Question

public class Account{

public int bal; // this store the balance amount in the account

public Account(int initialBalance)

{

bal = initialBalance;

}

public static void swap_1(int num1, int num2)

{

int temp = num1;

num1 = num2;

num2 = temp;

}

public static void swap_2(Account acc1, Account acc2)

{

int temp = acc1.bal;

acc1.bal = acc2.bal;

acc2.bal = temp;

}

public static void swap_3(Account acc1, Account acc2)

{

Account temp = acc1;

acc1 = acc2;

acc2 = temp;

}

}

Now, different students tried to use different swap_*() methods to swap the balances in their friends’ bank accounts. Predict the output of each of the following code snippets [1 pts each]

(a) Account tom = new Account(100);

Account jim = new Account(2000);

System.out.println(“1.Tom has $” + tom.bal + “Jim has $” + jim.bal);

swap_1(tom.bal, jim.bal);

System.out.println(“2.Tom has $” + tom.bal + “Jim has $” + jim.bal);

(b) Account tom = new Account(100);

Account jim = new Account(2000);

System.out.println(“1.Tom has $” + tom.bal + “Jim has $” + jim.bal);

swap_2(tom, jim);

System.out.println(“2.Tom has $” + tom.bal + “Jim has $” + jim.bal);

(c) Account tom = new Account(100);

Account jim = new Account(2000);

System.out.println(“1.Tom has $” + tom.bal + “Jim has $” + jim.bal);

swap_3(tom, jim);

System.out.println(“2.Tom has $” + tom.bal + “Jim has $” + jim.bal);

Section B: Fun with stacks [2 pts each 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”); }

Section C: Implementing Stack using Linked List

In this section, you will implement a generic Stack class implemented using linked list. Assume the linked list node class is already defined as below:
public class LLNode<T> {
public LLNode<T> link;

public T info;

public LLNode(T in) { info = in; link = null; }

}

Note that both class variables are public so any outside class can access them directly. Also assume that class StackUnderflowException has been defined that inherits Java’s Exception class. Your task is to implement four methods in the generic class LinkedListStack<T>.

public class LinkedListStack<T> {

private LLNode<T> head; // head of linked list, also stack top pointer

public LinkedListStack() { head = null; } // constructor

public boolean isEmpty() { // [1 pts]

// TODO: return true if stack is empty, false otherwise

// NO MORE THAN 1 LINE OF CODE!

}

public void push(T element) { // [2 pts]

// TODO: push an element to the stack

// NO MORE THAN 3 LINES of CODE!

}

public T peek() throws StackUnderflowException { // [2 pts]

// TODO: return the top element of the stack (but do NOT
// remove it). NO MORE THAN 4 LINES of CODE!

}

public T pop() throws StackUnderflowException { // [3 pts]

// TODO: remove and return the top element of the stack

// It throws StackUnderflowException if stack is empty

// NO MORE THAN 6 LINES of CODE!

}

Answer 1

Section A

Predict the output of each of the following code snippets

============================================================================================

(a) Account tom = new Account(100);

Account jim = new Account(2000);

System.out.println(“1.Tom has $” + tom.bal + “Jim has $” + jim.bal);

swap_1(tom.bal, jim.bal);

System.out.println(“2.Tom has $” + tom.bal + “Jim has $” + jim.bal);

Output

1.Tom has $100Jim has $2000
2.Tom has $100Jim has $2000

Reason: swap_1() is passing arguments as call by value method. all changes will only in the local variable of swap_1() function.

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(b) Account tom = new Account(100);

Account jim = new Account(2000);

System.out.println(“1.Tom has $” + tom.bal + “Jim has $” + jim.bal);

swap_2(tom, jim);

System.out.println(“2.Tom has $” + tom.bal + “Jim has $” + jim.bal);

Output

1.Tom has $100Jim has $2000
2.Tom has $2000Jim has $100

Reason: swap_2() is passing objects as parameters. so all changes are done on variables of objects in swap_2(). so they got swap in real.

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(c)

Account tom = new Account(100);

Account jim = new Account(2000);

System.out.println(“1.Tom has $” + tom.bal + “Jim has $” + jim.bal);

swap_3(tom, jim);

System.out.println(“2.Tom has $” + tom.bal + “Jim has $” + jim.bal);

Output

1.Tom has $100Jim has $2000
2.Tom has $100Jim has $2000

Reason: swap_3() is passing objects as parameters. But in function, objects are swapped. so value dont got swapped. we get same as output.

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