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Interface constants

Learn to define constants inside interfaces.

Interface constants

An interface can contain static final constants. Fields in interfaces are automatically public static final; these keywords apply even if you do not write them. This feature is used to group related constants together.

Example: interface Config { int MAX = 100; String NAME = "App"; }. Here MAX and NAME are automatically public static final. They are accessed as Config.MAX. Interface constants group related values under one umbrella. However, modern Java prefers enums or classes with static final for grouping constants; interface constants are an older approach.

Automatic public static final

Fields inside an interface automatically have public static final access even if you do not write these keywords. This guarantees that interface constants are accessible everywhere, at the class level and immutable.

Example: interface Keys { String ENTER = "Enter"; }. Here ENTER does not have public, static or final written, but all three apply. Keys.ENTER is accessible from everywhere; the value cannot be changed. This automatic behavior keeps interface constants consistent. Modern Java still supports this feature, but enums or classes are more commonly preferred for grouping constants.

Practice Add constants to the Keys interface

Define an interface named Keys. Add String ENTER = "Enter" and String ESC = "Esc" constants. In a main method print both constants' values on separate lines.

Starter code

Main.java
// Define the Keys interface
// Add two constants
// Print in main

Sample solution

Main.java
interface Keys {
    String ENTER = "Enter";
    String ESC = "Esc";
}
public class Main {
    public static void main(String[] args) {
        System.out.println(Keys.ENTER);
        System.out.println(Keys.ESC);
    }
}

Expected output

Enter
Esc

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Complete the interface constant

Question 
interface Config {
    int ____ = 100;
}
Answer 
interface Config {
    int MAX = 100;
}

Explanation: Interface constants are uppercase.

2 Match interface constant properties

Term Meaning
public Accessible everywhere
static Class level
final Immutability
automatic Applies without writing

3 In: interface Config { int MAX = 10; }, what access does MAX have?

  • protected
  • package-private
  • public static final Correct answer
  • private

Explanation: Public static final — interface fields automatically get this access.

All lessons in Intermediate

1 Classes

  1. class
  2. Attributes
  3. Methods
  4. new
  5. toString
  6. Summary - Classes

2 Constructor

  1. Constructor basics
  2. Parameterized constructor
  3. Constructor overloading
  4. this()
  5. Default constructor
  6. Summary - Constructor

3 Inheritance

  1. extends
  2. super
  3. Method overriding
  4. Object class
  5. final class and method
  6. Summary - Inheritance

4 Project: Library

  1. Class design
  2. Book model
  3. Member management
  4. Borrow flow
  5. Putting it together

5 Encapsulation

  1. private fields
  2. Getter
  3. Setter
  4. Package-private
  5. Record
  6. Summary - Encapsulation

6 Static

  1. static fields
  2. static methods
  3. static block
  4. Class variables
  5. instanceof
  6. Summary - Static

7 Constants

  1. static final
  2. Uppercase convention
  3. Constants with enum
  4. Interface constants
  5. Math constants
  6. Summary - Constants

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Interface

  1. interface
  2. implements
  3. default methods
  4. static methods
  5. Multiple interfaces
  6. Summary - Interface

10 Abstract class

  1. abstract class
  2. abstract methods
  3. Abstract and concrete
  4. Abstract vs interface
  5. Sealed class
  6. Summary - Abstract class

11 Polymorphism

  1. Polymorphism basics
  2. Dynamic binding
  3. Override rules
  4. Abstract reference
  5. Pattern matching
  6. Summary - Polymorphism

12 Project: Blog app

  1. Model design
  2. Blog class
  3. Adding comments
  4. Search and filter
  5. Putting it together

13 File I/O

  1. File class
  2. FileReader and FileWriter
  3. BufferedReader
  4. PrintWriter
  5. try-with-resources
  6. Summary - File I/O

14 Serialization

  1. Jackson ObjectMapper
  2. readValue
  3. writeValueAsString
  4. JsonProperty
  5. List and Map serialization
  6. Summary - Serialization

15 Generics

  1. Generic class
  2. Generic method
  3. Bounded type
  4. Wildcard
  5. Type erasure
  6. Summary - Generics

16 Generic collections

  1. List<E>
  2. Map<K,V>
  3. Set<E>
  4. Queue and Deque
  5. Collections class
  6. Summary - Generic collections

17 Iterator

  1. Iterator
  2. remove
  3. ListIterator
  4. Iterable
  5. Spliterator
  6. Summary - Iterator

18 Lambda expressions

  1. Lambda basics
  2. Parameterized lambda
  3. Method reference
  4. forEach with lambda
  5. Comparator with lambda
  6. Summary - Lambda expressions

19 Stream API basics

  1. stream()
  2. filter
  3. map
  4. forEach and count
  5. collect toList
  6. Summary - Stream API basics

20 Intermediate final

  1. Project design
  2. OOP model
  3. Stream layer
  4. File I/O
  5. Putting it together