Using a Stack data structure in Java

problem

You wish to use the Stack in Java. This data structure “stacks” data elements one on top of the other hence its name. Similar to the analogy of plates when we stack one on top of the other. It is also known as a LIFO (Last In First Out) data structure.

SOLUTION

Java’s collection library is giving us out of the box the Stack data structure. This data structure is generic and can easily be used. We will see how to stack a list of Strings representing laundry clothes ready for ironing.

initialize it
StackExample.java
				
					
        Stack<String> laundry = new Stack<>();

				
			

This data structure is generic meaning that can store any type. We specify the type of contents that will store in the “diamond syntax” Stack<String>. It will store only Strings in this case.

adding items
StackExample.java
				
					
        laundry.push("shirt");
        laundry.push("trousers");
        laundry.push("t-shirt");
        laundry.push("hoodie");
        laundry.push("shorts");
        laundry.push("jeans");

				
			

Simply invoke the push() method to insert a new item. Same value can be added more than once.

getting items

To get the last added item we invoke the pop() method. This method also removes it from the data collection.

StackExample.java
				
					String lastAddedItem = laundry.pop();
				
			

lastAddedItem should contain “jeans”

getting the size
StackExample.java
				
					System.out.println(laundry.size());
				
			

This line of code will print the number of elements currently in the Stack.

running

Let’s add all the above code in a main method and invoke it.

StackExample.java
				
					package com.programmerabroad;

import java.util.Stack;

public class StackExample {
    
    public static void main(String[] args) {

        Stack<String> laundry = new Stack<>();

        laundry.push("shirt");
        laundry.push("shirt");
        laundry.push("trousers");
        laundry.push("t-shirt");
        laundry.push("hoodie");
        laundry.push("shorts");
        laundry.push("jeans");
        
        System.out.println(laundry.size());

        String lastAddedItem = laundry.pop();
        System.out.println(lastAddedItem);

        System.out.println(laundry.size());
    }
}

				
			
output

It prints 7 that is the initial number of items.

After the invocation of pop, the last added it’s removed, reducing it’s size by 1.

It prints the “jeans” and then 6 which is the new size.

get item without removing

By using the get(index) method, the item is retrieved and not removed, for example:

StackExample.java
				
					String item = laundry.get(3);
				
			

conclusion

In this post we saw how to initialise, add items and get items from a Stack data structure. This is a LIFO data structure that usually is used when we want to use our data in a Last In First Out fashion. In contrast with FIFO – First In First Out.

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Ellion

Professional IT consultant, writer, programmer enthusiast interested in all sorts of coding.
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