Node, Stacks and Queues: Why am I getting a bizarre output?

**son012189** · 03-08-2012, 02:54 AM

The assignment asked me to write 4 classes: Car, Node, Lstack, and Lqueue

1) create an instance of the Lstack and place 7 values into it using a while loop that goes while the size is < 7, allowing the user to enter a String value for the license plate, and placing the character 'X' as the car's action code. Display it with labels.

2) create an instance of the Lqueue and place 5 values into it using a while loop that goes while the size is < 5, allowing the user to enter a String value for the license plate, and placing the character 'Y' as the car's action code. Display it with labels.

3) call the reverseQueue method on the queue and display the resulting contents.

4) call the reverseStack method on the stack and display the resulting contents.

Here's my code:

Java Code:

package assignment5;
import java.util.*;
import java.lang.String;

public class Assignment5 {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
//create instance objects of class Lstack
		System.out.println("Lstack ");
		Lstack myLstack = new Lstack();	
		int i = 0;
		while (i < 7) {
			System.out.print("Enter a license plate number: ");
			String licenseP = input.next();
			Car myCar = new Car(licenseP, 'X');
			myLstack.push(myCar);  //place into Lstack
			i++;					
		}
		System.out.println();
//create instance objects of class Lqueue		
		System.out.println("Lqueue ");
		Lqueue myLqueue = new Lqueue();
		int j = 0;
		while (j < 5) {
			System.out.print("Enter a license plate number: ");
			String licenseP = input.next();
			Car myCar = new Car(licenseP, 'Y');
			myLqueue.insert(myCar);  //place into Lqueue
			j++;			
		}
//display stuff
		myLstack.display();
		myLqueue.display();
		//reverse
		myLqueue.reverseQueue(myLqueue);
		myLqueue.display();
		myLstack.reverseStack(myLstack);
		myLstack.display();			
	}
}

class Car {
//define variables
	String licensePlate;
	char actionCode;
//constructor with specific license plate and action code
	Car(String newLicense, char newAction) {
		licensePlate = newLicense;
		actionCode = newAction;
	}
//getter and setter methods
	public String getLicensePlate() {
		return licensePlate;
	}
	public char getActionCode() {
		return actionCode;
	}
	public void setLicensePlate(String newLicense) {
		licensePlate = newLicense;
	}
	public void setActionCode(char newAction) {
		actionCode = newAction;
	}	
}

class Node {
//define variables
	Car data;
	Node next;
//constructor with specific size
	Node(Car newData, Node newNext) {
		data = newData;
		next = newNext;
	}
}

class Lstack {
//define variables
	private Node top;
	private int numelements;
//constructor with specific size
	Lstack() {
		top = null;
		numelements = 0;
	}		
//Lstack Methods
	public void push(Car x) {  //push a car on the stack
		top = new Node(x, top);
		numelements++;
	}
	public Car pop() {  //pop a car off the stack
		Car temp = top.data;
		top = top.next;
		numelements--;
		return temp;
	}
	public boolean isEmpty() {  //checks if empty
		return (top == null);
	}
	public int size() {  //returns number of elements in the stack
		return numelements;
	}
	public void display() {  //displays stack contents
		Node curr = top;
		System.out.print("TOP- ");
		while(curr != null) {
			System.out.print(curr.data + " | ");
			curr = curr.next;
		}
		System.out.println();
	}
	public void reverseStack(Lstack myS) {  //reverses stack's contents
		Lqueue q = new Lqueue();
		while(! myS.isEmpty()) {
			q.insert(myS.pop());
		}
		while(!q.isEmpty()) {
			myS.push(q.remove());
		}	
	}
}
class Lqueue {
//define variables
	private int numElements;
	private Node front;
	private Node rear;
//constructor with specific size
 	Lqueue() {
		front = null;
		rear = null;
	}
//Lqueue Methods
	public void insert(Car x) {  //add a car to rear of the queue
		if(front == null) {
			front = new Node(x, null);
			rear = front;
		} else {
			rear.next = new Node(x, null);
			rear = rear.next;
		}
	}
	public Car remove() {  //remove a car from the front of the queue
		Car temp = null;
		front = front.next;
		return temp;
	}
	public boolean isEmpty() {  //checks if empty
		return (front == null);
	}
	public int size() {  //returns number of elements in the queue
		return numElements;
	}
	public void display() {  //diplays contents of the queue
		Node curr = front;
		System.out.print("FRONT- ");
		while(curr != null) {
			System.out.print(curr.data + " | ");
			curr = curr.next;
		}
		System.out.println();	
	}
	public void reverseQueue(Lqueue myQ) {  //reverses the contents of the queue
		Lstack s = new Lstack();
		while(! myQ.isEmpty()) {
			s.push(myQ.remove());
		}
		while(!s.isEmpty()) {
			myQ.insert(s.pop());
		}
	}
}

Here is my output

Why am I getting an unusual output? Why am I getting assignment5.Car@1a758cb?? And why is null appearing in the last two lines of my output?

**quad64bit** · 03-08-2012, 10:48 PM

"assignment5.Car@1b67f74" and the like are memory addresses. This happens when you try to print a java object (rather than a primitive) that has no defined toString() method.

**Norm** · 03-08-2012, 11:19 PM

If you want to print the value of a Car variable, add a toString() method to it that returns a String containing what you would like to see.

**DarrylBurke** · 03-09-2012, 10:08 AM

Originally Posted by quad64bit

"assignment5.Car@1b67f74" and the like are memory addresses. This happens when you try to print a java object (rather than a primitive) that has no defined toString() method.

Misleading.

1) It's not documented to have any relation to the memory address. That's the typical return value from the toString() method inherited from java.lang.Object.

2) Every class has a 'defined' toString() method; either overridden in the class or inherited from a superclass. A class that needs to provide a human-readable description of itself should override the method accordingly.

db

**quad64bit** · 03-09-2012, 04:32 PM

It's not documented to have any relation to the memory address. That's the typical return value from the toString() method inherited from java.lang.Object.

I thought it was the address in virtual memory of the object, i.e. the memory address as it pertains the the memory space allocated by the JVM. I realize it isn't a machine memory address in physical memory, but is "1b67f74" not the hexadecimal address of the space reserved inside the JVM's memory space? A 'virtual' memory address if you will?

**JosAH** · 03-09-2012, 04:47 PM

Originally Posted by quad64bit

I thought it was the address in virtual memory of the object, i.e. the memory address as it pertains the the memory space allocated by the JVM. I realize it isn't a machine memory address in physical memory, but is "1b67f74" not the hexadecimal address of the space reserved inside the JVM's memory space? A 'virtual' memory address if you will?

That is only true for the Sun/Oracle implementation of the JVM; straight from the API docs:

Originally Posted by API toString()

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object.

Originally Posted by API hashCode()

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

kind regards,

Jos

**quad64bit** · 03-09-2012, 04:49 PM

Great information! Thank you :)

TIL the default toString returns a hash value of the object in question, not necessarily any useful memory address information.