Question

how would I complete this code without calling any built-in java collection framework classes like ArrayList, LinkedList, etc?

import java.util.Iterator;

class CallStack<T> implements Iterable<T> {

// You'll want some instance variables here

public CallStack() {

//setup what you need

}

public void push(T item) {

//push an item onto the stack

//you may assume the item is not null

//O(1)

}

public T pop() {

//pop an item off the stack

//if there are no items on the stack, return null

//O(1)

//replace this dummy return statement

return null;

}

public T peek() {

//return the top of the stack (but don't remove it)

//if there are no items on the stack, return null

//O(1)

//replace this dummy return statement

return null;

}

public String toString() {

//Create a string of the stack where each item

//is separated by a space. The top of the stack

//should be shown to the right and the bottom of

//the stack on the left.

//Hint: Reuse the provided code from another class

//instead of writing this yourself!

//O(n)

//replace this dummy return statement

return null;

}

public void clear() {

//remove everything from the stack

//O(1)

}

public int size() {

//return the number of items on the stack

//O(1)

//replace this dummy return statement

return -1;

}

public boolean isEmpty() {

//return whether or not the stack is empty

//O(1)

//replace this dummy return statement

return false;

}

@SuppressWarnings("unchecked")

public Object[] toArray() {

//Return an array representation of the stack.

//The top of the stack should be element 0

//in the array.

//O(n)

//replace this dummy return statement

return null;

}

public Iterator<T> iterator() {

//Return an iterator that traverses from

//the top of the stack to the bottom of

//the stack.

//The iterator's hasNext() and next() methods

//must both be O(1)

//next() should throw a NullPointerException

//if you try to use next when there are no

//more items

//replace this dummy return statement

return null;

}

public static void main(String[] args) {

CallStack<String> s1 = new CallStack<>();

s1.push("a");

s1.push("b");

CallStack<Integer> s2 = new CallStack<>();

s2.push(1);

s2.push(2);

s2.push(3);

if(s1.toString().equals("a b") && s1.toArray()[0].equals("b") && s1.toArray()[1].equals("a") && s1.toArray().length == 2) {

System.out.println("Yay 1");

}

if(s1.peek().equals("b") && s2.peek().equals(3) && s1.size() == 2 && s2.size() == 3) {

System.out.println("Yay 2");

}

if(s1.pop().equals("b") && s2.pop().equals(3) && s1.size() == 1 && s2.size() == 2) {

System.out.println("Yay 3");

}

if(s1.toString().equals("a") && s1.peek().equals("a") && s2.peek().equals(2) && s1.pop().equals("a") && s2.pop().equals(2) && s1.size() == 0 && s2.size() == 1) {

System.out.println("Yay 4");

}

if(s1.toString().equals("") && s1.peek() == null && s2.peek().equals(1) && s1.pop() == null && s2.pop().equals(1) && s1.size() == 0 && s2.size() == 0) {

System.out.println("Yay 5");

}

s2.push(10);

s2.push(20);

s2.push(30);

if(s1.isEmpty() && s1.toArray().length == 0 && !s2.isEmpty()) {

s2.clear();

if(s2.isEmpty()) {

System.out.println("Yay 6");

}

}

CallStack<Integer> s3 = new CallStack<>();

s3.push(3);

s3.push(2);

s3.push(1);

int i = 1;

for(Integer item : s3) {

if(i == item) System.out.println("Yay " + (6+i));

else

System.out.println(item);

i++;

}

}

}

Answer 1

   Given below is the completed code for the question. Please do rate the answer if it helped. Thank you.

   import java.util.Iterator;

   class CallStack<T> implements Iterable<T> {

       private T[] data;
       private int size;

       public CallStack() {
           data = (T[])new Object[10];
           size = 0;
       }

       public void push(T item) {

           //push an item onto the stack

           //you may assume the item is not null

           data[size] = item;
           size++;

       }

       public T pop() {

           //pop an item off the stack

           //if there are no items on the stack, return null

           //O(1)

           if(isEmpty())
               return null;
           else
               return data[--size];

       }

       public T peek() {

           //return the top of the stack (but don't remove it)

           //if there are no items on the stack, return null

           //O(1)

           if(isEmpty())
               return null;
           else
               return data[size-1];

       }

       public String toString() {

           //Create a string of the stack where each item

           //is separated by a space. The top of the stack

           //should be shown to the right and the bottom of

           //the stack on the left.

           //Hint: Reuse the provided code from another class

           //instead of writing this yourself!

           //O(n)

           //replace this dummy return statement
           String s = "";
           if(size > 0){
               s += data[0];
               for(int i = 1; i < size; i++)
                   s += " " + data[i];
           }
           return s;

       }

       public void clear() {

           //remove everything from the stack

           //O(1)
           size = 0;

       }

       public int size() {

           //return the number of items on the stack

           //O(1)

           return size;

       }

       public boolean isEmpty() {

           //return whether or not the stack is empty

           //O(1)

           return size == 0;
       }

       @SuppressWarnings("unchecked")

       public Object[] toArray() {

           //Return an array representation of the stack.

           //The top of the stack should be element 0

           //in the array.

           //O(n)

           Object[] arr = new Object[size];
           for(int i = size-1, j = 0; i >= 0; i--, j++)
               arr[j] = data[i];
           return arr;

       }

       public Iterator<T> iterator() {

           //Return an iterator that traverses from

           //the top of the stack to the bottom of

           //the stack.

           //The iterator's hasNext() and next() methods

           //must both be O(1)

           //next() should throw a NullPointerException

           //if you try to use next when there are no

           //more items

           return new CallStackIterator();

       }
      
       class CallStackIterator implements Iterator<T>{
           private int index;
           private CallStackIterator() {
               index = size - 1;
           }
           @Override
           public boolean hasNext() {
               return (index >= 0);
           }

           @Override
           public T next() {
               return data[index--];
           }
          
       }

       public static void main(String[] args) {

           CallStack<String> s1 = new CallStack<>();

           s1.push("a");

           s1.push("b");

           CallStack<Integer> s2 = new CallStack<>();

           s2.push(1);

           s2.push(2);

           s2.push(3);

           if(s1.toString().equals("a b") && s1.toArray()[0].equals("b") && s1.toArray()[1].equals("a") && s1.toArray().length == 2) {

               System.out.println("Yay 1");

           }

           if(s1.peek().equals("b") && s2.peek().equals(3) && s1.size() == 2 && s2.size() == 3) {

               System.out.println("Yay 2");

           }

           if(s1.pop().equals("b") && s2.pop().equals(3) && s1.size() == 1 && s2.size() == 2) {

               System.out.println("Yay 3");

           }

           if(s1.toString().equals("a") && s1.peek().equals("a") && s2.peek().equals(2) && s1.pop().equals("a") && s2.pop().equals(2) && s1.size() == 0 && s2.size() == 1) {

               System.out.println("Yay 4");

           }

           if(s1.toString().equals("") && s1.peek() == null && s2.peek().equals(1) && s1.pop() == null && s2.pop().equals(1) && s1.size() == 0 && s2.size() == 0) {

               System.out.println("Yay 5");

           }

           s2.push(10);

           s2.push(20);

           s2.push(30);

           if(s1.isEmpty() && s1.toArray().length == 0 && !s2.isEmpty()) {

               s2.clear();

               if(s2.isEmpty()) {

                   System.out.println("Yay 6");

               }

           }

           CallStack<Integer> s3 = new CallStack<>();

           s3.push(3);

           s3.push(2);

           s3.push(1);

           int i = 1;

           for(Integer item : s3) {

               if(i == item) System.out.println("Yay " + (6+i));

               else

                   System.out.println(item);

               i++;

           }

       }

   }

   output
   =====
   Yay 1
   Yay 2
   Yay 3
   Yay 4
   Yay 5
   Yay 6
   Yay 7
   Yay 8
   Yay 9