Exceptions

[c0426]

Hello all! I'm relatively new to Java, so I'm sorry if this question comes off as naive. I am creating an array-based stack class. The goal of it is to work on exceptions.

I have 2 classes, a test class and a regular class. The regular class has the methods object peek(), object push(), and object pop(). For the peek and the pop methods I am using an EmptyStackException. For some reason, the EmptyStackException will not allow me to enter in parameters for it like my exception for the push method does. Therefore, when I put it in the test class and my stack is empty, it doesn't check the exception. This part of my class looks like this:

public Object peek() // Returns object on top of this stack without removing it from the stack. Throws an EmptyStackException if this stack is empty
    {
       if( top ==  -1 )
       {
           throw new EmptyStackException();
       }
       return stk[top];
    }
 
    public Object pop() // Removes and returns object at top of this stack. Throws an EmptyStackException if the stack is empty
    {
        if( top == -1 )
        {
            throw new EmptyStackException();
        }
        return stk[top--];
 
    }
 

And in the test class I put..

       try
        {
            System.out.println(newStack.push("tart"));
            System.out.println(newStack.push("chocolate"));
            System.out.println(newStack.pop());
            System.out.println(newStack.pop());
            System.out.println(newStack.pop());
        }
        catch(EmptyStackException emptyEmpty)
        {
            System.out.println("Stack is empty - could not carry out operation");
        }
 

When complied, the output reads
tart
chocolate
chocolate
tart
null


Could somebody please tell me if they see something wrong in my coding, and perhaps lead me to a solution? Thank you so much!

[r035198x]

Most likely the top variable is not being updated properyly. What does your pop method look like?

[JosAH]

What happens when you do the following in your test class:

while (true)
   System.out.println(newStack.pop());
 

kind regards,

Jos

[c0426]

r, my pop method is as follows.
public Object pop() // Removes and returns object at top of this stack. Throws an EmptyStackException if the stack is empty
{
if( top == -1 )
{
throw new EmptyStackException();
}
return stk[top--];

}

Jos,
are you implying that I should put the while loop in the try/catch block?

Thank you both!

[c0426]

Oh Jos, I understand what you are saying. The goal of the assignment though is to utilize a try/catch block for an exception that is thrown. So I am supposed to handle the EmptyStackException using the try/catch block. I just don't see how this is possible if the complier will not allow me to pass a message in the exception's parameters.

[JosAH]

Oh Jos, I understand what you are saying. The goal of the assignment though is to utilize a try/catch block for an exception that is thrown. So I am supposed to handle the EmptyStackException using the try/catch block. I just don't see how this is possible if the complier will not allow me to pass a message in the exception's parameters.

That's because the EmptyStackException is (also?) an existing Exception.
Can you show us your entire Stack class? I suspect that the 'top' member
variable equals zero initially.

kind regards,

Jos

[c0426]

import java.util.*;
public class Stack
{
    private Object stk[];
    private int top;
 
 
    public Stack()
    {
        stk= new Stack[50]; //creates a default setting of 50 objects in the stack.
        top= -1; //sets top of stack to -1 to show there is no objects in the stack yet.
 
    }
 
    public Stack(int size)
    {
        if(size < 0) //if the size passed is less than zero, the size is set to fifty.
        {
          size=50; 
        }
 
        stk= new Object[size]; //otherwise, the size is set to whatever is passed in the parameters.
    }
 
    public boolean empty() //tests whether or not the stack is empty.
    {
        if(true)
        {
            return top==-1;
        }
        return false;
    }
 
    public Object peek() //looks at the top item without discarding it.
    {
       if( top ==  -1 ) //if the stack is empty, throws EmptyStackException.
       {
           throw new EmptyStackException(); 
       }
       return stk[top];
    }
 
    public Object pop() //removes and returns item on top of stack.
    {
        if( top == -1 ) 
        {
            throw new EmptyStackException();
        }
        return stk[top--]; 
 
    }
 
    public Object push(Object item) // pushes item on top of this stack and returns item.
    {
        if (top == stk.length -1) //if the stack is full, Throws IllegalStateException.
        {
            throw new IllegalStateException("The stack is too full!");
        }
 
        stk[++top]= item;
        return item;
    }
 
    public int getTop() //getter method for the top item on the stack.
    {
        return top;
    }
}

[JosAH]

import java.util.*;
public class Stack
{
    private Object stk[];
    private int top;
 
 
    public Stack()
    {
        stk= new Stack[50]; //creates a default setting of 50 objects in the stack.
        top= -1; //sets top of stack to -1 to show there is no objects in the stack yet.
 
    }
 
    public Stack(int size)
    {
        if(size < 0) //if the size passed is less than zero, the size is set to fifty.
        {
          size=50; 
        }
 
        stk= new Object[size]; //otherwise, the size is set to whatever is passed in the parameters.
    }

Both of these constructors are incorrect for different reasons:

1) it should be 'stk= new Object[50]' in your first constructor
2) in your second constructor you forgot to set top= -1.

Your test code most likely used the second constructor.

kind regards,

Jos

[c0426]

ah perfect! Thank you so much!