在自定义int堆栈数组中接收到错误的结果

Question

I am making my own Stack Array for ints. It works fine, except that when adding a value to the StackArray there is always a 0 that gets added. Every time I want to receive the value of the peek, the result is that it is empty. I have changed the code around many times, but when I change it to something different than what is given I get errors throughout execution

Here is the StackArray class

public class StackArray{
//declare variables
int top, size, length;
int array[];

//constructor to initialize variables
public StackArray(int _size)
{
    length = 0;
    top = -1;
    size = _size;
    array = new int[size];
}

//push method to add numbers to the stack
void push(int newNum)
{
    //if statement to check if the stack is full
    if(top != size)
    {
        //update top and length
        top++;
        length++;
        array[top] = newNum;
    }
}

//method to remove the top number in the stack
int pop()
{
    //declare local variable
    int temp;
    //if statement to check if stack is not empty
    if(!isEmpty())
    {
        temp = top;
        top--;
        length--;
    }
    else
        System.out.println("No more items in Stack.");

    return top;
}

//boolean method to check if the stack is empty
boolean isEmpty()
{
    if(top == -1)
        return true;
    else
        return false;
}

//method to return the size of the stack
int size()
{
    return length;
}

//method to print out the top number in the stack
void peek() {
    if (isEmpty())
        System.out.println(array[top]);
    else
        System.out.println("Stack is empty");

}

//method to turn the stack into a String
public String toString()
{
    System.out.print("Stack: [");
    for(int i = 0; i <= length; i++)
    {
        System.out.print(array[i] + ", ");
    }
    System.out.println("]");

    return "";
}}

Here is the Driver class that I am using to run the program

import java.util.Scanner;
public class Driver
{
    public static void main(String[] args)
    {
        //declare variables and initialize scanner
        Scanner key = new Scanner(System.in);
        int size, choice, value, end;
    end = 0;

    //ask user to enter the length of the stack
    System.out.print("Please enter the length of the stack: ");
    size = key.nextInt();

    //declate and initialize the stack
    StackArray stack1 = new StackArray(size);

    //loop to continue operations
    while(end == 0)
    {
        //print out menu for commands
        System.out.println("\t1) Push \n\t2) Pop \n\t3) Peek \n\t4) Size \n\t5) isEmpty \n\t6) End");
        System.out.print("Please choose an option: ");
        choice = key.nextInt();

        //switch the choice and execute commands
        switch (choice)
        {
            case 1: System.out.println("Please enter a value: ");
                    value = key.nextInt();
                    stack1.push(value);
                    stack1.toString();
                    break;
            case 2: stack1.pop();
                    stack1.toString();
                    break;
            case 3: stack1.peek();
                    stack1.toString();
                    break;
            case 4: System.out.println("Size: " + stack1.size());
                    stack1.toString();
                    break;
            case 5: if(!stack1.isEmpty())
                    {
                        System.out.println("Stack is empty.");
                    }
                    else
                        System.out.println("Stack is NOT empty.");
                    stack1.toString();
                    break;
            case 6: end = 1;
                    System.out.println("Goodbye!");
                    break;
        }
    }
}

}

Answer 1

The code has a few issues:

void peek() {
    //This was the change - you can peek as long as the stack
    // is NOT empty.
    if (!isEmpty())
        System.out.println(array[top]);
    else
        System.out.println("Stack is empty");

}

In this state, you would've had a out of bounds exception the minute your stack was empty. That's an easy fix. Also, your pop method is fundamentally wrong:

int pop()
{
    //declare local variable
    int temp;
    //if statement to check if stack is not empty
    if(!isEmpty())
    {
        temp = array[top]; //note the change here
        top--;
        length--;
    }
    else
        System.out.println("No more items in Stack.");

    return temp; //note the change here
}

Also - I would recommend that toString fulfills its original purpose - to return a string representation, and not to print it out.

public String toString()
{
    String returnStr = "Stack: [";
    for(int i = 0; i < length; i++)
    {
        returnStr+= array[i] + ", ";
    }
    returnStr += "]";

   return returnStr;
}

Now instead of writing array1.toString(); in your main code, just write System.out.println(array1); . The toString is implied. Note that I changed the <= into <, and now you won't have the extra 0 in the printout following the push!

Another issue is with your main case statement:

 //The check here should be positive - say the stack is empty when it is!
 case 5: if(stack1.isEmpty()) {
             System.out.println("Stack is empty.");
         }
         else
             System.out.println("Stack is NOT empty.");

I think this is all of it.