How do I fill array with non-default value?

This code snippet will show you how to create array variable and initialized it with a non-default value. By default, when we create an array of something in Java all entries will have its default value. For primitive types like int, long, float the default value are zero (0 or 0.0). For reference types (anything that holds an object in it) will have null as the default value. For boolean variable it will be false.

If you want to initialize the array to different value you can use the Arrays.fill() method. This method will help you to set the value for every elements of the array.

Let see the following code snippet as an example:

package org.kodejava.example.util;

import java.util.Arrays;

public class ArraysFillExample {
    public static void main(String[] args) {
        // Assign -1 to each elements of numbers array
        int[] numbers = new int[5];
        Arrays.fill(numbers, -1);
        System.out.println("Numbers: " + Arrays.toString(numbers));

        // Assign 1.0f to each elements of prices array
        float[] prices = new float[5];
        Arrays.fill(prices, 1.0f);
        System.out.println("Prices : " + Arrays.toString(prices));

        // Assign empty string to each elements of words array
        String[] words = new String[5];
        Arrays.fill(words, "");
        System.out.println("Words  : " + Arrays.toString(words));

        // Assign 9 to each elements of the multi array
        int[][] multi = new int[3][3];
        for (int[] array : multi) {
            Arrays.fill(array, 9);
        }
        System.out.println("Multi  : " + Arrays.deepToString(multi));
    }
}

In the code snippet above we utilize the Arrays.fill() utility method to assign value for each elements of the int, float and String array. To change the default value of multi dimensional array we can’t directly call the Arrays.fill() method. In the example we use for-loop to set each elements of the sub-array using the Arrays.fill() method.

The output of the code snippet above are:

Numbers: [-1, -1, -1, -1, -1]
Prices : [1.0, 1.0, 1.0, 1.0, 1.0]
Words  : [, , , , ]
Multi  : [[9, 9, 9], [9, 9, 9], [9, 9, 9]]

How can I insert an element in array at a given position?

As we know an array in Java is a fixed-size object, once it created its size cannot be changed. So if you want to have a resizable array-like object where you can insert an element at a given position you can use a java.util.List object type instead.

This example will show you how you can achieve array insert using the java.util.List and java.util.ArrayList object. Let see the code snippet below.

package org.kodejava.example.util;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class ArrayInsert {
    public static void main(String[] args) {
        // Creates an array of integer value and prints the original values.
        Integer[] numbers = new Integer[]{1, 2, 3, 8, 13, 21};
        System.out.println("Original numbers: " +
            Arrays.toString(numbers));

        // Creates an ArrayList object and add the entire content of numbers
        // array into the list. We use the add(index, element) method to add
        // element = 5 at index = 3.
        List<Integer> list = new ArrayList<>();
        list.addAll(Arrays.asList(numbers));
        list.add(3, 5);

        // Converts back the list into array object and prints the new values.
        numbers = list.toArray(new Integer[list.size()]);
        System.out.println("After insert    : " + Arrays.toString(numbers));
    }
}

In the code snippet above the original array of Integer numbers will be converted into a List, in this case we use an ArrayList, by adding all elements of the array into the list using the List.addAll() method. The Arrays.asList() can be use to convert an array into a collection type object.

Next we insert a new element into the List using the add(int index, E element) method. Where index is the insert / add position and element is the element to be inserted. After the new element inserted we convert the List back to the original array.

Below is the result of the code snippet above:

Original numbers: [1, 2, 3, 8, 13, 21]
After insert    : [1, 2, 3, 5, 8, 13, 21]

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 clear the content of an array?

In this example you’ll learn how to clear or reset the content of an array. We can use the java.util.Arrays.fill() method to replace to content of each element in the array. In the example below we create two arrays, names and numbers array. We initialize these arrays with some values and then clear the value by assigning null to each element of the array using the Arrays.fill() method.

package org.kodejava.example.util;

import java.util.Arrays;

public class ArrayClear {
    public static void main(String[] args) {
        String[] names = {"Alice", "Bob", "Carol"};
        System.out.println("Names = " + Arrays.toString(names));

        // Replace the contents of the names array to null for each array
        // element.
        Arrays.fill(names, null);
        System.out.println("Names = " + Arrays.toString(names));

        Integer[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        System.out.println("Numbers = " + Arrays.toString(numbers));

        // Replace the contents of the numbers array to null for each
        // array element.
        Arrays.fill(numbers, null);
        System.out.println("Numbers = " + Arrays.toString(numbers));
    }
}

The output of the code snippet:

Names = [Alice, Bob, Carol]
Names = [null, null, null]
Numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Numbers = [null, null, null, null, null, null, null, null, null, null]

How do I reverse the order of array elements?

In this code snippet you’ll learn how to reverse the order of array elements. To reverse to element order will be using the Collections.reverse() method. This method requires an argument with List type. Because of this we need to convert the array to a List type first. We can use the Arrays.asList() to do the conversion. And then we reverse it. To convert the List back to array we can use the Collection.toArray() method.

Let’s see the code snippet below:

package org.kodejava.example.util;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class ArrayReverse {
    public static void main(String[] args) {
        // Creates an array of Integers and print it out.
        Integer[] numbers = {0, 1, 2, 3, 4, 5, 6, 7, 8};
        System.out.println("Arrays.toString(numbers) = " + Arrays.toString(numbers));

        // Convert the int arrays into a List.
        List numberList = Arrays.asList(numbers);

        // Reverse the order of the List.
        Collections.reverse(numberList);

        // Convert the List back to arrays of Integers
        // and print it out.
        numberList.toArray(numbers);
        System.out.println("Arrays.toString(numbers) = " + Arrays.toString(numbers));
    }
}

The output of the code snippet above is:

Arrays.toString(numbers) = [0, 1, 2, 3, 4, 5, 6, 7, 8]
Arrays.toString(numbers) = [8, 7, 6, 5, 4, 3, 2, 1, 0]

How do I rotate elements of a collection?

This example demonstrate how to rotate the elements of a collection object. We can use the java.util.Collections class and call the rotate() method with the collection to rotate and the distance as the parameters.

package org.kodejava.example.util;

import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Arrays;

public class CollectionRotate {
    public static void main(String[] args) {
        List numbers = new ArrayList();

        // Add some items into the collection
        for (int i = 0; i < 25; i++) {
            numbers.add(i);
        }

        // Print the collection items        
        System.out.println(Arrays.toString(numbers.toArray()));

        // Rotates the elements in the collection by the 10.
        Collections.rotate(numbers, 10);

        // Print the rotated collection items
        System.out.println(Arrays.toString(numbers.toArray()));
    }
}

Here is the program result:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]
[15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]

How do I sort array values in descending order?

Here you will find an example on how to sort the values of an array in ascending or descending order.

package org.kodejava.example.util;

import java.util.Arrays;
import java.util.Collections;

public class SortArrayWithOrder {
    public static void main(String[] args) {
        Integer[] points = new Integer[5];
        points[0] = 94;
        points[1] = 53;
        points[2] = 70;
        points[3] = 44;
        points[4] = 64;
        System.out.println("Original  : " + Arrays.toString(points));

        // Sort the points array, the default order is in ascending order.
        // [44, 53, 64, 70, 94]
        Arrays.sort(points);
        System.out.println("Ascending : " + Arrays.toString(points));

        // Sort the points array in descending order.
        // [94, 70, 64, 53, 44]
        Arrays.sort(points, Collections.reverseOrder());
        System.out.println("Descending: " + Arrays.toString(points));
    }
}

The result of the code snippet above are:

Original  : [94, 53, 70, 44, 64]
Ascending : [44, 53, 64, 70, 94]
Descending: [94, 70, 64, 53, 44]

How do I split a string?

Prior to Java 1.4 we use java.util.StringTokenizer class to split a tokenized string, for example a comma separated string. Starting from Java 1.4 and later the java.lang.String class introduce a String.split(String regex) method that simplify this process.

Below is a code sample how to do it.

package org.kodejava.example.lang;

import java.util.Arrays;

public class StringSplit {
    public static void main(String[] args) {
        String data = "1,Diego Maradona,Footballer,Argentina";
        String[] items = data.split(",");

        // Iterates the array to print it out.
        for (String item : items) {
            System.out.println("item = " + item);
        }

        // Or simply use Arrays.toString() when print it out.
        System.out.println("item = " + Arrays.toString(items));
    }
}

The result of the code snippet:

item = 1
item = Diego Maradona
item = Footballer
item = Argentina
item = [1, Diego Maradona, Footballer, Argentina]

How do I sort an array of objects?

In this example we are going to learn how to sort an array of objects. We start by using an array of String objects as can be seen in the code snippet below. We sort the contents of the array using Arrays.sort() method and print the sorted result. It was really simple.

String names[] = {"Bob", "Mallory", "Alice", "Carol"};
Arrays.sort(names);
System.out.println("Names = " + Arrays.toString(names));

Next, we will sort an array of our own object. It is a bit different compared to sorting an array of primitives. The first rule is we need our object to implements the Comparable interface. This interface have one contract we need to implement, the compareTo() contract.

The basic rule of the compareTo() method is to return 0 when objects value are equals, 1 if this object value is greater and -1 if this object value is smaller. In the Person class below we simply call the String object compareTo() method. See the Person class below for more details.

package org.kodejava.example.util.support;

public class Person implements Comparable {
    private String name;

    public Person(String name) {
        this.name = name;
    }

    public int compareTo(Object o) {
        Person p = (Person) o;
        return this.name.compareTo(p.name);
    }

    public String toString() {
        return name;
    }
}

In the snippet below we create four Person objects. We sort the Person object based on their name using the Arrays.sort() method and print out the array values.

Person persons[] = new Person[4];
persons[0] = new Person("Bob");
persons[1] = new Person("Mallory");
persons[2] = new Person("Alice");
persons[3] = new Person("Carol");
Arrays.sort(persons);
System.out.println("Persons = " + Arrays.toString(persons));

This is the main class where you can run all the snippet above:

package org.kodejava.example.util;

import org.kodejava.example.util.support.Person;

import java.util.Arrays;

public class ObjectSortExample {
    public static void main(String[] args) {
        String names[] = {"Bob", "Mallory", "Alice", "Carol"};
        Arrays.sort(names);
        System.out.println("Names = " + Arrays.toString(names));

        Person persons[] = new Person[4];
        persons[0] = new Person("Bob");
        persons[1] = new Person("Mallory");
        persons[2] = new Person("Alice");
        persons[3] = new Person("Carol");
        Arrays.sort(persons);
        System.out.println("Persons = " + Arrays.toString(persons));
    }
}

This snippet will print the following output:

Names = [Alice, Bob, Carol, Mallory]
Persons = [Alice, Bob, Carol, Mallory]

How do I search specific value in an array?

package org.kodejava.example.util;

import java.util.Arrays;

public class ArraySearchExample {
    public static void main(String[] args) {
        // We create an array of ints where the search will be done.
        int items[] = {9, 5, 14, 6, 11, 28, 9, 16, 37, 3, 2};

        // The Arrays.binarySearch() require us to sort the array
        // items before we call the method. We can utilize the
        // Arrays.sort() method to do this. If we did not sort the
        // array the result will be undefined, the search process
        // will return a negative result.
        Arrays.sort(items);

        // To search we use Arrays.binarySearch() methods which accept
        // parameters of any array type. To search we passed the array
        // to be searched and the key to be searched for.
        //
        // When the search item exist more than one in the array, this
        // method gives no guarantee which one will be returned.
        int needle = 9;
        int index = Arrays.binarySearch(items, needle);

        // Print out where the 9 number is located in the array.
        System.out.println("Items: " + Arrays.toString(items));
        System.out.println("Item " + needle + " is at index " + index);
    }
}

There result of the code snippet:

Items: [2, 3, 5, 6, 9, 9, 11, 14, 16, 28, 37]
Item 9 is at index 5