java.util.function apply compose andThen 解析与应用（1）

原文：https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html

Type Parameters:

T - the type of the input to the function

R - the type of the result of the function

All Known Subinterfaces:

UnaryOperator<T>

Functional Interface:

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
```
@FunctionalInterface
public interface Function<T,R>
```
Represents a function that accepts one argument and produces a result.
This is a functional interface whose functional method is apply(Object).

Since:

1.8

Method Summary

All Methods Static Methods Instance Methods Abstract Methods Default Methods
Modifier and Type	Method and Description
`default <V> Function<T,V>`	`andThen(Function<? super R,? extends V> after)` Returns a composed function that first applies this function to its input, and then applies the `after` function to the result.
`R`	`apply(T t)` Applies this function to the given argument.
`default <V> Function<V,R>`	`compose(Function<? super V,? extends T> before)` Returns a composed function that first applies the `before` function to its input, and then applies this function to the result.
`static <T> Function<T,T>`	`identity()` Returns a function that always returns its input argument.

- Method Detail
  - apply
```
R apply(T t)
```
    Applies this function to the given argument.
    
    Parameters:
    
    t - the function argument
    
    Returns:
    
    the function result
  - compose
```
default <V> Function<V,R> compose(Function<? super V,? extends T> before)
```
    Returns a composed function that first applies the before function to its input, and then applies this function to the result. If evaluation of either function throws an exception, it is relayed to the caller of the composed function.
    
    Type Parameters:
    
    V - the type of input to the before function, and to the composed function
    
    Parameters:
    
    before - the function to apply before this function is applied
    
    Returns:
    
    a composed function that first applies the before function and then applies this function
    
    Throws:
    
    NullPointerException - if before is null
    
    See Also:
    
    andThen(Function)
  - andThen
```
default <V> Function<T,V> andThen(Function<? super R,? extends V> after)
```
    Returns a composed function that first applies this function to its input, and then applies the after function to the result. If evaluation of either function throws an exception, it is relayed to the caller of the composed function.
    
    Type Parameters:
    
    V - the type of output of the after function, and of the composed function
    
    Parameters:
    
    after - the function to apply after this function is applied
    
    Returns:
    
    a composed function that first applies this function and then applies the after function
    
    Throws:
    
    NullPointerException - if after is null
    
    See Also:
    
    compose(Function)
  - identity
```
static <T> Function<T,T> identity()
```
    Returns a function that always returns its input argument.
    
    Type Parameters:
    
    T - the type of the input and output objects to the function
    
    Returns:
    
    a function that always returns its input argument

在此文档中，很清楚了说明了 Function<T,R> 用途，我们编写一个demo边参考边应用

我们先看下

apply

R apply(T t)

Applies this function to the given argument.

Parameters:: t - the function argument
Returns:: the function result

demo:

public class FunctionApply {

	public static void main(String[] args) {
		int incr = 20;
		int myNumber = 10;
		
		modifyTheValue(myNumber, val -> val + incr);
		myNumber = 15;
		modifyTheValue(myNumber, val -> val * 10);
		modifyTheValue(myNumber, val -> val - 100);
		modifyTheValue(myNumber, val -> "somestring".length() + val - 100);

	}
	static void modifyTheValue(int valueToBeOperated, Function<Integer, Integer> function) {
		int newValue = function.apply(valueToBeOperated);//function对象应用到参数valueToBeOperated并返回结果
		System.out.println(newValue);
	}
}

compose

default <V> Function<V,R> compose(Function<? super V,? extends T> before)

Returns a composed function that first applies the before function to its input, and then applies this function to the result. If evaluation of either function throws an exception, it is relayed to the caller of the composed function.

Type Parameters:: V - the type of input to the before function, and to the composed function
Parameters:: before - the function to apply before this function is applied
Returns:: a composed function that first applies the before function and then applies this function
Throws:: NullPointerException - if before is null
See Also:: andThen(Function)

demo

public class FunctionApply {

	public static void main(String[] args) {
		/*int incr = 20;
		int myNumber = 10;

		modifyTheValue(myNumber, val -> val + incr);
		myNumber = 15;
		modifyTheValue(myNumber, val -> val * 10);
		modifyTheValue(myNumber, val -> val - 100);
		modifyTheValue(myNumber, val -> "somestring".length() + val - 100);*/
		System.out.println("compose:" + composeExam("2000"));
	}

	static void modifyTheValue(int valueToBeOperated, Function<Integer, Integer> function) {
		int newValue = function.apply(valueToBeOperated);// function对象应用到参数valueToBeOperated并返回结果
		System.out.println(newValue);
	}

	// Returns a composed function that first applies the before function to its
	// input, and then applies this function to the result. If evaluation of
	// either function throws an exception, it is relayed to the caller of the
	// composed function
	public static int composeExam(String value) {
		Function<Integer, String> converter = (i) -> Integer.toString(i);
		Function<String, Integer> reverse = (s) -> Integer.parseInt(s);//  that first applies the before function to its input
		return converter.compose(reverse).apply(value).length();//and then applies this function to the result
	}
}

andThen

default <V> Function<T,V> andThen(Function<? super R,? extends V> after)

Returns a composed function that first applies this function to its input, and then applies the after function to the result. If evaluation of either function throws an exception, it is relayed to the caller of the composed function.

Type Parameters:: V - the type of output of the after function, and of the composed function
Parameters:: after - the function to apply after this function is applied
Returns:: a composed function that first applies this function and then applies the after function
Throws:: NullPointerException - if after is null
See Also:: compose(Function)

demo

public static void main(String[] args) {
		/*int incr = 20;
		int myNumber = 10;

		modifyTheValue(myNumber, val -> val + incr);
		myNumber = 15;
		modifyTheValue(myNumber, val -> val * 10);
		modifyTheValue(myNumber, val -> val - 100);
		modifyTheValue(myNumber, val -> "somestring".length() + val - 100);*/
		System.out.println("compose:" + composeExam("2000"));
		System.out.println("compose:" + andThenExam(2000));
	}

	/*
	 * Returns a composed function that first applies this function to its
	 * input, and then applies the after function to the result
	 */
	public static int andThenExam(int value) {
	        Function<Integer, String> converter = (i) -> Integer.toString(i);
	        Function<String, Integer> reverse = (s) -> Integer.parseInt(s);// 后执行
	        // 因为有类型推断，也可以做如下使用：
	        // converter.andThen((s) -> Integer.parseInt(s)).apply(value).byteValue();
	        return converter.andThen(reverse).apply(value).byteValue();
	}