refactor: StackArray (#5349)
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Alex Klymenko
GitHub
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8605220721
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f5c0314111
@ -0,0 +1,51 @@
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package com.thealgorithms.datastructures.stacks;
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/**
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* A generic interface for Stack data structures.
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*
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* @param <T> the type of elements in this stack
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*/
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public interface Stack<T> {
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/**
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* Adds an element to the top of the stack.
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*
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* @param value The element to add.
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*/
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void push(T value);
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/**
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* Removes the element at the top of this stack and returns it.
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*
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* @return The element popped from the stack.
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* @throws IllegalStateException if the stack is empty.
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*/
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T pop();
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/**
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* Returns the element at the top of this stack without removing it.
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*
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* @return The element at the top of this stack.
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* @throws IllegalStateException if the stack is empty.
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*/
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T peek();
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/**
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* Tests if this stack is empty.
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*
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* @return {@code true} if this stack is empty; {@code false} otherwise.
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*/
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boolean isEmpty();
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/**
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* Returns the size of this stack.
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*
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* @return The number of elements in this stack.
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*/
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int size();
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/**
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* Removes all elements from this stack.
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*/
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void makeEmpty();
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}
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@ -3,170 +3,80 @@ package com.thealgorithms.datastructures.stacks;
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/**
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* This class implements a Stack using a regular array.
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*
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* <p>
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* A stack is exactly what it sounds like. An element gets added to the top of
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* the stack and only the element on the top may be removed. This is an example
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* of an array implementation of a Stack. So an element can only be
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* added/removed from the end of the array. In theory stack have no fixed size,
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* but with an array implementation it does.
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* @param <T> the type of elements in this stack
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*/
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public class StackArray {
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public class StackArray<T> implements Stack<T> {
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/**
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* Driver Code
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*/
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public static void main(String[] args) {
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// Declare a stack of maximum size 4
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StackArray myStackArray = new StackArray(4);
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assert myStackArray.isEmpty();
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assert !myStackArray.isFull();
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// Populate the stack
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myStackArray.push(5);
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myStackArray.push(8);
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myStackArray.push(2);
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myStackArray.push(9);
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assert !myStackArray.isEmpty();
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assert myStackArray.isFull();
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assert myStackArray.peek() == 9;
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assert myStackArray.pop() == 9;
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assert myStackArray.peek() == 2;
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assert myStackArray.size() == 3;
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}
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/**
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* Default initial capacity.
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*/
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private static final int DEFAULT_CAPACITY = 10;
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/**
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* The max size of the Stack
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*/
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private int maxSize;
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/**
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* The array representation of the Stack
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*/
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private int[] stackArray;
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/**
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* The top of the stack
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*/
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private T[] stackArray;
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private int top;
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/**
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* init Stack with DEFAULT_CAPACITY
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*/
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@SuppressWarnings("unchecked")
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public StackArray() {
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this(DEFAULT_CAPACITY);
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}
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/**
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* Constructor
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*
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* @param size Size of the Stack
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*/
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@SuppressWarnings("unchecked")
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public StackArray(int size) {
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maxSize = size;
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stackArray = new int[maxSize];
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top = -1;
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if (size <= 0) {
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throw new IllegalArgumentException("Stack size must be greater than 0");
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}
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this.maxSize = size;
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this.stackArray = (T[]) new Object[size];
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this.top = -1;
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}
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/**
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* Adds an element to the top of the stack
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*
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* @param value The element added
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*/
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public void push(int value) {
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if (!isFull()) { // Checks for a full stack
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top++;
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stackArray[top] = value;
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} else {
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@Override
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public void push(T value) {
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if (isFull()) {
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resize(maxSize * 2);
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push(value); // don't forget push after resizing
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}
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stackArray[++top] = value;
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}
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/**
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* Removes the top element of the stack and returns the value you've removed
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*
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* @return value popped off the Stack
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*/
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public int pop() {
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if (!isEmpty()) { // Checks for an empty stack
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return stackArray[top--];
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@Override
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public T pop() {
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if (isEmpty()) {
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throw new IllegalStateException("Stack is empty, cannot pop element");
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}
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if (top < maxSize / 4) {
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T value = stackArray[top--];
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if (top + 1 < maxSize / 4 && maxSize > DEFAULT_CAPACITY) {
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resize(maxSize / 2);
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return pop(); // don't forget pop after resizing
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} else {
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System.out.println("The stack is already empty");
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return -1;
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}
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return value;
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}
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/**
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* Returns the element at the top of the stack
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*
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* @return element at the top of the stack
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*/
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public int peek() {
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if (!isEmpty()) { // Checks for an empty stack
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return stackArray[top];
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} else {
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System.out.println("The stack is empty, cant peek");
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return -1;
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@Override
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public T peek() {
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if (isEmpty()) {
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throw new IllegalStateException("Stack is empty, cannot peek element");
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}
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return stackArray[top];
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}
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private void resize(int newSize) {
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int[] transferArray = new int[newSize];
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for (int i = 0; i < stackArray.length; i++) {
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transferArray[i] = stackArray[i];
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}
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// This reference change might be nice in here
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stackArray = transferArray;
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@SuppressWarnings("unchecked") T[] newArray = (T[]) new Object[newSize];
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System.arraycopy(stackArray, 0, newArray, 0, top + 1);
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stackArray = newArray;
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maxSize = newSize;
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}
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/**
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* Returns true if the stack is empty
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*
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* @return true if the stack is empty
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*/
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public boolean isEmpty() {
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return (top == -1);
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}
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/**
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* Returns true if the stack is full
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*
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* @return true if the stack is full
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*/
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public boolean isFull() {
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return (top + 1 == maxSize);
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return top + 1 == maxSize;
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}
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/**
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* Deletes everything in the Stack
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*
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* <p>
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* Doesn't delete elements in the array but if you call push method after
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* calling makeEmpty it will overwrite previous values
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*/
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public void makeEmpty() { // Doesn't delete elements in the array but if you call
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@Override
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public boolean isEmpty() {
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return top == -1;
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}
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@Override public void makeEmpty() { // Doesn't delete elements in the array but if you call
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top = -1; // push method after calling makeEmpty it will overwrite previous values
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}
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/**
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* Return size of stack
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*
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* @return size of stack
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*/
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@Override
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public int size() {
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return top + 1;
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}
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@ -0,0 +1,121 @@
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package com.thealgorithms.datastructures.stacks;
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import org.junit.jupiter.api.Assertions;
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import org.junit.jupiter.api.BeforeEach;
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import org.junit.jupiter.api.Test;
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class StackArrayTest {
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private Stack<Integer> stack;
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@BeforeEach
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void setUp() {
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stack = new StackArray<>(5); // Initialize a stack with capacity of 5
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}
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@Test
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void testPushAndPop() {
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stack.push(1);
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stack.push(2);
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stack.push(3);
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stack.push(4);
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stack.push(5);
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Assertions.assertEquals(5, stack.pop()); // Stack follows LIFO, so 5 should be popped first
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Assertions.assertEquals(4, stack.pop()); // Next, 4 should be popped
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Assertions.assertEquals(3, stack.pop()); // Followed by 3
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Assertions.assertEquals(2, stack.pop()); // Then 2
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Assertions.assertEquals(1, stack.pop()); // Finally 1
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}
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@Test
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void testPeek() {
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stack.push(10);
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stack.push(20);
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stack.push(30);
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Assertions.assertEquals(30, stack.peek()); // Peek should return 30, the top of the stack
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Assertions.assertEquals(3, stack.size()); // Size should remain 3 after peek
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stack.pop();
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Assertions.assertEquals(20, stack.peek()); // After popping, peek should return 20
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}
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@Test
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void testIsEmpty() {
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Assertions.assertTrue(stack.isEmpty()); // Initially, the stack should be empty
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stack.push(42);
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Assertions.assertFalse(stack.isEmpty()); // After pushing an element, the stack should not be empty
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stack.pop();
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Assertions.assertTrue(stack.isEmpty()); // After popping the only element, the stack should be empty again
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}
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@Test
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void testResizeOnPush() {
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StackArray<Integer> smallStack = new StackArray<>(2); // Start with a small stack size
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smallStack.push(1);
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smallStack.push(2);
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Assertions.assertTrue(smallStack.isFull()); // Initially, the stack should be full
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smallStack.push(3); // This push should trigger a resize
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Assertions.assertFalse(smallStack.isFull()); // The stack should no longer be full after resize
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Assertions.assertEquals(3, smallStack.size()); // Size should be 3 after pushing 3 elements
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Assertions.assertEquals(3, smallStack.pop()); // LIFO behavior check
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Assertions.assertEquals(2, smallStack.pop());
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Assertions.assertEquals(1, smallStack.pop());
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}
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@Test
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void testResizeOnPop() {
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StackArray<Integer> stack = new StackArray<>(4);
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stack.push(1);
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stack.push(2);
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stack.push(3);
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stack.push(4);
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stack.pop(); // Removing elements should trigger a resize when less than 1/4 of the stack is used
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stack.pop();
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stack.pop();
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Assertions.assertEquals(1, stack.size()); // After popping, only one element should remain
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stack.pop();
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Assertions.assertTrue(stack.isEmpty()); // The stack should be empty now
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}
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@Test
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void testMakeEmpty() {
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stack.push(1);
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stack.push(2);
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stack.push(3);
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stack.makeEmpty();
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Assertions.assertTrue(stack.isEmpty()); // The stack should be empty after calling makeEmpty
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Assertions.assertThrows(IllegalStateException.class, stack::pop); // Popping from empty stack should throw exception
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}
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@Test
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void testPopEmptyStackThrowsException() {
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Assertions.assertThrows(IllegalStateException.class, stack::pop); // Popping from an empty stack should throw an exception
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}
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@Test
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void testPeekEmptyStackThrowsException() {
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Assertions.assertThrows(IllegalStateException.class, stack::peek); // Peeking into an empty stack should throw an exception
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}
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@Test
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void testConstructorWithInvalidSizeThrowsException() {
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Assertions.assertThrows(IllegalArgumentException.class, () -> new StackArray<>(0)); // Size 0 is invalid
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Assertions.assertThrows(IllegalArgumentException.class, () -> new StackArray<>(-5)); // Negative size is invalid
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}
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@Test
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void testDefaultConstructor() {
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StackArray<Integer> defaultStack = new StackArray<>(); // Using default constructor
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Assertions.assertEquals(0, defaultStack.size()); // Initially, size should be 0
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defaultStack.push(1);
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Assertions.assertEquals(1, defaultStack.size()); // After pushing, size should be 1
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}
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}
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