Java Stack class

Java Stack class is a part of the Java Collection Framework that represents a stack data structure based on the Last-In, First-Out (LIFO) principle. It is used to store and manage elements where the last inserted element is accessed first.

In this chapter, you will learn about the Java Stack class, its features, working, and how it is used to store and manage data using the LIFO principle.

What is Java Stack?

The Stack is a class in the Collection Framework that extends the Vector class. It represents a Last-In, First-Out (LIFO) stack of objects, where the last inserted element is the first one to be removed.

It implements List, Collection, Iterable, Cloneable, and Serializable interfaces. Stack uses generics (<E>) to allow storing elements of any data type.

The Stack class provides various methods to perform operations like push, pop, peek, and search, making it useful for handling structured data efficiently.

Stack Class Constructor

The Stack class has only one constructor which creates an empty stack.

Here is the syntax:

Creating a Stack

If we want to create a stack, first, import the java.util package and create an object of the Stack class.

Or

Where type denotes the type of stack like Integer, String, etc.

Stack Class Methods

The Stack class provides methods to perform operations such as push, pop, peek, and search on stack elements.

Stack class mainly provides five methods to perform these operations. In addition to these, it also inherits all the methods of the Vector class.

Stack Class empty() Method

The empty() method of the Stack class check the stack is empty or not. If the stack is empty, it returns true if the stack contains no elements, otherwise it returns false. We can also use the isEmpty() method of the Vector class.p>

Syntax

It has the following syntax:

Returns: The method returns true if the stack is empty, else returns false.

Example

In the following example, we have created an instance of the Stack class. After that, we have invoked the empty() method two times. The first time it returns true because we have not pushed any element into the stack. After that, we have pushed elements into the stack. Again we have invoked the empty() method that returns false because the stack is not empty.

Output:

Is the stack empty? true
Elements in Stack: [78, 113, 90, 120]
Is the stack empty? false

Stack Class push() Method

The method inserts an item onto the top of the stack. It works the same as the method addElement(item) method of the Vector class. It passes a parameter item to be pushed into the stack.

Syntax

It has the following syntax:

Parameter: An item to be pushed onto the top of the stack.

Returns: The method returns the argument that we have passed as a parameter.

Stack Class pop() Method

The method removes an object at the top of the stack and returns the same object. It throws EmptyStackException if the stack is empty.

Syntax

It has the following syntax:

Returns: It returns an object that is at the top of the stack.

Example

Let's implement the stack in a Java program and perform push and pop operations.

Output:

stack: []
push -> 20
stack: [20]
push -> 13
stack: [20, 13]
push -> 89
stack: [20, 13, 89]
push -> 90
stack: [20, 13, 89, 90]
push -> 11
stack: [20, 13, 89, 90, 11]
push -> 45
stack: [20, 13, 89, 90, 11, 45]
push -> 18
stack: [20, 13, 89, 90, 11, 45, 18]
pop -> 18
stack: [20, 13, 89, 90, 11, 45]
pop -> 45
stack: [20, 13, 89, 90, 11]
pop -> 11
stack: [20, 13, 89, 90]

Stack Class peek() Method

The peek() method returns the top element of the stack without removing it. It allows us to inspect the element at the top of the stack without modifying the stack's contents. It looks at the element that is at the top in the stack. It also throws EmptyStackException if the stack is empty.

Syntax

It has the following syntax:

Returns: It returns the top elements of the stack.

Example

Let's see an example of the peek() method.

Output:

Stack: [Apple, Grapes, Mango, Orange]
Element at the top of the stack: Orange

Stack Class search() Method

The method searches the object in the stack from the top. It parses a parameter that we want to search for. It returns the 1-based location of the object in the stack. These topmost object of the stack is considered at distance 1.

Suppose, o is an object in the stack that we want to search for. The method returns the distance from the top of the stack of the occurrence nearest the top of the stack. It uses equals() method to search an object in the stack.

Syntax

It has the following syntax:

Parameter: o is the desired object to be searched.

Returns: It returns the object location from the top of the stack. If it returns -1, it means that the object is not on the stack.

Example

Let's see an example of the search() method.

Output:

Stack: [Mac Book, HP, DELL, Asus]
Location of Dell: 3

Java Stack Operations

Java Stack provides various operations to manipulate and access elements such as finding size and iterating through elements.

Size of the Stack

This operation is used to find the number of elements present in the stack using the size() method.

Syntax

Here is the syntax:

It has the following syntax

Example: Finding Stack Size

This example demonstrates how to check whether the stack is empty and find its size.

Output:

Is the stack empty or not? false
The stack size is: 5

Iterate Elements

Iteration means accessing each element of the stack. We can iterate through the stack using different methods.

Using iterator() Method

IThis method returns an iterator to traverse the elements of the stack.

Syntax

It has the following syntax

Example: Iteration using iterator()

This example demonstrates how to iterate over stack elements using the iterator.

Output:

BMW
Audi
Ferrari
Bugatti
Jaguar

Using forEach() Method

This method is used to iterate elements using a lambda expression.

Syntax

It has the following syntax

Example: Iteration using forEach()

This example demonstrates how to iterate over stack elements using the forEach() method.

Output:

Iteration over the stack using forEach() Method:
119
203
988

Using listIterator() Method

This method returns a list iterator and is used to iterate the stack from top to bottom.

Syntax

It has the following syntax:

Example: Iteration using listIterator()

This example demonstrates how to iterate over the stack from top to bottom.

Output:

Iteration over the Stack from top to bottom:
988
203
119