Question

Create a LIFO class with following methods in java

- public LifoList()

The default constructor for LifoList class which will initialize your class variables maxSize to 0 and the int array to null.

public LifoList(int maxSize)

The constructor for LifoList class which takes the int input maxSize to initialize the size of the array. You should NOT reinitialize the array elsewhere. For example, creating an object as new LifoList(5) should create a LifoList whose maximum size is 5. If the parameters passed are invalid (negative numbers) then initialize your class variables maxSize to 0 and the int array to null.

public LifoList(LifoList s)

The deep copy constructor for LifoList class which takes the LifoList object s as an input and creates a deep copy of the LifoList. If the parameters passed are invalid (null object), then initialize your class variables maxSize to 0 and the integer array to null.

public void add(int element)

A method to add an int element to the LifoList. If LifoList is already full or LifoList is null, print the contents of MAX_SIZE_ERROR string and return.

public int delete()

A method to delete the top element. Since elements in LifoList are maintained in LIFO order, this method should delete the latest element that was inserted into the LifoList and return it. If LifoList is empty or LifoList is null, print the contents of EMPTY_ERROR and return -1.

public int peek()

A method that returns the top element. Since elements in LifoList are maintained in LIFO order, this method should return the latest element that was inserted into the LifoList. If LifoList is empty or LifoList is null, print the contents of EMPTY_ERROR and return -1.

public int size()

A method that returns the size of the LifoList based on how many elements have been added.

public String toString()

A method to print the LifoList as a string in the specified format. Since elements are stored in LIFO order, this method should return string where elements are in LIFO order. If LifoList is empty or LifoList is null, this method should return an empty string. For example:

if the LifoList has elements 2, 5, 4 (in the order they are inserted), it should return: 
4-5-2
If 6 is added to the LifoList and if toString() method is called, it should return: 
6-4-5-2

Tests:

Creating LifoList of maxSize 5 and adding 2, then 7, and then 4:
4, 7, 2

Add 5 to the LifoList:
5, 4, 7, 2

Peek operation (returns the top element):
Since LifoList maintains elements in LIFO, the top element is 5. Hence, returns 5. 

Delete operation (deletes the top element):
4, 7, 2

Answer 1

Here is the completed code for this problem. Comments are included, go through it, learn how things work and let me know if you have any doubts or if you need anything to change. If you are satisfied with the solution, please rate the answer. Thanks

// LifoList.java

public class LifoList {

    // array to store elements

    private int array[];

    // max size

    private int maxSize;

    // current count of numbers in array

    private int count;

    // constants for error messages

    private static final String MAX_SIZE_ERROR = "ERROR! LifoList is full!";

    private static final String EMPTY_ERROR = "ERROR! LifoList is empty!";

    // default constructor for LifoList class which will initialize your class

    // variables maxSize to 0 and the int array to null.

    public LifoList() {

         maxSize = 0;

         array = null;

         count = 0;

    }

    // constructor for LifoList class which takes the int input maxSize to

    // initialize the size of the array.

    public LifoList(int maxSize) {

         // using default values if maxSize is invalid

         if (maxSize < 0) {

             this.maxSize = 0;

             array = null;

             count = 0;

         } else {

             this.maxSize = maxSize;

             array = new int[maxSize];

             count = 0;

         }

    }

    // deep copy constructor for LifoList class which takes the LifoList object

    // s as an input and creates a deep copy of the LifoList

    public LifoList(LifoList s) {

         // using default values if maxSize is invalid

         if (s == null) {

             maxSize = 0;

             array = null;

             count = 0;

         } else {

             // otherwise copying values from s to this list

             maxSize = s.maxSize;

             array = new int[s.maxSize];

             for (int i = 0; i < count; i++) {

                 array[i] = s.array[i];

             }

             count = s.count;

         }

    }

    // method to add an int element to the LifoList

    public void add(int element) {

         // printing error if full

         if (count == maxSize || array == null) {

             System.out.println(MAX_SIZE_ERROR);

         } else {

             // adding to array at count index and updating count

             array[count] = element;

             count++;

         }

    }

    // method to delete and return the top element

    public int delete() {

         // printing error and returning -1 if empty

         if (count == 0) {

             System.out.println(EMPTY_ERROR);

             return -1;

         }

         // decrementing count and returning element at index count (previously

         // count-1)

         count--;

         return array[count];

    }

    // method that returns the top element.

    public int peek() {

         // printing error and returning -1 if empty

         if (count == 0) {

             System.out.println(EMPTY_ERROR);

             return -1;

         }

         // returning element at index count-1

         return array[count - 1];

    }

    // returns the size

    public int size() {

         return count;

    }

    // return string where elements are in LIFO order

    public String toString() {

         String s = "";

         for (int i = count - 1; i >= 0; i--) {

             s += array[i];

             // appending a '-' if this is not the first element

             if (i != 0) {

                 s += "-";

             }

         }

         return s;

    }

    // main method for testing

    public static void main(String[] args) {

         // Creating LifoList of maxSize 5 and adding 2, then 7, and then 4:

         LifoList list = new LifoList(5);

         list.add(2);

         list.add(7);

         list.add(4);

         // printing list

         System.out.println("list: " + list);

         // adding 5

         list.add(5);

         // printing list

         System.out.println("list: " + list);

         // peeking top element

         System.out.println("peek: " + list.peek());

         // removing top

         System.out.println("delete: " + list.delete());

         // printing list

         System.out.println("list: " + list);

    }

}

/*OUTPUT*/

list: 4-7-2

list: 5-4-7-2

peek: 5

delete: 5

list: 4-7-2