Java 8 Function Interface

Function Interface (Functional Interface) is a method of having a common interface.

Function interface can be implicitly converted to a lambda expression.

Function interface can support existing functions friendly lambda.

Before JDK 1.8 has a function interface:

java.lang.Runnable
java.util.concurrent.Callable
java.security.PrivilegedAction
java.util.Comparator
java.io.FileFilter
java.nio.file.PathMatcher
java.lang.reflect.InvocationHandler
java.beans.PropertyChangeListener
java.awt.event.ActionListener
javax.swing.event.ChangeListener

JDK 1.8 newly added function interface:

java.util.function

java.util.function It contains many classes, functions to support Java programming, this package of functional interfaces:

No.	Interface and Description
1	BiConsumer <T, U> It represents an operation that takes two input parameters, and does not return any results
2	BiFunction <T, U, R> It represents a method accepts two input parameters, and returns a result
3	BinaryOperator <T> Acting on behalf of an operator in two of the same type of operation, and the operator returns the same type of results
4	BiPredicate <T, U> It represents a boolean value method two parameters
5	BooleanSupplier It represents the boolean value of the result of the provider
6	Consumer <T> Represents accepts an input parameter and no return operation
7	DoubleBinaryOperator It is acting on behalf of the two double values operator operation, and returns the results of a double value.
8	DoubleConsumer Accepts an operation on behalf of a double value of the parameter, and does not return a result.
9	DoubleFunction <R> Accepting on behalf of a double parameter method, and returns the result
10	DoublePredicate It represents a boolean value of the parameter has a double value method
11	DoubleSupplier Value represents a double structure Provider
12	DoubleToIntFunction Accepts a double type input and returns an int result.
13	DoubleToLongFunction Accepts a double type input and returns a long type of result
14	DoubleUnaryOperator Accepts a parameter for the same type of double, return type is also double.
15	Function <T, R> Accepts an input parameter and returns a result.
16	IntBinaryOperator Accepts two parameters with the same type int, the return value type is also int.
17	IntConsumer Accepts an int input parameters, no return value.
18	IntFunction <R> Accepts an input parameter of type int, and returns a result.
19	IntPredicate : Accepts an int input parameter and returns a Boolean value of the result.
20	IntSupplier No arguments and returns an int result.
twenty one	IntToDoubleFunction Accepts an input of type int, and returns a result of type double.
twenty two	IntToLongFunction Accepts an input of type int, and returns a long type of result.
twenty three	IntUnaryOperator Accepts a parameter for the same type int, the return value type is also int.
twenty four	LongBinaryOperator It accepts two parameters with the same type of long, return type for long.
25	LongConsumer It takes a long type of input parameters, no return value.
26	LongFunction <R> To accept a long type of input parameters and returns a result.
27	LongPredicate R takes a long input parameter and returns a Boolean value type results.
28	LongSupplier No parameters and returns a result value of type long.
29	LongToDoubleFunction To accept a long type of input and returns a result of type double.
30	LongToIntFunction To accept a long type of input and returns an int result.
31	LongUnaryOperator It accepts a parameter for the same type of long, return type for long.
32	ObjDoubleConsumer <T> Accepts an object type and a double type of input parameters, no return value.
33	ObjIntConsumer <T> Accepts an object type and an int input parameters, no return value.
34	ObjLongConsumer <T> Accepts an object type and a long type of input parameters, no return value.
35	Predicate <T> Accepts an input parameter and returns a Boolean result.
36	Supplier <T> No parameters and returns a result.
37	ToDoubleBiFunction <T, U> It accepts two input parameters and returns a result of type double
38	ToDoubleFunction <T> Accepts an input parameter and returns a result of type double
39	ToIntBiFunction <T, U> It accepts two input parameters and returns a result of type int.
40	ToIntFunction <T> Accepts an input parameter and returns a result of type int.
41	ToLongBiFunction <T, U> It accepts two input parameters and returns a long type of result.
42	ToLongFunction <T> Accepts an input parameter and returns a long type of result.
43	UnaryOperator <T> Accept a parameter of type T, the type of the return value for T.

Functional interface instance

Predicate <T> interface is a functional interface that accepts an input parameter T, returns a boolean result.

The interface contains several default methods will be combined into other complex Predicate logic (for example: AND, OR, NOT).

This interface is used to test objects are true or false.

We can by the following examples (Java8Tester.java) to understand the function interface Predicate <T> use:

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 是一个参数传递到 Predicate 接口的 test 方法
      // n 如果存在则 test 方法返回 true
		
      System.out.println("输出所有数据:");
		
      // 传递参数 n
      eval(list, n->true);
		
      // Predicate<Integer> predicate1 = n -> n%2 == 0
      // n 是一个参数传递到 Predicate 接口的 test 方法
      // 如果 n%2 为 0 test 方法返回 true
		
      System.out.println("输出所有偶数:");
      eval(list, n-> n%2 == 0 );
		
      // Predicate<Integer> predicate2 = n -> n > 3
      // n 是一个参数传递到 Predicate 接口的 test 方法
      // 如果 n 大于 3 test 方法返回 true
		
      System.out.println("输出大于 3 的所有数字:");
      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 + " ");
         }
      }
   }
}

Implementation of the above script, output is:

$ javac Java8Tester.java 
$ java Java8Tester
输出所有数据:
1 
2 
3 
4 
5 
6 
7 
8 
9 
输出所有偶数:
2 
4 
6 
8 
输出大于 3 的所有数字:
4 
5 
6 
7 
8 
9

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Functional interface instance