How Is Abstract Data Type Defined in Java?


Heather Bennett

An abstract data type (ADT) in Java is a class or interface that defines a set of operations without specifying how these operations are implemented. It provides a high-level description of the behavior of a data structure, allowing programmers to use and manipulate the data without worrying about the underlying details.

Defining an Abstract Data Type

To define an abstract data type in Java, you can create an interface or an abstract class. Both approaches serve the same purpose of defining the operations that can be performed on the data type.

The key difference:

  • An interface only provides method signatures, without any implementation details.
  • An abstract class can provide default implementations for some of the methods, allowing for code reuse among subclasses.

Example: Defining an Abstract Data Type using an Interface

Let’s say we want to define an ADT for a stack. We can start by creating an interface called Stack:

<u>public interface Stack<E> {
    void push(E element);
    E pop();
    E peek();
    boolean isEmpty();

The <u>Stack<E> is a generic interface where <E> represents the type of elements stored in the stack. The interface defines four methods:

  • <u>push(E element): Adds an element to the top of the stack.
  • <u>E pop(): Removes and returns the element at the top of the stack.
  • <u>E peek(): Returns the element at the top of the stack without removing it.
  • <u>boolean isEmpty(): Checks if the stack is empty.

Example: Defining an Abstract Data Type using an Abstract Class

Alternatively, we can define the stack ADT using an abstract class:

<u>public abstract class AbstractStack<E> {
    public abstract void push(E element);
    public abstract E pop();
    public abstract E peek();
    public boolean isEmpty() {
        return size() == 0;
    protected abstract int size();

In this example, the <u>AbstractStack<E> class provides a default implementation for the <u>isEmpty() method. It also introduces a new method called <u>size(), which is declared as protected and needs to be implemented by subclasses. This allows subclasses to provide their specific implementation of the <u>size() method while reusing other common behavior defined in the abstract class.

Implementing an Abstract Data Type

To use an abstract data type, you need to implement it by providing concrete implementations for all its methods. Multiple classes can implement the same ADT, as long as they adhere to its contract defined by the interface or abstract class.


  • If you choose to implement the ADT using an interface, you must use the <u>implements keyword when defining the class.
  • If you choose to implement the ADT using an abstract class, you must use the <u>extends keyword when defining the class.

Example: Implementing a Stack

Let’s implement the stack ADT using an array-based approach:

<u>public class ArrayStack<E> implements Stack<E> {
    private E[] elements;
    private int top;
    public ArrayStack(int capacity) {
        elements = (E[]) new Object[capacity];
        top = -1;
    public void push(E element) {
        if (top == elements.length - 1) {
            throw new IllegalStateException("Stack is full");
        elements[++top] = element;
    public E pop() {
        if (isEmpty()) {
            throw new NoSuchElementException("Stack is empty");
        return elements[top--];
    public E peek() {
        if (isEmpty()) {
            throw new NoSuchElementException("Stack is empty");
        return elements[top];
    public boolean isEmpty() {
        return top == -1;

In this implementation, we create a class called <u>ArrayStack<E> that implements the <u>Stack<E> interface. The stack is internally represented using an array of type E. The methods are implemented according to the stack’s behavior, with appropriate exceptions thrown for edge cases.


  • The code above uses type casting to create the array. This might result in an unchecked cast warning, but it is necessary due to type erasure in Java generics.

Using Abstract Data Types

Once you have defined and implemented an abstract data type, you can use it in your Java programs. Here’s an example of how to use the <u>ArrayStack<E> class:

<u>public class Main {
    public static void main(String[] args) {
        Stack<Integer> stack = new ArrayStack<>(5);
        System.out.println(stack.pop()); // Output: 30
        System.peek()); // Output: 20

In this example, we create a new <u>ArrayStack<Integer> object and perform various operations on it. The output demonstrates the behavior of the stack ADT.


In Java, abstract data types provide a way to define high-level descriptions of data structures and their operations. By using interfaces or abstract classes, programmers can design reusable and flexible code that promotes encapsulation and separation of concerns.

The power of abstract data types lies not only in their ability to define behavior but also in their visual appeal when properly structured with HTML styling elements like <b>, <u>, <ul>, <li>, <h2>, and others. These elements help make the content more engaging and organized, enhancing the learning experience for readers.

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