Java8 - Array Stream Lambda Expression How to Examples

Java8 - Array Stream Lambda Expression How to Examples

Stream API Array Lambda Examples

In my previous post, we covered and learned lambda expressions in java8 . This post focuses on frequently used Array lambda expression examples using Streams API.

Here are examples that we covered with Java 8 lambda expressions:

  • Trimming white spaces in an array of strings
  • Counting words in a string
  • Converting primitive types to Object types in an array of elements
  • Converting Object List to Primitive Array
  • Sorting an array using lambda expression in Java 8
  • Converting an array to a Stream of Arrays

Convert Array to List using Java 8

You can convert an array to a list in Java before Java 8 using the following lines of code.

Arrays.asList(array)

Java 8 simplifies this using lambda expressions and streams.

Here’s an example to convert an array of numbers and strings into a List

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Sort {
    public static void main(String[] args) {

        int numbers[] = {1, 2, 3, 4, 5};
        List<Integer> numbersList = Arrays.stream(numbers).boxed().collect(Collectors.toList());
        System.out.println("List of Integers: " + numbersList);

        String words[] = {"one","two","three","four"};
        List<String> wordsList = Arrays.stream(words).collect(Collectors.toList());
        System.out.println("List of Strings: " + wordsList);

    }
}

In this example,

  • Array is declared and initialized with values
  • Convert Array to Stream using Arrays.stream() method.
  • Convert the Stream of numbers into a Stream of integers using the boxed() method
  • Finally, Collect the data from streams using the collect method

Output:

List of Integers: [1, 2, 3, 4, 5]
List of Strings: [one, two, three, four]

How to Remove White Spaces from Array String in Java 8?

Remove or trim each string object from an array and return the string array without whitespaces.

Here’s a step-by-step approach using lambda Stream map function in Java 8:

  • Declare an array of string objects; some elements contain whitespaces.
  • Convert the array to a Stream using the Arrays.stream(array) method.
  • Use the stream class map() method with a lambda expression function to trim each string of the stream.
  • Once stream modification is done, convert the stream to an array using the toArray() method.
import java.util.Arrays;
public class Main {
    public static void main(String[] args) {
        String[] stringArray = { "one ", "two ", " three ", " four " };
        System.out.println("Original Array: " + Arrays.toString(stringArray));
        String[] result = Arrays.stream(stringArray).map(value -> value.trim()).toArray(size -> new String[size]);
        System.out.println("Trim Array: " + Arrays.toString(result));
    }
}
Original Array: [one, two ,  three,  four ]
Trim Array: [one, two, three, four]

How to Find the Count of Words in a String in Java 8

Given a string as a set of words separated by space, this code returns the count of each word repeated in the string.

Following are step by steps

  • Create a word string array using a regular expression.
  • Create a Stream array using Arrays.stream(array) method.
  • Group the same strings using Collectors.groupBy with Function.identity(); it is similar to the SQL group by keyword.
  • Supply grouping elements to the reduction operator with the collect method to return a Map with words and their count.
import java.util.Arrays;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        String stringText = "This is a testing java stream example of a lamda expression example";
        String[] word = stringText.trim().split("\\s+");
        Map<String, Long> mapWordCount = Arrays.stream(word)
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
        System.out.println("Map String with count:"+ mapWordCount);
    }
}

Output:

Map String with count:{a=2, expression=1, java=1, stream=1, of=1, testing=1, This=1, lamda=1, is=1, example=2}

How to Convert Primitive int Array to Object Array in Java 8?

Here’s how you can convert a primitive array to an Object array in Java 8.

  • First, convert the primitive array to a Stream using the Arrays.stream() method.
  • Next, use the boxed() method, which takes the stream array and converts it to an Integer stream.
  • Finally, convert the stream to an array using toArray() to return an Integer object Array.
import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] primitiveIntArray = {11, 21, 13, 41, 15,61};
        Integer[] integerObjectArray=Arrays.stream(primitiveIntArray).boxed().toArray(Integer[]::new);
        System.out.println("Integer Object Array: " + Arrays.toString(integerObjectArray));
    }
}

Output:

Integer Object Array: [11, 21, 13, 41, 15, 61]

How to Convert Object List to Primitive Array in Java 8?

This example demonstrates converting a List<Object> to int[] in Java 8.

  • First, create a List of Employee objects, with each employee object containing an id and name.
  • Next, convert the list to a stream using the list.stream() method. Pass this stream of objects to the mapToInt function to convert to primitive values of streams.
  • Finally, convert to an array from the stream using the toArray() method.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class LamdaExpressionThisExample {

    public static void main(String[] args) {
        List<Emp> emps = new ArrayList<Emp>();
        emps.add(new Emp(1, "one"));
        emps.add(new Emp(4, "four"));
        emps.add(new Emp(6, "six"));
        emps.add(new Emp(9, "nine"));
        emps.add(new Emp(12, "tweleve"));
        int[] idsArray = emps.stream().mapToInt(Emp::getId).toArray();
        System.out.println(Arrays.toString(idsArray));
    }
}

class Emp {
    Integer id;
    String name;

    Emp(Integer id, String name) {
        this.id = id;
        this.name = name;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Output:

[1, 4, 6, 9, 12]

How to Sort an Array of Numbers in Java 8 Using Stream and Lambda?

Here is a sequence of steps to sort array numbers in Java 8.

  • First, convert IntStream using IntStream.of() method.
  • Sort using sorted, does boxed, and mapToInt reduce operations with a Comparator.
  • For descending order, use Comparator.reverseOrder().
  • For ascending order, use the default Comparator.

The below example sorts numbers in ascending and descending order using lambda and stream classes.

import java.util.Arrays;
import java.util.Comparator;
import java.util.stream.IntStream;

public class Main {
    public static void main(String[] args) {
        int intArray[] = {56,11,1,67,34,3,90,2};
// Sort Descending Order
        int[] sortedDescOutput = IntStream.of(intArray)
                .boxed()
                .sorted(Comparator.reverseOrder())
                .mapToInt(i -> i)
                .toArray();
        // Sort Asecnding Order
        int[] sortedAscOutput = IntStream.of(intArray)
                .boxed()
                .sorted()
                .mapToInt(i -> i)
                .toArray();
        System.out.println(Arrays.toString(sortedDescOutput));
        System.out.println(Arrays.toString(sortedAscOutput));
    }
}

Output:

[90, 67, 56, 34, 11, 3, 2, 1]
[1, 2, 3, 11, 34, 56, 67, 90]

Conclusion

Learned multiple examples on array streams using lambda expression in java8.