Stack implementation in Java

Stacks operate in LIFO model.

Stack plays vital role in many data structures, some of them are

  • in parsing arithmetic expressions
  • to help traverse nodes of binary tree
  • searching vertices of a graph
  • in java, every method’s return type and arguments are pushed on to a stack and when method returns they are popped off.

Efficiency of stack is that it performs push and pop operations in O(1) complexity.

package algorithm.stack;
/**
 * @author ntallapa
 *
 */
public class TNStack {
	private int size;
	private int[] stackArr;
	private int top = -1;
	
	public TNStack(int size) {
		this.size = size;
		stackArr = new int[size];
	}
	
	/**
	 * increment the ctr and push element into stack 
	 * @param i element to be pushed
	 */
	public void push(int i) {
		top++;
		System.out.println("Pushing "+i);
		stackArr[top] = i;
	}
	
	/**
	 * pop the element from stack and decrement the ctr 
	 * @return the popped element
	 */
	public int pop() {
		int i = stackArr[top];
		top--;
		System.out.println("Popping "+i);
		return i;
	}
	
	public int peek() {
		System.out.println("Peek "+stackArr[top]);
		return stackArr[top];
	}
	
	public boolean isFull() {
		return (top == size-1);
	}
	
	public boolean isEmpty() {
		return (top == -1);
	}
}

package algorithm.stack;
/**
 * @author ntallapa
 *
 */
public class TNStackClient {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		TNStack tns = new TNStack(3);
		// push some elements
		if(!tns.isFull())
			tns.push(4);
		if(!tns.isFull())
			tns.push(5);
		if(!tns.isFull())
			tns.push(3);
		if(!tns.isFull())
			tns.push(6);
		else 
			System.out.println("Stack is full, cannot push element");
		
		// pop some elements
		if(!tns.isEmpty())
			tns.pop();
		if(!tns.isEmpty())
			tns.pop();
		if(!tns.isEmpty())
			tns.pop();
		if(!tns.isEmpty())
			tns.pop();
		else 
			System.out.println("Stack is empty, cannot pop element");
		
		//reinsert to verify peek method
		if(!tns.isFull())
			tns.push(6);
		
		// peek couple of times; result should be same
		tns.peek();
		tns.peek();
	}
}

Output:
Pushing 4
Pushing 5
Pushing 3
Stack is full, cannot push element
Popping 3
Popping 5
Popping 4
Stack is empty, cannot pop element
Pushing 6
Peek 6
Peek 6

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