Question

Consider a generic Stack class implemented using linked list with the following methods:

• public boolean isEmpty(), which returns true if and only if the stack is empty;

• public T pop() throws StackUnderflowException, which removes and returns the top element of the stack (if the stack is empty, it throws StackUnderflowException);

• public T peek() throws StackUnderflowException, which returns the top element of the stack (but does not remove it; it throws exception if stack is empty);

• public void push(T element), which pushes an element onto the stack

Predict the output of each of the following code snippets

(a) Stack<Integer> s = new Stack<Integer>();

try{

s.push(5);

s.push(10);

System.out.println(s.peek());

System.out.println(s.pop());

s.push(20);

System.out.println(s.pop());

}
catch(Exception e) { System.out.print(“An exception was thrown”); }

(b) Stack<Integer> s = new Stack<Integer>();

try{

s.push(5);

s.push(50);

System.out.println(s.pop());

System.out.println(s.pop());

System.out.println(s.peek());

s.push(30);

}
catch(Exception e) { System.out.println(“An exception was thrown”); }

(c) Stack<Integer> s = new Stack<Integer>();

try{

s.push(5);

System.out.println(s.pop());

System.out.println(s.isEmpty());

s.push(10);

s.push(43);

s.push(s.pop());

System.out.println(s.peek());

} catch(Exception e) {

System.out.println(“An exception was thrown”);

}

(d) Stack<Integer> s = new Stack<Integer>();

try {

s.push(5);

s.push(10);

while(!s.isEmpty())

{

System.out.println(s.peek());

}

}
catch(Exception e) { System.out.println(“An exception was thrown”); }

Answer 1

(a) Stack<Integer> s = new Stack<Integer>();
try{
s.push(5); // Stores value 5 on to stack. Now Stack contains: 5
s.push(10); // Stores value 10 on to stack. Now Stack contains: 5 10
System.out.println(s.peek()); // Prints element at top of the stack which is 10 => Prints value 10
System.out.println(s.pop()); // Prints element at top of the stack which is 10 and removes it from stack. Now stack contains: 5 => Prints value 10
s.push(20); // Now stack contains: 5 20
System.out.println(s.pop()); // Prints element at top of the stack which is 20 and removes it from stack. Now stack contains: 5 => Prints value 20
}
catch(Exception e) { System.out.print(“An exception was thrown”); }

Hence output will be:

10
10
20

______________________________________________________________________________________________________________________________________________________

(b) Stack<Integer> s = new Stack<Integer>();
try{
s.push(5); // Stores value 5 on to stack. Now Stack contains: 5
s.push(50); // Stores value 50 on to stack. Now Stack contains: 5 50
System.out.println(s.pop()); // Prints element at top of the stack which is 50 and removes it from stack. Now stack contains: 5 => Prints value 50
System.out.println(s.pop()); // Prints element at top of the stack which is 5 and removes it from stack. Now stack contains: EMPTY => Prints value 5
System.out.println(s.peek()); // As stack is empty Raises an exception and prints the exception
s.push(30);
}
catch(Exception e) { System.out.println(“An exception was thrown”); }

Hence output will be:
50
5
An exception was thrown

______________________________________________________________________________________________________________________________________________________

(c) Stack<Integer> s = new Stack<Integer>();
try{
s.push(5); // Stores value 5 on to stack. Now Stack contains: 5
System.out.println(s.pop()); // Prints element at top of the stack which is 5 and removes it from stack. Now stack contains: EMPTY => Prints value 5
System.out.println(s.isEmpty()); // As stack is empty prints true
s.push(10); // Stores value 10 on to stack. Now Stack contains: 10
s.push(43); // Stores value 43 on to stack. Now Stack contains: 10 43
s.push(s.pop()); // Pushes element present at top of stack which is 43 => Stack contains: 10 43
System.out.println(s.peek()); // Element present at top of the stack is 43 => Prints value 43
} catch(Exception e) {
System.out.println(“An exception was thrown”);
}

Hence output will be:
5
true
43

______________________________________________________________________________________________________________________________________________________

(d) Stack<Integer> s = new Stack<Integer>();
try {
s.push(5); // Stores value 5 on to stack. Now Stack contains: 5
s.push(10); // Stores value 10 on to stack. Now Stack contains: 5 10
while(!s.isEmpty())
{
System.out.println(s.peek()); // Prints element 10 which is at top of the stack infinite times
}
}
catch(Exception e) { System.out.println(“An exception was thrown”); }

Hence output will be:
10
10
10
.. Infinite Times