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About Mary Zheng

Mary Zheng
Mary has graduated from Mechanical Engineering department at ShangHai JiaoTong University. She also holds a Master degree in Computer Science from Webster University. During her studies she has been involved with a large number of projects ranging from programming and software engineering. She works as a senior Software Engineer in the telecommunications sector where she acts as a leader and works with others to design, implement, and monitor the software solution.

Java Array Contains Example

1. Introduction

An array is a container object that holds a fixed number of elements with a single type. Java not only provides Arrays to copy, sort, and search elements, it also provides the collection framework to check if a certain value in a collection with the contains method.

In this example, I will demonstrate:

  • Find an element in a sorted array via Arrays.binarySearch.
  • Find an element in an array via Stream.anyMatch method.
  • Find an element in an array via AbstractCollection.contains method.
  • Find an element in an array via a loop.

2. Technologies Used

The example code in this article was built and run using:

  • Java 11
  • Maven 3.3.9
  • Eclipse Oxygen
  • Junit 4.12

3. Maven Project

3.1 Dependencies

I will include Junit in the pom.xml.

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>jcg-zheng-demo</groupId>
	<artifactId>array-demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<build>
		<sourceDirectory>src</sourceDirectory>
		<plugins>
			<plugin>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.8.0</version>
				<configuration>
					<release>11</release>
				</configuration>
			</plugin>
		</plugins>
	</build>
	<dependencies>
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<version>4.12</version>
		</dependency>
	</dependencies>
</project>

3.2 POJO

I will create a POJO class which has name and age members and can be sorted based on its age. It has three methods:

  • equals – generated by Eclipse IDE based on both name and age members.
  • hashCode – generated by Eclipse IDE based on both name and age members.
  • compareTo – compare to other POJO object based on the age member.

POJO.java

package jcg.zheng.demo.model;

public class POJO implements Comparable<POJO> {

	private int age;

	private String name;

	public POJO() {
		super();
	}

	public POJO(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		POJO other = (POJO) obj;
		if (age != other.age)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	@Override
	public String toString() {
		return "POJO [name=" + name + ", age=" + age + "]";
	}

	@Override
	public int compareTo(POJO o) {
		if (this.age == (o.age))
			return 0;
		else if ((this.age) > (o.age))
			return 1;
		else
			return -1;
	}

}

3.3 FindService

I will create a FindService class which finds an element from an array via a loop. This is the most common way prior to Java 8.

It has three methods:

  • containsInt – find an element from an integer array.
  • containsObject – find an element from an object array.
  • containsString – find an element from a string array.

FindService.java

package jcg.zheng.demo.api;

import jcg.zheng.demo.model.POJO;

public class FindService {

	public boolean containsInt(int[] numbers, int searchingItem) {
		boolean found = false;
		for (int element : numbers) {
			if (element == searchingItem) {
				found = true;
				break;
			}
		}

		return found;
	}

	public boolean containsObject(POJO[] objects, POJO searchingItem) {
		boolean found = false;
		for (POJO element : objects) {
			if (element.equals(searchingItem)) {
				found = true;
				break;
			}
		}

		return found;
	}

	public boolean containsString(String[] strings, String searchingItem) {
		boolean found = false;
		for (String element : strings) {
			if (element.equalsIgnoreCase(searchingItem)) {
				found = true;
				break;
			}
		}

		return found;
	}

}

4. JUnit Test

4.1 Find Integer Test

I will create a FindIntTest class which includes three tests to find an element from a sorted integer array:

  • findvia_binarySearch – uses Arrays.binarySearch to find an element.
  • findvia_stream – uses IntStream.anyMatch to find an element.
  • findvia_loop – uses FindService.containsInt to find an element.

FindIntTest.java

package jcg.zheng.demo.api;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

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

import org.junit.Test;

import jcg.zheng.demo.api.FindService;

public class FindIntTest {
	int[] digits = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

	@Test
	public void findvia_binarySearch() {
		int foundOne = Arrays.binarySearch(digits, 1);
		assertEquals(1, foundOne);
		
		int foundTwo = Arrays.binarySearch(digits, 2);
		assertEquals(2, foundTwo);
		
		int foundNone = Arrays.binarySearch(digits, 11);
		assertEquals(-11, foundNone);
	}

	@Test
	public void findvia_list_isbad() {
		boolean shouldContain = Arrays.asList(digits).contains(1); 
		assertFalse(shouldContain);
	}

	@Test
	public void findvia_loop() {
		FindService foundService = new FindService();
		assertTrue(foundService.containsInt(digits, 1));
		assertFalse(foundService.containsInt(digits, 100));
	}
	
	@Test
	public void findvia_stream() {
		assertTrue(IntStream.of(digits).anyMatch(x -> x == 1));
		assertFalse(IntStream.of(digits).anyMatch(x -> x == 10));
	}

}

Please note that the List.containts method should not be used for the primitive data type as the findvia_list_isbad shows.

Execute mvn test -Dtest=FindIntTest and capture the output here:

Junit Output

Running jcg.zheng.demo.api.FindIntTest
Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.132 sec

Results :

Tests run: 4, Failures: 0, Errors: 0, Skipped: 0

4.2 Find String Test

I will create a FindStriingTest class which contains four tests to find an element from a sorted string array:

  • findvia_binarySearch – uses Arrays.binarySearch to find an element.
  • findvia_stream – uses Stream.anyMatch to find an element.
  • findvia_loop – uses FindService.containsString to find an element.
  • findvia_list – uses List.contains to find an element.

FindStringTest.java

package jcg.zheng.demo.api;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import java.util.Arrays;

import org.junit.Test;

import jcg.zheng.demo.api.FindService;

public class FindStringTest {
	String[] alphabet = new String[] { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P",
			"Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z" };

	@Test
	public void findvia_binarySearch() {
		int foundA = Arrays.binarySearch(alphabet, "A");
		assertEquals(0, foundA);
		
		int foundB = Arrays.binarySearch(alphabet, "B");
		assertEquals(1, foundB);
		
		int foundNone = Arrays.binarySearch(alphabet, "1");
		assertEquals(-1, foundNone);
	}

	@Test
	public void findvia_list() {
		assertTrue(Arrays.asList(alphabet).contains("A"));
		assertFalse(Arrays.asList(alphabet).contains("1"));
	}

	@Test
	public void findvia_loop() {
		FindService foundService = new FindService();
		assertTrue(foundService.containsString(alphabet, "A"));
		assertFalse(foundService.containsString(alphabet, "1"));
	}
	
	@Test
	public void findvia_stream() {
		assertTrue(Arrays.stream(alphabet).anyMatch("A"::equals));
		assertFalse(Arrays.stream(alphabet).anyMatch("1"::equals));
	}
}

Execute mvn test -Dtest=FindStringTest and capture output here”

Junit Output

Running jcg.zheng.demo.api.FindStringTest
Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.155 sec

Results :

Tests run: 4, Failures: 0, Errors: 0, Skipped: 0

4.3 Find Object Test

I will create a FindObjectTest class which has four tests to find an element from a sorted object array:

  • findvia_binarySearch – uses Arrays.binarySearch to find an element.
  • findvia_stream – uses Stream.anyMatch to find an element.
  • findvia_loop – uses FindService.containsObject to find an element.
  • findvia_list – uses List.contains to find an element.

Please note that the object class must have equals and hashCode methods.

FindObjectTest

package jcg.zheng.demo.api;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import java.util.Arrays;

import org.junit.Before;
import org.junit.Test;

import jcg.zheng.demo.api.FindService;
import jcg.zheng.demo.model.POJO;

public class FindObjectTest {
	POJO badMark = new POJO("Mark_NA", 60);

	POJO mark = new POJO("Mark", 10);
	POJO[] objects = new POJO[5];

	@Test
	public void findvia_binarySearch() {
		int foundOne = Arrays.binarySearch(objects, mark);
		assertEquals(0, foundOne);

		int foundTwo = Arrays.binarySearch(objects, badMark);
		assertEquals(-6, foundTwo);

	}

	@Test
	public void findvia_list() {
		assertTrue(Arrays.asList(objects).contains(mark));
		assertFalse(Arrays.asList(objects).contains(badMark));
	}

	@Test
	public void findvia_loop() {
		FindService foundService = new FindService();
		assertTrue(foundService.containsObject(objects, mark));
		assertFalse(foundService.containsObject(objects, badMark));
	}

	@Test
	public void findvia_stream() {
		assertTrue(Arrays.stream(objects).anyMatch(mark::equals));
		assertFalse(Arrays.stream(objects).anyMatch(badMark::equals));
	}

	@Before
	public void setup() {
		objects[0] = mark;
		objects[1] = new POJO("Mary", 20);
		objects[2] = new POJO("Terry", 30);
		objects[3] = new POJO("Tom", 40);
		objects[4] = new POJO("John", 50);
	}
}

Execute mvn test -Dtest=FindObjectTest and capture output here:

Junit Output

Running jcg.zheng.demo.api.FindObjectTest
Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.26 sec

Results :

Tests run: 4, Failures: 0, Errors: 0, Skipped: 0

5. Summary

In the example, I demonstrated four ways to find an element in an array:

  • Utilize Java AbstractCollection.contains. Note: does not work for the primitive data type.
  • Utilize Java Arrays.binarySearch. Note: only works for a sorted array
  • Utilize Java Stream.anyMatch. Note: works for Java 8 and higher version.
  • Utilize a loop to find the element.

Please note when finding an element in an object array, the object class must have an equal method.

6. Download the Source Code

Download
You can download the full source code of this example here: Java Array Contains Example
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