Java, Practice

Java. Examples of solving tasks on data streams Stream API

Examples of solving tasks on data streams Stream API

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1. Determining the number of even numbers in the data stream. Example

Task. For any set of randomly generated numbers, you need to determine the number of paired ones. To solve the problem, use the tools of the Stream API programming interface.

Solution. The solution to the problem is displayed in the static method DemoStream(), which is called from the main() function.

import java.util.stream.*;
import java.util.*;
import java.util.function.*;

public class TrainStreamAPI {

  public static void DemoStream() {
    // Task. For any set of randomly generated numbers,
    // you need to determine the number of paired ones.

    // 1. Create data stream from random array of numbers
    ArrayList<Integer> AL = new ArrayList<Integer>();
    int number;
    Random rnd = new Random();

    for (int i=0; i<10; i++) {
      number = rnd.nextInt() % 100;
      AL.add(number);
    }

    System.out.println("Array AL:");
    System.out.println(AL);

    // 2. Determine the number of paired numbers. Method 1.
    // 2.1. Create stream from AL array - stream() method
    Stream<Integer> st = AL.stream();

    // 2.2. Declare a reference to the Predicate<Integer>
    // standard functional interface
    Predicate<Integer> fn; // this reference will be passed to the filter() method

    // 2.3. Assign a lambda expression to the reference that determines if the number is even
    fn = (n) -> (n%2) == 0;

    // 2.4. Call the filter() method, which selects only pairs of numbers from stream st
    Stream<Integer> resStream = st.filter(fn);

    // 2.5. Print the number of paired numbers
    System.out.println("n = " + resStream.count());

    // 3. Determine the number of paired numbers. Method 2 - quick way
    int n2 = (int)(AL.stream().filter((n)->(n%2)==0)).count();
    System.out.println("n2 = " + n2);
  }

  public static void main(String[] args) {
    DemoStream();
  }
}

The result of the program

Array AL:
[-79, 99, -83, -75, -78, -22, -57, 84, 11, 15]
n = 3
n2 = 3

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2. Working with streams of strings. Search in a string. Example

Task. Many names of employees are set. Develop a program that displays all surnames starting with the letter “J”. Solve the problem using the Stream API.

Solution.

import java.util.stream.*;
import java.util.*;
import java.util.function.*;

public class TrainStreamAPI {

  public static void DemoStream() {
    // Task. Many names of employees are set.
    // Develop a program that displays all the names that begin with the letter 'J'

    // 1. Organize the input of surnames from the keyboard.
    // End of input - empty string ""
    Scanner scanner = new Scanner(System.in);
    String s;
    ArrayList<String> AL = new ArrayList<String>();

    System.out.println("Enter names: ");
    while (true) {
      System.out.print("name = ");
      s = scanner.nextLine();
      if (s.equals("")==true)
        break;
      AL.add(s);
    }
    System.out.println();

    // 2. Display an array of entered names
    System.out.println("AL = " + AL);

    // 3. Calculate the number of names that start with 'J'
    // 3.1. Declare a reference to the functional interface Predicate<T>
    Predicate<String> fn;

    // 3.2. Set lambda expression for fn
    fn = (str) -> {
      if (str.charAt(0)=='J')
        return true;
      return false;
    };

    // 3.3. Convert AL to stream of strings
    Stream<String> stream = AL.stream();

    // 3.4. Get the filtered stream
    // according to the predicate defined in fn()
    Stream<String> resStream = stream.filter(fn);

    // 3.5. Print the number of names starting with J
    System.out.println("count = " + resStream.count());
  }

  public static void main(String[] args) {
    DemoStream();
  }
}

The result of the program

Enter names:
name = abc
name = Jlskjf
name = JJ
name = lskdjlsk
name = sldkfjs
name = dflskdj
name = sdsd
name = ss
name =

AL = [abc, Jlskjf , JJ, lskdjlsk, sldkfjs, dflskdj, sdsd, ss]
count = 2

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3. Sorting strings. Example

Task. An array of strings is given. Sort the strings in lexicographic order using StreamAPI.

Solution. The text of the program that solves this problem is as follows.

import java.util.*;
import java.util.stream.*;
import java.util.function.*;

public class StreamAPI {

  public static void main(String[] args) {
    // 1. Declare an arbitrary array of strings
    String[] AS = {
      "abcd", "bcfk", "def", "jklmn", "jprst",
      "afc", "ambn", "kmk", "qrbd", "jus"
    };

    // 2. Create collection from array of strings
    ArrayList<String> AL = new ArrayList<String>(AS.length);

    for (int i=0; i<AS.length; i++)
      AL.add(i, AS[i]);

    // 3. Create data stream from collection AL
    Stream<String> stream = AL.stream();

    // 4. Sort stream in natural order (ascending)
    Stream<String> streamSorted = stream.sorted();

    // 5. Display stream streamSorted
    // 5.1. Define an action that will display one stream item.
    // For this, the Consumer<T> interface is used.
    Consumer<String> action = (str) -> {
      System.out.println(str);
    };

    // 5.2. Display each element of the stream
    streamSorted.forEach(action);

    // 6. Sort stream elements in descending order
    // 6.1. Implement a comparator - a method that compares two strings
    Comparator<String> comparator;

    comparator = (str1, str2) -> {
      return str2.compareTo(str1); // <0, ==0, >0
    };

    // 6.2. Create the new stream
    stream = AL.stream();

    // 6.3. Sort stream
    Stream<String> streamSortedDesc = stream.sorted(comparator);

    // 6.4. Display new stream
    System.out.println("-----------------------");
    streamSortedDesc.forEach(action);
  }
}

The result of the program

abcd
afc
ambn
bcfk
def
jklmn
jprst
jus
kmk
rbd
-----------------------
rbd
kmk
jus
jprst
jklmn
def
bcfk
ambn
afc
abcd

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