObjIntConsumer<T>
Represents an operation that accepts an object-valued and an int-valued argument, and returns no result.
34
ObjLongConsumer<T>
Represents an operation that accepts an object-valued and a long-valued argument, and returns no result.
35
Predicate<T>
Represents a predicate (Boolean-valued function) of one argument.
36
Supplier<T>
Represents a supplier of results.
37
ToDoubleBiFunction<T,U>
Represents a function that accepts two arguments and produces a double-valued result.
38
ToDoubleFunction<T>
Represents a function that produces a double-valued result.
39
ToIntBiFunction<T,U>
Represents a function that accepts two arguments and produces an int-valued result.
40
ToIntFunction<T>
Represents a function that produces an int-valued result.
41
ToLongBiFunction<T,U>
Represents a function that accepts two arguments and produces a long-valued result.
42
ToLongFunction<T>
Represents a function that produces a long-valued result.
43
UnaryOperator<T>
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.
import 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
