Java define function in function

How to do function composition in Java?

Function composition is a technique in functional programming where the output of one function is used as the input for another function. In Java, function composition can be achieved through various methods such as method references, lambda expressions, and functional interfaces. However, the specific implementation of function composition may vary depending on the use case and desired behavior.

Method 1: Using Method References

Assuming you have two functions f and g that you want to compose, you can use method references to create a new function that applies f to the result of g . Here’s an example:

FunctionInteger, Integer> f = x -> x * 2; FunctionInteger, Integer> g = x -> x + 1; FunctionInteger, Integer> h = g.andThen(f); int result = h.apply(1); // result = f(g(1)) = 4

In this example, h is a new function that applies g to its input, and then applies f to the result. We use the andThen method to create this composition.

The andThen method is a default method on the Function interface, which takes another function as an argument and returns a new function that applies the first function and then the second function to its input.

We can also use method references to create functions from existing methods. Here’s an example:

class StringUtils  public static String reverse(String s)  return new StringBuilder(s).reverse().toString(); > > FunctionString, String> f = StringUtils::reverse; FunctionString, Integer> g = String::length; FunctionString, Integer> h = f.andThen(g); int result = h.apply("hello"); // result = g(f("hello")) = 5

In this example, we define a StringUtils class with a reverse method that takes a string and returns the reversed string. We use a method reference to create a function f that applies this method to its input.

We also use a method reference to create a function g that returns the length of a string.

Finally, we compose f and g using the andThen method to create a new function h that applies f to its input and then applies g to the result.

We can then apply h to a string to get the length of the reversed string.

That’s it! Using method references is a concise and elegant way to compose functions in Java.

Method 2: Using Lambda Expressions

Java Function Composition using Lambda Expressions

Function composition is a powerful technique in programming that allows you to combine two or more functions into a single function. In Java, you can use lambda expressions to achieve function composition.

Here is an example of how to compose two functions using lambda expressions:

FunctionInteger, Integer> add = x -> x + 1; FunctionInteger, Integer> multiply = x -> x * 2; FunctionInteger, Integer> addAndMultiply = add.andThen(multiply); int result = addAndMultiply.apply(5); // result = (5 + 1) * 2 = 12

In the above example, we have two functions add and multiply that add 1 and multiply by 2, respectively. We then use the andThen method to compose these two functions into a single function addAndMultiply . Finally, we apply this composed function to the value 5, which results in 12.

You can also use the compose method to compose functions in the opposite order:

FunctionInteger, Integer> add = x -> x + 1; FunctionInteger, Integer> multiply = x -> x * 2; FunctionInteger, Integer> multiplyAndAdd = multiply.compose(add); int result = multiplyAndAdd.apply(5); // result = (5 * 2) + 1 = 11

In this example, we have two functions add and multiply as before. However, this time we use the compose method to compose these two functions in the opposite order to create a new function multiplyAndAdd . We then apply this composed function to the value 5, which results in 11.

You can also chain multiple functions together using the andThen method:

FunctionInteger, Integer> add = x -> x + 1; FunctionInteger, Integer> multiply = x -> x * 2; FunctionInteger, Integer> subtract = x -> x - 3; FunctionInteger, Integer> addMultiplyAndSubtract = add.andThen(multiply).andThen(subtract); int result = addMultiplyAndSubtract.apply(5); // result = ((5 + 1) * 2) - 3 = 7

In this example, we have three functions add , multiply , and subtract . We use the andThen method to chain them together and create a new function addMultiplyAndSubtract . We then apply this composed function to the value 5, which results in 7.

In summary, function composition is a powerful technique that can help you to write more concise and reusable code. In Java, you can use lambda expressions and the andThen or compose methods to compose functions together.

Method 3: Using Functional Interfaces

Function composition is a technique that allows developers to combine two or more functions to create a new function. In Java, we can use functional interfaces to accomplish function composition. Here are the steps to do function composition using functional interfaces:

Step 1: Define the Functional Interfaces

We need to define the functional interfaces that we want to use for function composition. For example, let’s define two functional interfaces: Function1 and Function2 .

@FunctionalInterface interface Function1T, R>  R apply(T t); > @FunctionalInterface interface Function2T, U, R>  R apply(T t, U u); >

Step 2: Implement the Functions

Next, we need to implement the functions that we want to compose. For example, let’s implement two functions: addOne and multiplyByTwo .

Function1Integer, Integer> addOne = x -> x + 1; Function1Integer, Integer> multiplyByTwo = x -> x * 2;

Step 3: Compose the Functions

Now, we can compose the functions using the andThen method of the Function interface. For example, let’s compose the addOne and multiplyByTwo functions.

Function1Integer, Integer> addOneAndMultiplyByTwo = addOne.andThen(multiplyByTwo);

Step 4: Use the Composed Function

Finally, we can use the composed function to apply the two functions in order. For example, let’s apply the addOneAndMultiplyByTwo function to the integer value 2 .

int result = addOneAndMultiplyByTwo.apply(2); // result is 6

That’s it! We have successfully composed two functions using functional interfaces in Java.

Here is the complete code example:

@FunctionalInterface interface Function1T, R>  R apply(T t); > @FunctionalInterface interface Function2T, U, R>  R apply(T t, U u); > public class Main  public static void main(String[] args)  Function1Integer, Integer> addOne = x -> x + 1; Function1Integer, Integer> multiplyByTwo = x -> x * 2; Function1Integer, Integer> addOneAndMultiplyByTwo = addOne.andThen(multiplyByTwo); int result = addOneAndMultiplyByTwo.apply(2); // result is 6 System.out.println(result); > >

Method 4: Using a Utility Library

Function composition is a technique used in functional programming to combine multiple functions into a single function. In Java, we can achieve function composition using a Utility Library. Here is how to do it:

Step 1: Import the necessary classes

import java.util.function.Function;

Step 2: Define your functions

FunctionInteger, Integer> addTwo = x -> x + 2; FunctionInteger, Integer> multiplyByThree = x -> x * 3;

Step 3: Compose the functions using the andThen method

FunctionInteger, Integer> addTwoAndMultiplyByThree = addTwo.andThen(multiplyByThree);

In the above code, we are using the andThen method to compose the addTwo and multiplyByThree functions. The andThen method applies the first function and then applies the second function to the result.

Step 4: Test the composed function

int result = addTwoAndMultiplyByThree.apply(4); System.out.println(result); // Output: 18

In the above code, we are testing the composed function addTwoAndMultiplyByThree by passing an integer value of 4 to it. The output of the function should be 18 because addTwo adds 2 to 4 and multiplyByThree multiplies the result by 3 .

That’s it! You have successfully composed functions using a Utility Library in Java.

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Как передать функцию в функцию java

Передать функцию в качестве параметра в другую функцию можно разными способами. Рассмотрим некоторые из них.

1) Воспользуемся функциональным интерфейсом Predicate :

import java.util.List; import java.util.function.Predicate; import java.util.stream.Collectors; public class App  public static void main(String[] args)  ListString> list1 = List.of("1", "22", "333", "4444"); ListString> filteredList1 = filterList(list1, x -> x.length() >= 3); System.out.println(filteredList1); // => [333, 4444] ListInteger> list2 = List.of(1, 2, 3, 4); ListInteger> filteredList2 = filterList(list2, x -> x >= 3); System.out.println(filteredList2); // => [3, 4] > public static T> ListT> filterList(ListT> list, PredicateT> rool)  return list.stream() .filter(x -> rool.test(x)) .collect(Collectors.toList()); > > 

2) Воспользуемся готовым функциональным интерфейсом UnaryOperator :

public static void superMethod(UnaryOperatorString> method)  String str = "Hexlet"; String result = method.apply(str); System.out.println(result); > // вызов с передачей методов public class App  public static void main(String[] args) throws Exception  // передадим стандартный метод superMethod(String::toUpperCase); // => HEXLET // передадим лямбда-функцию superMethod(s -> s + "!"); // => hexlet! // передадим собственный метод superMethod(App::reverse); // => telxeh > public static String reverse(String str)  StringBuilder builder = new StringBuilder(); builder.append(str); return builder.reverse().toString(); > > 

3) Создадим собственный интерфейс и передадим объект этого типа в нашу функцию :

interface MyInterface  int count(int a, int b, int c); > public static void superMethodInterface(MyInterface method)  int a = 5, b = 10, c = 20; int result = method.count(a, b, c); System.out.println(result); > // вызов с передачей методов public class App  public static void main(String[] args) throws Exception  MyInterface count = new MyInterface()  @Override public int count(int a, int b, int c)  return a + b + c; > >; superMethodInterface(count); // => 35 superMethodInterface((a,b,c) -> a * b * c); // => 1000 superMethodInterface((a,b,c) -> a + b - c); // => -5 > > 

4) Получим метод с помощью рефлексии и передадим его :

public static void superMethodReflection(Object object, Method method) throws Exception  int a = 10; int b = 20; int result = (int) method.invoke(object, a, b); System.out.println(result); > // вызов с передачей методов public class App  public static void main(String[] args) throws Exception  // передадим стандартный метод Method method = Integer.class.getDeclaredMethod("max", int.class, int.class); superMethodReflection(0, method); // => 20 method = Integer.class.getDeclaredMethod("sum", int.class, int.class); superMethodReflection(0, method); // => 30 // передадим собственный метод method = App.class.getDeclaredMethod("concate", int.class, int.class); superMethodReflection(new App(), method); // => 1020 > public static int concate(int a, int b)  return Integer.parseInt("" + a + b); > > 

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