Data Types in Java

Java type-system overview

Java is statically typed: the compiler checks whether values and operations are compatible with their declared types. The two broad families are primitive types and reference types.

The eight primitive data types

Primitive types are predefined by the Java language. Their values are not objects, although wrapper classes can represent primitive values as objects.

Integral types: byte, short, int, long, and char

The integral types represent exact integer values. int is the normal choice for whole-number arithmetic, while long is used when the required range exceeds int.

public class IntegralTypes {
    public static void main(String[] args) {
        byte temperature = 28;
        short year = 2026;
        int population = 1_500_000;
        long distance = 9_876_543_210L;
        char grade = 'A';

        System.out.println(population);
        System.out.println(distance);
        System.out.println(grade);
    }
}

Underscores can improve the readability of numeric literals. A long literal that exceeds the int range requires the L suffix.

Floating-point types: float and double

float and double represent approximate binary floating-point values. double is the usual choice because it provides greater precision. A float literal requires an f or F suffix.

float discount = 12.5f;
double pi = 3.141592653589793;

The boolean type

A boolean value is either true or false. Java does not treat numeric values such as 0 and 1 as boolean values.

boolean loggedIn = true;
if (loggedIn) {
    System.out.println("Welcome");
}

Default values of fields and array components

Instance fields, static fields, and array components receive default values. Local variables do not receive a value that can be read automatically; they must be definitely assigned first.

public class DefaultValues {
    int number;
    double price;
    char letter;
    boolean active;
    String name;

    public static void main(String[] args) {
        DefaultValues value = new DefaultValues();
        System.out.println(value.number);
        System.out.println(value.price);
        System.out.println((int) value.letter);
        System.out.println(value.active);
        System.out.println(value.name);
    }
}

Output

0
0.0
0
false
null

The character default is the null character '\u0000', which is usually invisible. Casting it to int displays its numeric value, 0.

Reference data types

A reference variable holds a reference value that can refer to an object or array, or it can hold null. Common reference types include classes, interfaces, arrays, enums, records, and type variables.

• String and other class types
• Array types such as int[]
• Interface types such as List<String>
• Enum and record types
• User-defined classes and interfaces

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

public class ReferenceTypes {
    public static void main(String[] args) {
        String language = "Java";
        int[] scores = {82, 91, 76};
        List<String> topics = new ArrayList<>();
        topics.add("Data Types");

        System.out.println(language.toUpperCase());
        System.out.println(scores.length);
        System.out.println(topics.get(0));
    }
}

Primitive vs reference types

Literals, suffixes, and type inference

• An integer literal such as 42 is normally an int.
• Use L for a long literal when required: 9_000_000_000L.
• A floating-point literal such as 3.14 is a double.
• Use f or F for a float literal: 3.14f.
• A character literal uses single quotes: 'A'.
• A string literal uses double quotes: "Java".
• var can request local-variable type inference, but Java still assigns a static compile-time type.

Primitive wrapper classes

Each primitive type has a corresponding wrapper class. Wrappers are useful with generic collections and APIs that require objects.

Complete data-types example

public class DataTypeExample {
    public static void main(String[] args) {
        int lessons = 124;
        long visitors = 2_500_000L;
        double completion = 87.5;
        char grade = 'A';
        boolean published = true;
        String course = "Java Tutorial";

        System.out.println(course);
        System.out.println("Lessons: " + lessons);
        System.out.println("Visitors: " + visitors);
        System.out.println("Completion: " + completion + "%");
        System.out.println("Grade: " + grade);
        System.out.println("Published: " + published);
    }
}

Common mistakes with Java data types

float rate = 12.5;

float rate = 12.5f;