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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.
Java8Tester.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
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