Stack

This collection is implements LIFO: the last in is the first out. We push elements onto the top of a stack and then pop them for retrieval.

This collection is deprecated—its use is discouraged in favor of Deque. It is based on Vector, which has worse performance than newer lists.

An example

Let us begin working upon a Stack. We import java.util.Stack to make the Stack type accessible. We use diamond inference syntax to declare the stack.

import java.util.Stack;

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

        Stack<String> stack = new Stack<>();
        stack.push("fly");
        stack.push("worm");
        stack.push("butterfly");

        // Peek at the top of the stack.
        String peekResult = stack.peek();
        System.out.println(peekResult);

        // Pop the stack and display the result.
        String popResult = stack.pop();
        System.out.println(popResult);

        // Pop again.
        popResult = stack.pop();
        System.out.println(popResult);
    }
}
butterfly
butterfly
worm

Add, loop

A Stack is based on the Vector type. We can invoke all the Vector's methods: here we call add() instead of push() to add elements.

import java.util.Stack;

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

        Stack<Integer> stack = new Stack<>();
        stack.add(10);
        stack.add(20);
        stack.add(1, 100); // We can add at a position.

        // Loop over int values.
        for (int value : stack) {
            System.out.println(value);
        }
    }
}
10
100
20

Avoid this collection

Stack is deprecated. It still exists in the Java libraries for compatibility. When I looked for a Stack collection, it appeared in Eclipse.

Stack and queues

Implementing parsers is often done with stacks. When each element is encountered, it is pushed to the stack. In older programs, this Stack is used.