Arrays and Strings in Java

Arrays and strings are two of the first data structures you use in Java. An array stores multiple values of the same type, while a String stores a sequence of characters. Together, they help you work with lists of data, text input, and many common programming problems.

What Is an Array?

An array is a fixed-size container that holds values of the same type. Each value is called an element, and each element has an index.

int[] scores = {85, 90, 78, 92};

Array indexes start at 0, so the first element is at index 0, the second is at index 1, and so on.

System.out.println(scores[0]); // 85
System.out.println(scores[3]); // 92

If an array has 4 elements, the valid indexes are 0 through 3. Accessing an invalid index causes an ArrayIndexOutOfBoundsException.

Declaring and Creating Arrays

You can create an array with values immediately:

String[] names = {"Alice", "Bob", "Charlie"};

Or you can create an array with a fixed size and fill it later:

int[] numbers = new int[3];
numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;

The size of an array cannot change after it is created. If you need a collection that can grow or shrink, use an ArrayList.

Default Values

When you create an array with new, Java fills it with default values.

Type	Default value
Numeric types	`0`
`boolean`	`false`
`char`	`\u0000`
Object references	`null`

int[] numbers = new int[3];
System.out.println(numbers[0]); // 0

String[] names = new String[3];
System.out.println(names[0]); // null

Array Length

Use the length property to find the number of elements in an array.

int[] scores = {85, 90, 78, 92};
System.out.println(scores.length); // 4

Because indexes start at 0, the last valid index is array.length - 1.

System.out.println(scores[scores.length - 1]); // 92

Looping Through Arrays

A regular for loop is useful when you need the index.

int[] scores = {85, 90, 78, 92};

for (int i = 0; i < scores.length; i++) {
    System.out.println("Score " + i + ": " + scores[i]);
}

An enhanced for loop is cleaner when you only need the values.

for (int score : scores) {
    System.out.println(score);
}

Updating Array Elements

Array elements can be changed by assigning a new value at a specific index.

int[] scores = {85, 90, 78, 92};
scores[2] = 88;

System.out.println(scores[2]); // 88

This changes the element at index 2 from 78 to 88.

Multi-Dimensional Arrays

A multi-dimensional array is an array of arrays. The most common example is a two-dimensional array, which can represent a table or grid.

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

System.out.println(matrix[0][0]); // 1
System.out.println(matrix[1][2]); // 6

The first index selects the row, and the second index selects the column.

for (int row = 0; row < matrix.length; row++) {
    for (int col = 0; col < matrix[row].length; col++) {
        System.out.print(matrix[row][col] + " ");
    }
    System.out.println();
}

What Is a String?

A String is an object that represents text. Strings are made of characters and are written inside double quotes.

String message = "Hello, Java";
System.out.println(message);

Although strings look simple, they are objects from the java.lang.String class.

String Immutability

Strings in Java are immutable, which means their contents cannot be changed after creation.

String name = "Java";
name = name + " Programming";

System.out.println(name); // Java Programming

This does not modify the original "Java" string. Instead, Java creates a new string and makes name refer to it.

Immutability makes strings safer to share, but repeated string changes inside loops can create many temporary objects. For heavy text building, use StringBuilder.

StringBuilder builder = new StringBuilder();
builder.append("Java");
builder.append(" Programming");

System.out.println(builder.toString()); // Java Programming

Common String Methods

The String class provides many useful methods.

String text = "Learning Java";

System.out.println(text.length());        // 13
System.out.println(text.charAt(0));       // L
System.out.println(text.toUpperCase());   // LEARNING JAVA
System.out.println(text.toLowerCase());   // learning java
System.out.println(text.contains("Java")); // true
System.out.println(text.startsWith("Learn")); // true
System.out.println(text.endsWith("Java"));    // true

Use substring() to extract part of a string.

String text = "Learning Java";
String language = text.substring(9);

System.out.println(language); // Java

Use trim() to remove leading and trailing spaces.

String input = "  hello  ";
System.out.println(input.trim()); // hello

Comparing Strings

Use equals() to compare string contents.

String a = "Java";
String b = new String("Java");

System.out.println(a.equals(b)); // true

Do not use == to compare string contents. The == operator checks whether two references point to the same object.

System.out.println(a == b); // false

For case-insensitive comparison, use equalsIgnoreCase().

String input = "java";
System.out.println(input.equalsIgnoreCase("Java")); // true

Splitting and Joining Strings

Use split() to break a string into an array.

String csv = "red,green,blue";
String[] colors = csv.split(",");

for (String color : colors) {
    System.out.println(color);
}

Use String.join() to combine strings with a separator.

String[] words = {"Java", "is", "fun"};
String sentence = String.join(" ", words);

System.out.println(sentence); // Java is fun

Converting Between Arrays and Strings

A string can be converted to a character array.

String word = "Java";
char[] letters = word.toCharArray();

for (char letter : letters) {
    System.out.println(letter);
}

You can also create a string from a character array.

char[] letters = {'J', 'a', 'v', 'a'};
String word = new String(letters);

System.out.println(word); // Java

Common Mistakes

Forgetting that array indexes start at 0.
Accessing array[array.length] instead of array[array.length - 1].
Trying to resize an array after it has been created.
Using == instead of equals() to compare strings.
Forgetting that string methods return new strings instead of changing the original string.

Practice Exercises

Create an array of five numbers and print the largest value.
Create an array of names and print each name in uppercase.
Count how many vowels appear in a string.
Reverse a string using a character array.
Split a sentence into words and print one word per line.

Summary

Arrays let you store multiple values of the same type under one variable name. They are fixed in size, indexed from 0, and commonly processed with loops. Strings represent text, provide many useful methods, and are immutable. Once you understand arrays and strings, you can solve many beginner Java problems involving lists, input, and text processing.

Dung (Donny) Nguyen

Senior Software Engineer