Java 8 Function Examples

By mkyong | Updated: February 26, 2020

Viewed: 85,096 (+468 pv/w)

Tags: convert function functional interface hashing java 8 sha256

In Java 8, Function is a functional interface; it takes an argument (object of type T) and returns an object (object of type R). The argument and output can be a different type.

@FunctionalInterface
public interface Function<T, R> {

      R apply(T t);

}

• T – Type of the input to the function.
• R – Type of the result of the function.

1. Function<T, R>

1.1 This example takes a <T> String and returns the length of the string as <R> Integer.

package com.mkyong;

import java.util.function.Function;

public class JavaMoney {

    public static void main(String[] args) {

        Function<String, Integer> func = x -> x.length();

        Integer apply = func.apply("mkyong");   // 6

        System.out.println(apply);

    }

}

Output

6

2. Chain Function<T, R>

2.1 This example chains the Function with andThen().

package com.mkyong;

import java.util.function.Function;

public class Java8Function2 {

    public static void main(String[] args) {

        Function<String, Integer> func = x -> x.length();

        Function<Integer, Integer> func2 = x -> x * 2;

        Integer result = func.andThen(func2).apply("mkyong");   // 12

        System.out.println(result);

    }

}

Output

12

3. List -> Map

3.1 This example accepts Function as an argument, convert a List into a Map.

package com.mkyong;

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;

public class Java8Function3 {

    public static void main(String[] args) {

        Java8Function3 obj = new Java8Function3();

        List<String> list = Arrays.asList("node", "c++", "java", "javascript");

        // lambda
        Map<String, Integer> map = obj.convertListToMap(list, x -> x.length());

        System.out.println(map);    // {node=4, c++=3, java=4, javascript=10}

        // method reference
        Map<String, Integer> map2 = obj.convertListToMap(list, obj::getLength);

        System.out.println(map2);
    }

    public <T, R> Map<T, R> convertListToMap(List<T> list, Function<T, R> func) {

        Map<T, R> result = new HashMap<>();
        for (T t : list) {
            result.put(t, func.apply(t));
        }
        return result;

    }

    public Integer getLength(String str) {
        return str.length();
    }

}

Output

{node=4, c++=3, java=4, javascript=10}
{node=4, c++=3, java=4, javascript=10}

4. List -> List

4.1 This example accepts Function as an argument, convert a List of String into another List of String, which was hashed with SHA256.

package com.mkyong;

import org.apache.commons.codec.digest.DigestUtils;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Function;

public class Java8Function4 {

    public static void main(String[] args) {

        Java8Function4 obj = new Java8Function4();

        List<String> list = Arrays.asList("node", "c++", "java", "javascript");

        // lambda
        //List<String> result = obj.map(list, x -> obj.sha256(x));

        // method reference
        List<String> result = obj.map(list, obj::sha256);

        result.forEach(System.out::println);

    }

    public <T, R> List<R> map(List<T> list, Function<T, R> func) {

        List<R> result = new ArrayList<>();
        for (T t : list) {
            result.add(func.apply(t));
        }
        return result;

    }

    // sha256 a string
    public String sha256(String str) {
        return DigestUtils.sha256Hex(str);
    }

}

Output

545ea538461003efdc8c81c244531b003f6f26cfccf6c0073b3239fdedf49446
cedb1bac7efcd7db47e9f2f2250a7c832aba83b410dd85766e2aea6ec9321e51
38a0963a6364b09ad867aa9a66c6d009673c21e182015461da236ec361877f77
eda71746c01c3f465ffd02b6da15a6518e6fbc8f06f1ac525be193be5507069d

P.S The DigestUtils.sha256Hex is from the commons-codec.

<dependency>
  <groupId>commons-codec</groupId>
  <artifactId>commons-codec</artifactId>
  <version>1.14</version>
</dependency>

References

• Function JavaDoc
• Java 8 Tutorials
• Java 8 Predicate Examples
• Java 8 BiPredicate Examples
• Java 8 Consumer Examples
• Java 8 BiConsumer Examples