Thread: Infinite loop on double linked list

I've been stuck for the last couple hours trying to understand why the "printInReverse" method is getting into an infinite loop. I have no idea. I was supposed to make a double linked list that you can insert numbers and it orders both ascending and descending; in this case, descending is the "printInReverse" method, which takes the already ordered lists and prints it reversed, like if it was descending, and that's where the problem lives. Here follows the whole code:

import java.util.Scanner;
 
public class MyList {
 
private Scanner userInput;
private Integer selectedOption;
 
private MyListElement firstElement = null;
private boolean exitRequested = false;
 
public static void main(final String[] args) {
    new MyList(new Scanner(System.in)).run();
}
 
private MyList(final Scanner userInput) {
    this.userInput = userInput;
}
 
public void run() {
    do {
        promptUserForOption();
        processSelectedOption();
    } while (!exitRequested);
}
 
private void promptUserForOption() {
    System.out.println("");
    System.out.println("1 - insert number in the beginning list");
    System.out.println("2 - insert in the end of the list");
    System.out.println("3 - query list");
    System.out.println("4 - remove from list");
    System.out.println("5 - empty list");
    System.out.println("6 - exit");
    System.out.println("7 - sort reverse");
    System.out.print("Please choose option: ");
 
    try {
        selectedOption = Integer.parseInt(userInput.nextLine());
    } catch (final Exception e) {
        printWrongOperationSelected();
        selectedOption = -1;
    }
}
 
private void printWrongOperationSelected() {
    System.out.println("Wrong operation selected.");
}
 
private void processSelectedOption() {
    switch (selectedOption) {
        case 1:
        case 2: {
            addNewElement();
        }
        break;
        case 3: {
            printList();
        }
        break;
        case 4: {
            removeElement();
        }
        break;
        case 5: {
            clearList();
        }
        break;
        case 6: {
            exit();
        }
        case 7: {
            printInReverse();
        }
        break;
        default: {
            printWrongOperationSelected();
        }
    }
}
 
private void addNewElement() {
    System.out.print("Please type the number to be added to the list: ");
 
    Integer newValue = null;
    while (newValue == null) {
        try {
            newValue = Integer.parseInt(userInput.nextLine());
        } catch (final Exception e) {
            System.out.println("Wrong value. Please insert new value.");
        }
    }
 
    MyListElement newElement = new MyListElement(newValue);
 
    if (firstElement != null) {
        placeElementInList(newElement);
    } else {
        firstElement = newElement;
    }
}
 
private void placeElementInList(final MyListElement newElement) {
    if (newElement.value < firstElement.value) {
        newElement.nextElement = firstElement;
        firstElement = newElement;
    } else {
        MyListElement previousElement = firstElement;
        MyListElement elementInList = firstElement.nextElement;
        while (elementInList != null) {
            if (newElement.value < elementInList.value) {
                break;
            }
            previousElement = elementInList;
            elementInList = elementInList.nextElement;
        }
        previousElement.nextElement = newElement;
        newElement.nextElement = elementInList;
    }
}
 
private void printList() {
    if (firstElement == null) {
        System.out.println("No elements in the list.");
    } else {
        MyListElement elementInList = firstElement;
        while (elementInList != null) {
            System.out.print(elementInList.value + ", ");
            elementInList = elementInList.nextElement;
        }
        System.out.println("");
    }
}
 
private void removeElement() {
    System.out.print("Please type the number to be removed from the list: ");
    Integer valueToRemove = null;
    while (valueToRemove == null) {
        try {
            valueToRemove = Integer.parseInt(userInput.nextLine());
        } catch (final Exception e) {
            System.out.println("Wrong value. Please insert value to remove.");
        }
    }
 
    if (firstElement == null) {
        System.out.println("No elements in the list. None can be removed.");
    } else {
        boolean found = false;
 
        if (firstElement.value.equals(valueToRemove)) {
            firstElement = firstElement.nextElement;
            found = true;
        } else {
            MyListElement previousElement = firstElement;
            MyListElement elementInList = firstElement.nextElement;
            while (elementInList != null) {
                if (elementInList.value.equals(valueToRemove)) {
                    previousElement.nextElement = elementInList.nextElement;
                    found = true;
                    break;
                }
                previousElement = elementInList;
                elementInList = elementInList.nextElement;
            }
        }
 
        if (!found) {
            System.out.println("Value " + valueToRemove + " is not in the list.");
            return;
        } else {
            System.out.println("Value removed.");
        }
    }
}
 
private void clearList() {
    firstElement = null;
}
 
private void exit() {
    exitRequested = true;
}
 
private void printInReverse() {
    MyListElement tmpElement = firstElement;
    MyListElement previousElement = tmpElement.nextElement;
    while (previousElement != null) {
        previousElement.nextElement = tmpElement;
        tmpElement = previousElement;
        previousElement = previousElement.nextElement;
    }
    MyListElement firstReverseElement = tmpElement;
    MyListElement elementInList = firstReverseElement;
    while (elementInList != null) {
        System.out.print(elementInList.value + ", ");
        elementInList = elementInList.nextElement;
    }
    System.out.println("");
}
 
private class MyListElement {
 
    private final Integer value;
    private MyListElement nextElement;
 
    private MyListElement(final Integer value) {
        this.value = value;
    }
}

Okay, got it. But how could I declare a variable that points to the last element in the list? Thank you.

Okay, got it. But how could I get the previousElement from my lastElement? I tried to do create an auxElement variable, storing firstElement and progressing through the list until lastElement is null but it did not work.

lastElement.previousElement; // ???

Yes I know. I mean, I'm not managing to make my previousElement to effectively point to an element's previous element. I guess you'll understand better if I post the code:

private void printInReverse() {
        MyListElement tmpElement = firstElement;
        while (tmpElement.nextElement != null) {
            tmpElement = tmpElement.nextElement;
        }
        //tmpElement gets the last value on list
        MyListElement firstReverseElement = tmpElement;
 
        MyListElement elementInList = firstReverseElement;
        while (elementInList != null) {
            System.out.print(elementInList.value + ", ");
            elementInList = elementInList.prevElement;
        }
        System.out.println("");
 
    }

It's only printing my last element.

It stops when the prev link is null because it's when the first element of the list is reached so the list was percurred.

It's not. Exactly, it's only printing my last element because I don't know how to get the previousElement pointer to actually point to an element's previous element.

It prints always the same address for the pointers. Here it is:

private void printInReverse() {
        MyListElement tmpElement = firstElement;
        while (tmpElement.nextElement != null) {
            tmpElement = tmpElement.nextElement;
        }
 
        MyListElement firstReverseElement = tmpElement;
 
        MyListElement elementInList = firstReverseElement;
        System.out.println("tmpElement => " + tmpElement);
        System.out.println("elementInList => " + elementInList);
        System.out.println("firstReverseElement => " + firstReverseElement);
        System.out.println("prevElement => " + elementInList.prevElement);
        while (elementInList != null) {
            System.out.print(elementInList + " => " + elementInList.value + "\n ");
            elementInList = elementInList.prevElement;
        }
        System.out.println("");
 
    }