I have a problem with 2 classes that I have created for a program the uses the stack. The first problem that I get is that when I try to run the program I get a runtime error. Its kind of a difficult thing to ask because it doing several things. It asks for user input to add numbers to the stack and checking if the stack is full or empty. I also may need help to copy the array.
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
at IntegerStack.push(IntegerStack.java:24)
at Lab15.main(Lab15.java:38)
This is the main class that runs the program.
import java.util.Scanner;
public class Lab15 {
public static void main(String[] args)
{
System.out.println("***** Playing with an Integer Stack *****");
final int SIZE = 5;
IntegerStack myStack = new IntegerStack(SIZE);
Scanner scan = new Scanner(System.in);
//Pushing integers onto the stack
System.out.println("Please enter an integer to push onto the stack - OR - 'q' to Quit");
while(scan.hasNextInt())
{
int i = scan.nextInt();
myStack.push(i);
System.out.println("Pushed "+ i);
}
//Pop a couple of entries from the stack
System.out.println("Lets pop 2 elements from the stack");
int count = 0;
while(!myStack.isEmpty() && count<2)
{
System.out.println("Popped "+myStack.pop());
count++;
}
scan.next(); //Clearing the Scanner to get it ready for further input.
//Push a few more integers onto the stack
System.out.println("Push in a few more elements - OR - enter q to quit");
while(scan.hasNextInt())
{
int i = scan.nextInt();
myStack.push(i);
System.out.println("Pushed "+ i);
}
System.out.println("\nThe final contentes of the stack are:");
while(!myStack.isEmpty())
{
System.out.println("Popped "+myStack.pop());
}
}
}
This is the class that is adding the numbers to the stack which is what has the problems. This is where I may need help copying the array. At the end.
import java.util.Arrays;
public class IntegerStack
{
private int stack [];
private int top;
public IntegerStack(int SIZE)
{
stack = new int [SIZE];
top = -1;
}
public void push(int i)
{
if (top == stack.length)
{
extendStack();
}
stack[top]= i;
top++;
}
public int pop()
{
top --;
return stack[top];
}
public int peek()
{
return stack[top];
}
public boolean isEmpty()
{
if ( top == -1);
{
return true;
}
}
private void extendStack()
{
int [] copy = Arrays.copyOf(stack, stack.length);
}
}
Any help or direction will be appreciated.
Better solution for your Stack implementation
import java.util.List;
import java.util.ArrayList;
public class IntegerStack
{
private List<Integer> stack;
public IntegerStack(int SIZE)
{
stack = new ArrayList<Integer>(SIZE);
}
public void push(int i)
{
stack.add(0,i);
}
public int pop()
{
if(!stack.isEmpty()){
int i= stack.get(0);
stack.remove(0);
return i;
} else{
return -1;// Or any invalid value
}
}
public int peek()
{
if(!stack.isEmpty()){
return stack.get(0);
} else{
return -1;// Or any invalid value
}
}
public boolean isEmpty()
{
stack.isEmpty();
}
}
If you have to use Array... Here are problems in your code and possible solutions
import java.util.Arrays;
public class IntegerStack
{
private int stack [];
private int top;
public IntegerStack(int SIZE)
{
stack = new int [SIZE];
top = -1; // top should be 0. If you keep it as -1, problems will arise when SIZE is passed as 0.
// In your push method -1==0 will be false and your code will try to add the invalid element to Stack ..
/**Solution top=0; */
}
public void push(int i)
{
if (top == stack.length)
{
extendStack();
}
stack[top]= i;
top++;
}
public int pop()
{
top --; // here you are reducing the top before giving the Object back
/*Solution
if(!isEmpty()){
int value = stack[top];
top --;
return value;
} else{
return -1;// OR invalid value
}
*/
return stack[top];
}
public int peek()
{
return stack[top]; // Problem when stack is empty or size is 0
/*Solution
if(!isEmpty()){
return stack[top];
}else{
return -1;// Or any invalid value
}
*/
}
public boolean isEmpty()
{
if ( top == -1); // problem... we changed top to 0 above so here it need to check if its 0 and there should be no semicolon after the if statement
/* Solution if(top==0) */
{
return true;
}
}
private void extendStack()
{
int [] copy = Arrays.copyOf(stack, stack.length); // The second parameter in Arrays.copyOf has no changes, so there will be no change in array length.
/*Solution
stack=Arrays.copyOf(stack, stack.length+1);
*/
}
}
Because you initialized the top variable to -1 in your constructor, you need to increment the top variable in your push() method before you access the array. Note that I've changed the assignment to use ++top:
public void push(int i)
{
if (top == stack.length)
{
extendStack();
}
stack[++top]= i;
}
That will fix the ArrayIndexOutOfBoundsException you posted about. I can see other issues in your code, but since this is a homework assignment I'll leave those as "an exercise for the reader." :)
Stack Implementation in Java
class stack
{ private int top;
private int[] element;
stack()
{element=new int[10];
top=-1;
}
void push(int item)
{top++;
if(top==9)
System.out.println("Overflow");
else
{
top++;
element[top]=item;
}
void pop()
{if(top==-1)
System.out.println("Underflow");
else
top--;
}
void display()
{
System.out.println("\nTop="+top+"\nElement="+element[top]);
}
public static void main(String args[])
{
stack s1=new stack();
s1.push(10);
s1.display();
s1.push(20);
s1.display();
s1.push(30);
s1.display();
s1.pop();
s1.display();
}
}
Output
Top=0
Element=10
Top=1
Element=20
Top=2
Element=30
Top=1
Element=20
Here is an example of implementing stack in java (Array Based implementation):
public class MyStack extends Throwable{
/**
*
*/
private static final long serialVersionUID = -4433344892390700337L;
protected static int top = -1;
protected static int capacity;
protected static int size;
public int stackDatas[] = null;
public MyStack(){
stackDatas = new int[10];
capacity = stackDatas.length;
}
public static int size(){
if(top < 0){
size = top + 1;
return size;
}
size = top+1;
return size;
}
public void push(int data){
if(capacity == size()){
System.out.println("no memory");
}else{
stackDatas[++top] = data;
}
}
public boolean topData(){
if(top < 0){
return true;
}else{
System.out.println(stackDatas[top]);
return false;
}
}
public void pop(){
if(top < 0){
System.out.println("stack is empty");
}else{
int temp = stackDatas[top];
stackDatas = ArrayUtils.remove(stackDatas, top--);
System.out.println("poped data---> "+temp);
}
}
public String toString(){
String result = "[";
if(top<0){
return "[]";
}else{
for(int i = 0; i< size(); i++){
result = result + stackDatas[i] +",";
}
}
return result.substring(0, result.lastIndexOf(",")) +"]";
}
}
calling MyStack:
public class CallingMyStack {
public static MyStack ms;
public static void main(String[] args) {
ms = new MyStack();
ms.push(1);
ms.push(2);
ms.push(3);
ms.push(4);
ms.push(5);
ms.push(6);
ms.push(7);
ms.push(8);
ms.push(9);
ms.push(10);
System.out.println("size: "+MyStack.size());
System.out.println("List---> "+ms);
System.out.println("----------");
ms.pop();
ms.pop();
ms.pop();
ms.pop();
System.out.println("List---> "+ms);
System.out.println("size: "+MyStack.size());
}
}
output:
size: 10
List---> [1,2,3,4,5,6,7,8,9,10]
----------
poped data---> 10
poped data---> 9
poped data---> 8
poped data---> 7
List---> [1,2,3,4,5,6]
size: 6
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