How do I clone an array variable?

You have an array variable and you want to make a clone of this array into a new array variable. To do this you can use Apache Commons Lang ArrayUtils.clone() method. The code snippet below demonstrates the cloning of a primitive array that contains some integers elements in it.

package org.kodejava.example.commons.lang;

import org.apache.commons.lang3.ArrayUtils;

public class PrimitiveArrayClone {
    public static void main(String[] args) {
        int[] fibonacci = new int[] {1, 1, 2, 3, 5, 8, 13, 21, 34, 55};
        System.out.println("fibonacci = " + ArrayUtils.toString(fibonacci));

        int[] clone = ArrayUtils.clone(fibonacci);
        System.out.println("clone = " + ArrayUtils.toString(clone));
    }
}

The fibonacci array contents were cloned into the clone array and we print out the content using ArrayUtils.toString() method.

fibonacci = {1,1,2,3,5,8,13,21,34,55}
clone = {1,1,2,3,5,8,13,21,34,55}

In the code snippet above the clone() method create a reference to a new array. The clone() method it self doesn’t not change the original array. In addition to clone primitive arrays the clone() method also work for cloning array of objects.

As an example we will create an array of String objects and clone it using the ArrayUtils.clone() method. To display the contents of the array we will again use the ArrayUtils.toString() method.

package org.kodejava.example.commons.lang;

import org.apache.commons.lang3.ArrayUtils;

public class ObjectArrayClone {
    public static void main(String[] args) {
        String[] colors = new String[] {"Red", "Green", "Blue", "Yellow"};
        System.out.println("colors = " + ArrayUtils.toString(colors));

        String[] clone = ArrayUtils.clone(colors);
        System.out.println("clone = " + ArrayUtils.toString(clone));
    }
}

And here is the result:

colors = {Red,Green,Blue,Yellow}
clone = {Red,Green,Blue,Yellow}

The only different from the when cloning a primitive array and object array using the ArrayUtils.clone() method is that when cloning an object such as String, Date, etc we need to cast the result of the clone() method into the targeted object type.

Maven Dependencies

<!-- https://search.maven.org/remotecontent?filepath=org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.9</version>
</dependency>

Maven Central

How do I print the contents of an array variable?

You need to print the contents of an array variable. The long way to it is to user a loop to print each element of the array. To simplify this you can user Apache Commons Lang ArrayUtils.toString() method. This method can takes any array as a parameter and print out the contents separated by commas and surrounded by curly brackets. When you need to print a specific string when the array is null, you can provide the second string argument to this method.

package org.kodejava.example.commons.lang;

import org.apache.commons.lang3.ArrayUtils;

public class ArrayUtilsToString {
    public static void main(String[] args) {
        // Print int array as string.
        int[] numbers = {1, 2, 3, 5, 8, 13, 21, 34};
        System.out.println("Numbers = " + ArrayUtils.toString(numbers));

        // Print string array as string.
        String[] grades = {"A", "B", "C", "D", "E", "F"};
        System.out.println("Grades = " + ArrayUtils.toString(grades));

        // Print multidimensional array as string.
        int[][] matrix = {{0, 1, 2}, {1, 2, 3}, {2, 3, 4}};
        System.out.println("Matrix = " + ArrayUtils.toString(matrix));

        // Return "Empty" when the array is null.
        String[] colors = null;
        System.out.println("Colors = " + ArrayUtils.toString(colors, "None"));
    }
}

The output of the code snippet above:

Numbers = {1,2,3,5,8,13,21,34}
Grades = {A,B,C,D,E,F}
Matrix = {{0,1,2},{1,2,3},{2,3,4}}
Colors = None

If you are using the JDK 1.5 or later you can actually use the java.util.Arrays class to do the same thing as the org.apache.commons.lang.ArrayUtils class does.

Maven Dependencies

<!-- https://search.maven.org/remotecontent?filepath=org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.9</version>
</dependency>

Maven Central

How do I find specific elements or object in an array?

This example demonstrate how to find specific items in array. We will use the org.apache.commons.lang.ArrayUtils class. This class provides method called contains(Object[] array, Object objectToFind) method to check if an array contains the objectToFind in it.

We can also use the indexOf(Object[] array, Object objectToFind) method and the lastIndexOf(Object[] array, Object objectToFind) method to get the index of an array element where our objectToFind is located.

package org.kodejava.example.commons.lang;

import org.apache.commons.lang3.ArrayUtils;

public class ArrayUtilsIndexOfDemo {
    public static void main(String[] args) {
        String[] colours = { "Red", "Orange", "Yellow", "Green",
            "Blue", "Violet", "Orange", "Blue" };

        // Does colours array contains the Blue colour?
        boolean contains = ArrayUtils.contains(colours, "Blue");
        System.out.println("Contains Blue? " + contains);

        // Can you tell me the index of each colour defined bellow?
        int indexOfYellow = ArrayUtils.indexOf(colours, "Yellow");
        System.out.println("indexOfYellow = " + indexOfYellow);

        int indexOfOrange = ArrayUtils.indexOf(colours, "Orange");
        System.out.println("indexOfOrange = " + indexOfOrange);

        int lastIndexOfOrange = ArrayUtils.lastIndexOf(colours, "Orange");
        System.out.println("lastIndexOfOrange = " + lastIndexOfOrange);
    }
}

Here are the result of the code above.

Contains Blue? true
indexOfYellow = 2
indexOfOrange = 1
lastIndexOfOrange = 6

Maven Dependencies

<!-- https://search.maven.org/remotecontent?filepath=org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.9</version>
</dependency>

Maven Central

How do I convert array of object to array of primitive?

In the code example below we demonstrate the ArrayUtils.toPrimitive() method to convert an array of Integer object to an array of its primitive type. Besides converting array of Integer objects this method is overloaded to accept other types of object array.

package org.kodejava.example.commons.lang;

import org.apache.commons.lang3.ArrayUtils;

public class ArrayObjectToPrimitiveDemo {
    public static void main(String[] args) {
        // An array of Integer objects.
        Integer[] integers = {1, 2, 3, 5, 8, 13, 21, 34, 55};
        Boolean[] booleans = {Boolean.TRUE, Boolean.TRUE, Boolean.FALSE, Boolean.FALSE};

        // Convert array of Integer objects into array of type int.
        int[] ints = ArrayUtils.toPrimitive(integers);
        System.out.println(ArrayUtils.toString(ints));

        // Convert array of Boolean objects into array of type boolean.
        boolean[] bools = ArrayUtils.toPrimitive(booleans);
        System.out.println(ArrayUtils.toString(bools));
    }
}

The output of our code snippet:

{1,2,3,5,8,13,21,34,55}
{true,true,false,false}

Maven Dependencies

<!-- https://search.maven.org/remotecontent?filepath=org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.9</version>
</dependency>

Maven Central

How do I convert an array to a Map?

This example use the Apache Commons Lang’s ArrayUtils.toMap() method to convert a two dimensional array into a Map object.

To convert a two dimensional array into a Map object, each element of the two dimensional array must be an array with at least two elements where the first element will be the key and the second element will be the value.

package org.kodejava.example.commons.lang;

import java.util.Map;

import org.apache.commons.lang3.ArrayUtils;

public class ArrayToMapExample {

    public static void main(String[] args) {
        // A two dimensional array of countries capital.
        String[][] countries = {{"United States", "Washington, D.C."},
            {"United Kingdom", "London"},
            {"Netherlands", "Amsterdam"},
            {"Japan", "Tokyo"},
            {"France", "Paris"}};

        // Convert an array to a Map.
        Map capitals = ArrayUtils.toMap(countries);

        for (Object key : capitals.keySet()) {
            System.out.printf("%s is the capital of %s.%n", capitals.get(key), key);
        }
    }
}

The result of our code snippet:

London is the capital of United Kingdom.
Amsterdam is the capital of Netherlands.
Paris is the capital of France.
Washington, D.C. is the capital of United States.
Tokyo is the capital of Japan.

Maven Dependencies

<!-- https://search.maven.org/remotecontent?filepath=org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.9</version>
</dependency>

Maven Central

How do I reverse array elements order?

In this example we are going to use the ArraysUtils helper class from Apache Commons Lang library to reverse the order of array elements. The method to reverse the order of array elements is ArrayUtils.reverse() method.

The ArrayUtils.reverse() method is overloaded so we can reverse other types of array such as java.lang.Object, long, int, short, char, byte, double, float and boolean.

package org.kodejava.example.commons.lang;

import org.apache.commons.lang3.ArrayUtils;

public class ArrayReverseExample {

    public static void main(String[] args) {
        // Define colors array.
        String[] colors = {"Red", "Green", "Blue", "Cyan", "Yellow", "Magenta"};
        System.out.println(ArrayUtils.toString(colors));

        // Now we reverse the order of array elements.
        ArrayUtils.reverse(colors);
        System.out.println(ArrayUtils.toString(colors));
    }
}

Here is the output of the code snippet above:

{Red,Green,Blue,Cyan,Yellow,Magenta}
{Magenta,Yellow,Cyan,Blue,Green,Red}

Maven Dependencies

<!-- https://search.maven.org/remotecontent?filepath=org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.9</version>
</dependency>

Maven Central

How do I convert array of primitives into array of objects?

To convert from primitive arrays into object type arrays we can user Apache Commons Lang library. The commons lang provides an ArrayUtils class that does this conversion. To convert the other way just use the toPrimitive() method.

package org.kodejava.example.commons.lang;

import org.apache.commons.lang3.ArrayUtils;

public class ArrayPrimitiveObjectConversionDemo {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 4, 5};
        boolean[] booleans = {true, false, false, true};
        float[] decimals = {10.1f, 3.14f, 2.17f};

        Integer[] numbersObjects = ArrayUtils.toObject(numbers);
        Boolean[] booleansObjects = ArrayUtils.toObject(booleans);
        Float[] decimalsObjects = ArrayUtils.toObject(decimals);

        numbers = ArrayUtils.toPrimitive(numbersObjects);
        booleans = ArrayUtils.toPrimitive(booleansObjects);
        decimals = ArrayUtils.toPrimitive(decimalsObjects);
    }
}

Maven Dependencies

<!-- https://search.maven.org/remotecontent?filepath=org/apache/commons/commons-lang3/3.9/commons-lang3-3.9.jar -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.9</version>
</dependency>

Maven Central