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 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

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