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Java - JUnit testing for balanced BinarySearchTree

I've written a working Binary Search Tree and want to construct some JUnit tests to go along with it. I'm working on three: one to find the maximum value (InOrder traversal), one to remove this maximum value, and one to check if my Binary Tree is balanced. I've written the first two, but can't quite figure out how to nail the last test -- checking for balance. I'd appreciate some guidance, as I feel like I've overlooked something.
My test methods:
public class BSTreePreLabTest {
#Test
public void testFindMax() {
BSTree<Integer> tree = new BSTree<Integer>();
tree.addElement(15);
tree.addElement(16);
tree.addElement(17);
tree.addElement(18);
tree.addElement(19);
tree.addElement(20);
assertEquals("20", tree.findMax().toString());
}
#Test
public void testRemoveMax() {
BSTree<Integer> tree = new BSTree<Integer>();
tree.addElement(15);
tree.addElement(16);
tree.addElement(17);
tree.addElement(18);
tree.addElement(19);
tree.addElement(20);
tree.removeMax();
assertEquals("Inorder traversal: [15, 16, 17, 18, 19]", tree.toString());
}
And my main BinarySearchTree method, for reference, if needed:
public class BSTree<T> {
private BSTreeNode<T> root = null;
private int count;
public BSTree(T element) {
root = new BSTreeNode<T>(element);
count = 1;
}
public BSTree() {
root = null;
count = 0;
}
public void addElement(T element) {
if (isEmpty()) {
root = new BSTreeNode<T>(element);
}
else {
BSTreeNode<T> current = root;
BSTreeNode<T> previous = null;
Comparable<T> comparableElement = (Comparable<T>) element;
while (current != null) {
if (comparableElement.compareTo(current.getElement()) < 0) {
previous = current;
current = current.getLeft();
}
else {
previous = current;
current = current.getRight();
}
}
BSTreeNode<T> newNode = new BSTreeNode<T>(element);
if (comparableElement.compareTo(previous.getElement()) < 0)
previous.setLeft(newNode);
else
previous.setRight(newNode);
}
count++;
}
public boolean isEmpty() {
return root == null;
}
public int size() {
return count;
}
public T find(T targetElement) throws ElementNotFoundException {
BSTreeNode<T> current = findNode(targetElement, root);
if (current == null)
throw new ElementNotFoundException("BSTree");
return (current.getElement());
}
private BSTreeNode<T> findNode(T targetElement, BSTreeNode<T> next) {
if (next == null)
return null;
if (next.getElement().equals(targetElement))
return next;
BSTreeNode<T> temp = findNode(targetElement, next.getLeft());
if (temp == null)
temp = findNode(targetElement, next.getRight());
return temp;
}
public T removeElement(T targetElement) throws ElementNotFoundException {
T result = null;
if (isEmpty())
throw new ElementNotFoundException("BSTree");
else {
BSTreeNode<T> parent = null;
if (((Comparable<T>) targetElement).equals(root.getElement())) {
result = root.getElement();
BSTreeNode<T> temp = replacement(root);
if (temp == null)
root = null;
else {
root.setElement(temp.getElement());
root.setRight(temp.getRight());
root.setLeft(temp.getLeft());
}
} else {
parent = root;
if (((Comparable) targetElement).compareTo(root.getElement()) < 0)
result = removeElement(targetElement, root.getLeft(), parent);
else
result = removeElement(targetElement, root.getRight(), parent);
}
}
count--;
return result;
}
private T removeElement(T targetElement, BSTreeNode<T> node,
BSTreeNode<T> parent) throws ElementNotFoundException {
T result = null;
if (node == null)
throw new ElementNotFoundException("BSTree");
else {
if (((Comparable<T>) targetElement).equals(node.getElement())) {
result = node.getElement();
BSTreeNode<T> temp = replacement(node);
if (parent.getRight() == node)
parent.setRight(temp);
else
parent.setLeft(temp);
} else {
parent = node;
if (((Comparable) targetElement).compareTo(node.getElement()) < 0)
result = removeElement(targetElement, node.getLeft(),
parent);
else
result = removeElement(targetElement, node.getRight(),
parent);
}
}
return result;
}
private BSTreeNode<T> replacement(BSTreeNode<T> node) {
BSTreeNode<T> result = null;
if ((node.getLeft() == null) && (node.getRight() == null))
result = null;
else if ((node.getLeft() != null) && (node.getRight() == null))
result = node.getLeft();
else if ((node.getLeft() == null) && (node.getRight() != null))
result = node.getRight();
else {
BSTreeNode<T> current = node.getRight();
BSTreeNode<T> parent = node;
while (current.getLeft() != null) {
parent = current;
current = current.getLeft();
}
current.setLeft(node.getLeft());
if (node.getRight() != current) {
parent.setLeft(current.getRight());
current.setRight(node.getRight());
}
result = current;
}
return result;
}
public String toString()
{
ArrayList<T> temp = new ArrayList<T>();
inOrder(root, temp);
return "Inorder traversal: " + temp.toString();
}
public Iterator<T> iterator()
{
return iteratorInOrder();
}
public Iterator<T> iteratorInOrder()
{
ArrayList<T> tempList = new ArrayList<T>();
inOrder(root, tempList);
return tempList.iterator();
}
public T findMax(){
T result = null;
if (isEmpty())
throw new ElementNotFoundException ("binary tree");
else {
BSTreeNode<T> current = root;
while (current.getRight() != null)
current = current.getRight();
result = current.getElement();
}
return result;
}
public T removeMax(){
T result = null;
if (isEmpty())
throw new ElementNotFoundException("binary tree");
else
{
if (root.getRight() == null)
{
result = root.getElement();
root = root.getLeft();
}
else
{
BSTreeNode<T> parent = root;
BSTreeNode<T> current = root.getRight();
while (current.getRight() != null)
{
parent = current;
current = current.getRight();
}
result = current.getElement();
parent.setRight(current.getLeft());
}
count--;
}
return result;
}
protected void inOrder(BSTreeNode<T> node, ArrayList<T> tempList) {
if (node != null) {
inOrder(node.getLeft(), tempList);
tempList.add(node.getElement());
inOrder(node.getRight(), tempList);
}
}
}

You can write a function to find the height of left and right sub-tree
int height(Node node)
{
if (node == null)
return 0;
return 1 + Math.max(height(node.left), height(node.right));
}
then, you can write another method to check if the tree is balanced
boolean isBalanced(Node node)
{
int lh;
int rh;
if (node == null)
return true;
lh = height(node.left);
rh = height(node.right);
if (Math.abs(lh - rh) <= 1
&& isBalanced(node.left)
&& isBalanced(node.right)) {
return true;
}
return false;
}
and then, you can write a JUnit test case to test your isBalanced().
I hope this helps!

Couting leaves in a binary tree

I have been trying to figure out why my countingLeaves method cannot be found when I call it from my tester class.
My compiler gives me the error TreeTester.java:25: error: cannot find symbol
countLeaves();
^
symbol: method countLeaves()
location: class TreeTester
public class BinarySearchTree
{
private Node root;
public BinarySearchTree()
{
root = null;
}
public void add(Comparable obj)
{
Node newNode = new Node();
newNode.data = obj;
newNode.left = null;
newNode.right = null;
if (root == null) { root = newNode; }
else { root.addNode(newNode); }
}
public boolean find(Comparable obj)
{
Node current = root;
while (current != null)
{
int d = current.data.compareTo(obj);
if (d == 0) { return true; }
else if (d > 0) { current = current.left; }
else { current = current.right; }
}
return false;
}
public void remove(Comparable obj)
{
Node toBeRemoved = root;
Node parent = null;
boolean found = false;
while (!found && toBeRemoved != null)
{
int d = toBeRemoved.data.compareTo(obj);
if (d == 0) { found = true; }
else
{
parent = toBeRemoved;
if (d > 0) { toBeRemoved = toBeRemoved.left; }
else { toBeRemoved = toBeRemoved.right; }
}
}
if (!found) { return; }
if (toBeRemoved.left == null || toBeRemoved.right == null)
{
Node newChild;
if (toBeRemoved.left == null)
{
newChild = toBeRemoved.right;
}
else
{
newChild = toBeRemoved.left;
}
if (parent == null) // Found in root
{
root = newChild;
}
else if (parent.left == toBeRemoved)
{
parent.left = newChild;
}
else
{
parent.right = newChild;
}
return;
}
Node smallestParent = toBeRemoved;
Node smallest = toBeRemoved.right;
while (smallest.left != null)
{
smallestParent = smallest;
smallest = smallest.left;
}
toBeRemoved.data = smallest.data;
if (smallestParent == toBeRemoved)
{
smallestParent.right = smallest.right;
}
else
{
smallestParent.left = smallest.right;
}
}
public void print()
{
print(root);
System.out.println();
}
private static void print(Node parent)
{
if (parent == null) { return; }
print(parent.left);
System.out.print(parent.data + " ");
print(parent.right);
}
public int countLeaves(Node node)
{
if(node == null)
return 0;
else if(node.left == null && node.right == null)
{
return 1;
}
else
{
return countLeaves(node.left) + countLeaves(node.right);
}
}
class Node
{
public Comparable data;
public Node left;
public Node right;
public void addNode(Node newNode)
{
int comp = newNode.data.compareTo(data);
if (comp < 0)
{
if (left == null) { left = newNode; }
else { left.addNode(newNode); }
}
else if (comp > 0)
{
if (right == null) { right = newNode; }
else { right.addNode(newNode); }
}
}
}
}
The tester class used
public class TreeTester
{
public static void main(String[] args)
{
BinarySearchTree t = new BinarySearchTree();
t.add("D");
t.add("B");
t.add("A");
t.add("C");
t.add("F");
t.add("E");
t.add("I");
t.add("G");
t.add("H");
t.add("J");
t.remove("A"); // Removing leaf
t.remove("B"); // Removing element with one child
t.remove("F"); // Removing element with two children
t.remove("D"); // Removing root
t.print();
System.out.println("Expected: C E G H I J");
countLeaves(t);
}
}

The countLeaves needs a Node, not a BinarySearchTree.
You may add a method in BinarySearchTree as this:
Node getRoot(){return root;}
Amd use countLeaves(t.getRoot()).

The countLeaves needs a Node, not a BinarySearchTree.

java-recursive binary search tree

I've written a boolean insert method that inserts values into a binary search tree which inserts the value if the value is not already there and returns true if so, and returns false if the value is already there so inserts nothing. I am trying to convert this iterative method into all recursion with no loops at all but I am having trouble with figuring out how. Any help is appreciated!
public boolean insert(int value) {
Node travel= root, prev= null, newNode;
boolean result= true;
while (travel != null && travel.data != value) {
prev= travel;
if (value < travel.data)
travel= travel.left;
else travel= travel.right;
}
if (travel != null)
result= false;
else
if (root == null)
root= new Node(value);
else
if (value < prev.data)
prev.left= new Node(value);
else prev.right= new Node(value);
return result;
}

http://www.java2s.com/Code/Java/Collections-Data-Structure/BinaryTree.htm
public class BinarySearchTree {
private Node root;
public boolean insert(int value) {
if (root == null) {
root = new Node(value);
return true;
} else {
return insert(root, value);
}
}
private boolean insert(Node node, int value) {
if (value < node.value) {
if (node.left != null) {
return insert(node.left, value);
} else {
node.left = new Node(value);
return true;
}
} else if (value > node.value) {
if (node.right != null) {
return insert(node.right, value);
} else {
node.right = new Node(value);
return true;
}
} else {
return false;
}
}
public void printInOrder(Node node) {
if (node != null) {
printInOrder(node.left);
System.out.println("Traversed " + node.value);
printInOrder(node.right);
}
}
public static void main(String[] args) {
BinarySearchTree t = new BinarySearchTree();
System.out.println("insert 5: " + t.insert(5));
System.out.println("insert 4: " + t.insert(4));
System.out.println("insert 7: " + t.insert(7));
System.out.println("insert 4: " + t.insert(4));
t.printInOrder(t.root);
}
}
class Node {
Node left;
Node right;
int value;
public Node(int value) {
this.value = value;
}
}

You may try the following:
public boolean insert(int value) {
return insert(value, root);
}
public boolean insert(int value, Node explore) {
if (explore != null) {
if (value < explore.data) {
if (explore.left != null) {
return insert(value, explore.left);
} else {
explore.left = new Node(value);
return true;
}
} else if (value > explore.data) {
if (explore.right != null) {
return insert(value, explore.right);
} else {
explore.right = new Node(value);
return true;
}
} else {
// In this case the value already exists
return false;
}
} else {
explore = new Node(value);
}
return true;
}

avl tree rotation

I am trying to do a an avl tree which updates itself everytime the tree is unbalanced. The rotations are working but i have a bug where if for example the tree node 7, leftChild 6 , leftchild of leftchild 5 becomes node 6, leftchild 5, rightchild 7, and after balancing I add a new node, the node is first compared with 7 and not with 6. How do I fix this problem?
This is the main class:
import java.io.*;
import javax.swing.*;
import java.util.*;
import java.lang.*;
public class NewMain implements Cloneable{
/**
* #param args the command line arguments
*/
public static void main(String[] args)
{
File file = new File ("AVLTree.txt");
ArrayList <TreeNode> array = new ArrayList ();
Scanner kb = new Scanner (System.in);
int num = 0;
TreeNode root = new TreeNode ();
do {
System.out.print(" AVL Tree \n\n\n\n");
System.out.println("1. Create a new binary tree");
System.out.println("2. Save Tree");
System.out.println("3. Load Tree");
System.out.println("4. Enter a new node in the tree");
System.out.println("5. Show current AVL tree");
System.out.println("6. Show inorder traversal");
System.out.println("7. Search");
System.out.println("8. Quit \n\n\n\n\n\n\n");
System.out.print("Enter a number: ");
num = kb.nextInt ();
if (num == 1){
if (array.isEmpty ())
{
System.out.print ("Enter the root value: ");
int value = kb.nextInt ();
root = new TreeNode();
root.setValue(value);
array.add(root);
}
else
{
array.clear();
System.out.print ("Enter the root value: ");
int value = kb.nextInt ();
root = new TreeNode();
root.setValue(value);
array.add(root);
}
}
if (num == 2)
{
FileOutputStream outFile = null;
ObjectOutputStream oos = null;
try
{
outFile = new FileOutputStream(file);
oos = new ObjectOutputStream(outFile);
for (TreeNode list : array)
{
oos.writeObject(list);
}
oos.close();
}
catch (Exception e)
{
System.out.print("Save Not Successful!");
}
}
if (num == 3)
{
if (file.exists())
{
FileInputStream inFile = null;
ObjectInputStream ios = null;
try
{
Object obj = null;
inFile = new FileInputStream(file);
ios = new ObjectInputStream(inFile);
while ((obj = ios.readObject()) != null) {
if (obj instanceof TreeNode)
{
array.add((TreeNode) obj);
}
}
ios.close();
}
catch(EOFException e)
{
}
catch (Exception e)
{
System.out.print("File was not found while loading");
}
}
}
if (num == 4)
{
System.out.print ("Enter a new child node: ");
int value = kb.nextInt ();
try
{
array.add(root.insert(value));
root.balance();
}
catch (Exception e)
{
System.out.print (e.getMessage());
}
}
if (num == 5){
System.out.print ("Pointer Number\t\tLeft\t\tNode\t\tRight\t\tLeft Height\t\tRight Height\n");
for (int i=0; i<array.size();i++)
{
System.out.print (i+"\t\t\t"+array.indexOf(array.get(i).getLeftChild())+"\t\t"+array.get(i).getValue()+"\t\t"+array.indexOf(array.get(i).getRightChild())+"\t\t"+array.get(i).getLeftHeight()+"\t\t\t"+array.get(i).getRightHeight()+"\n");
}
}
if (num == 6)
{
System.out.print("Inorder traversal: ");
System.out.println(root.InOrderTraversal());
System.out.print("Postorder traversal: ");
System.out.println(root.PostOrderTraversal());
System.out.print("Preorder traversal: ");
System.out.println(root.PreOrderTraversal());
}
if (num == 7)
{
System.out.print("Enter node to be searched: ");
int node = kb.nextInt ();
for (int i = 0; i<array.size();i++)
{
if (node == array.get(i).getValue())
{
System.out.print ("Node is in index "+i+"\n");
break;
}
if (i == array.size()-1 && node != array.get(i).getValue())
{
System.out.print ("Node not found in the tree!"+"\n");
break;
}
}
}
}while (num != 8);
}
}
This is from a normal java class:
import java.lang.StringBuilder;
import java.util.ArrayList;
import java.io.*;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.*;
public class TreeNode implements Serializable, Cloneable
{
public Integer value;
public TreeNode leftChild;
public TreeNode rightChild;
public TreeNode()
{
this.value = null;
this.leftChild = null;
this.rightChild = null;
}
public TreeNode(int value)
{
this.value = value;
this.leftChild = null;
this.rightChild = null;
}
public int getValue()
{
return this.value;
}
public void setValue(int value)
{
this.value = value;
}
public TreeNode getLeftChild()
{
return this.leftChild;
}
public void setLeftChild(TreeNode leftChild)
{
this.leftChild = leftChild;
}
public TreeNode getRightChild()
{
return this.rightChild;
}
public void setRightChild(TreeNode rightChild)
{
this.rightChild = rightChild;
}
public int getLeftHeight()
{
if (this.leftChild == null)
{
return 0;
}
else
{
return this.getLeftChild().getHeight() + 1;
}
}
public int getRightHeight()
{
if (this.rightChild == null)
{
return 0;
}
else
{
return this.getRightChild().getHeight() + 1;
}
}
public TreeNode insert(int value) throws Exception
{
if(this.value == null && this.leftChild == null && this.rightChild == null)
{
this.value = value;
return this;
}
else
{
TreeNode node = new TreeNode (value);
if(value < this.value)
{
if(this.getLeftChild() == null)
{
this.setLeftChild (node);
return node;
}
else
{
return this.getLeftChild().insert(value);
}
}
else if(value > this.value)
{
if(this.getRightChild() == null)
{
this.setRightChild(node);
return node;
}
else
{
return this.getRightChild().insert(value);
}
}
else
{
return null;
}
}
}
public TreeNode balance() throws Exception
{
if (Math.abs(this.getLeftHeight() - this.getRightHeight())==2)
{
if (this.rightChild == null)
{
if(this.leftChild.leftChild != null)
{
return this.LLRotation ();
}
if(this.leftChild.rightChild != null)
{
return this.LRRotation ();
}
}
if (this.leftChild == null)
{
if (this.rightChild.rightChild != null)
{
return this.RRRotation ();
}
if (this.rightChild.leftChild != null)
{
return this.RLRotation ();
}
}
}
else
{
if (this.getLeftChild () != null)
{
return this.getLeftChild().balance();
}
if (this.getRightChild () != null)
{
return this.getRightChild().balance();
}
}
return this;
}
public int getHeight ()
{
if (this.leftChild == null && this.rightChild == null)
{
return 0;
}
else
{
int leftH = 0;
int rightH = 0;
if (this.leftChild != null)
{
leftH++;
leftH += this.getLeftChild().getHeight();
}
if (this.rightChild != null)
{
rightH++;
rightH += this.getRightChild().getHeight();
}
return Math.max(leftH,rightH);
}
}
public TreeNode LLRotation ()
{
TreeNode temp = this.leftChild;
this.leftChild = null;
temp.rightChild = this;
return temp;
}
public TreeNode RRRotation ()
{
TreeNode temp = this.rightChild;
this.rightChild = temp.leftChild;
try
{
temp.leftChild = (TreeNode)this.clone();
}
catch (CloneNotSupportedException ex)
{
}
this.value = temp.value;
this.rightChild = temp.rightChild;
this.leftChild = temp.leftChild;
return temp;
}
public TreeNode LRRotation ()
{
this.leftChild = this.leftChild.RRRotation();
return LLRotation();
}
public TreeNode RLRotation ()
{
this.rightChild = this.rightChild.RRRotation();
return RRRotation();
}
public String InOrderTraversal ()
{
StringBuilder sb = new StringBuilder ();
if (this.leftChild == null && this.rightChild == null)
{
sb.append(this.value).append(" ");
}
else
{
if(this.leftChild != null)
{
sb.append(this.getLeftChild().InOrderTraversal());
}
sb.append(this.value).append(" ");
if (this.rightChild != null)
{
sb.append(this.getRightChild().InOrderTraversal());
}
}
return sb.toString();
}
public String PostOrderTraversal ()
{
StringBuilder sb = new StringBuilder ();
if (this.leftChild == null && this.rightChild == null)
{
sb.append(this.value).append(" ");
}
else
{
if(this.leftChild != null)
{
sb.append(this.getLeftChild().PostOrderTraversal());
}
if (this.rightChild != null)
{
sb.append(this.getRightChild().PostOrderTraversal());
}
sb.append(this.value).append(" ");
}
return sb.toString();
}
public String PreOrderTraversal ()
{
StringBuilder sb = new StringBuilder ();
if (this.leftChild == null && this.rightChild == null)
{
sb.append(this.value).append(" ");
}
else
{
sb.append(this.value).append(" ");
if(this.leftChild != null)
{
sb.append(this.getLeftChild().PreOrderTraversal());
}
if (this.rightChild != null)
{
sb.append(this.getRightChild().PreOrderTraversal());
}
}
return sb.toString();
}
}

The code is a bit more complex it needs to be. I hope here to give you a simpler version, which with you yourself may correctly balance. As maybe you should better do pointer rotation / rebalancing inside the insertion. Don't feel obliged to give points; this is but a half answer.
Remark only: the field value may be an ìnt.
It is definitely easier for recursive algorithms, if the "this" object might be null. This can be achieved by wrapping the entire tree in one public class, which uses the TreeNode class internally.
public class Tree {
private static class TreeNode {
private int value;
private TreeNode left;
private TreeNode right;
private TreeNode(int value, TreeNode left, TreeNode right) {
this.value = value;
this.left = left;
this.right = right;
}
}
private static class AlreadyExistsException extends Exception {
private TreeNode node;
private AlreadyExistsException(TreeNode node) {
this.node = node;
}
public TreeNode getNode() {
return node;
}
}
private TreeNode root;
private int size;
public boolean insert(int value) {
try {
root = insertInto(root, value);
++size;
return true;
} catch (AlreadyExistsException e) {
// Fine.
return false;
}
}
private TreeNode insertInto(TreeNode node, int value) throws AlreadyExistsException {
if (node == null) {
return new TreeNode(value, null, null);
}
if (value < node.value) {
node.left = insertInto(node.left, value);
return node;
} else if (value > node.value) {
node.right = insertInto(node.right, value);
return node;
} else {
throw new AlreadyExistsException(node);
}
}
}
As you see the for balancing immediately during the insertion, there could be done an insertion with pointer rotation at < and > (3 pointers: left's right-most leaf or right's left-most leaf). Comming back from recursion one could have collected the parent of the left's right-most leaf, and perform a rotation. Or at any other point. There exist 3 variants!

Probably because root is still pointing at the old node. Are you sure you want to ignore the return value of balance() at the marked line?
if (num == 4) {
System.out.print ("Enter a new child node: ");
int value = kb.nextInt ();
try {
array.add(root.insert(value));
root.balance(); // look here
} catch (Exception e) {
System.out.print (e.getMessage());
}
}
BTW, an AVL tree as described in the literature does not recurse of the entirety of the tree to find the balance of a node.

linked list implementation java

I have implemented a Linked List into Java. I have created everything, but I am having difficulty removing a specific node with specific data. It is throwing a NullPointerException. I believe, I am getting a NullPointerException because the next node is null. If someone could please point me in the right direction that would be great.
Input
anything
one
two
three
exception:
Exception in thread "main" java.lang.NullPointerException
at LinkedList.remove(LinkedList.java:28)
at Main.main(Main.java:29)
Classes:
Linked list class
public class LinkedList {
// fields
private Node head;
private Node last;
private int size = 0;
// constructor, used when the class is first called
public LinkedList() {
head = last = new Node(null);
}
// add method
public void add(String s) {
last.setNext(new Node(s));
last = last.getNext();
size++;
}
// remove method, if it returns false then the specified index element doens not exist
// otherwise will return true
public boolean remove(String data) {
Node current = head;
last = null;
while(current != null) {
if(current.getData().equals(data)) {
current = current.getNext();
if(last == null) {
last = current;
}else {
last.getNext().setNext(current);
size--;
return true;
}
}else {
last = current;
current = current.getNext();
}
}
return false;
}
//will return the size of the list - will return -1 if list is empty
public int size() {
return size;
}
// will check if the list is empty or not
public boolean isEmpty() {
return true;
}
// #param (index) will get the data at specified index
public String getData(int index) {
if(index <= 0) {
return null;
}
Node current = head.getNext();
for(int i = 1;i < index;i++) {
if(current.getNext() == null) {
return null;
}
current = current.getNext();
}
return current.getData();
}
//#param will check if the arguement passed is in the list
// will return true if the list contains arg otherwise false
public boolean contains(String s) {
for(int i = 1;i<=size();i++) {
if(getData(i).equals(s)) {
return true;
}
}
return false;
}
//#return contents of the list - recursively
public String toString() {
Node current = head.getNext();
String output = "[";
while(current != null) {
output += current.getData()+",";
current = current.getNext();
}
return output+"]";
}
//#return first node
public Node getHead() {
return head;
}
// #return (recursively) list
public void print(Node n) {
if(n == null) {
return;
}else {
System.out.println(n.getData());
print(n.getNext());
}
}
}
Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class Main {
static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
public static void main(String[] args) throws IOException{
LinkedList list = new LinkedList(); // declaring main linked list
LinkedList b_List = new LinkedList(); // declaring the backup list
String input = null;
// getting input from user, will stop when user has entered 'fin'
while(!(input = br.readLine()).equals("fin")) {
list.add(input); // adding to main list
b_List.add(input);
}
list.print(list.getHead().getNext());
System.out.println("Input Complete.");
if(list.size() == 1) {
System.out.println("You have entered only one name. He/She is the survior");
}else {
System.out.println("Enter the name(s) would like to remove: ");
while(b_List.size() != 1) {
String toRemove = br.readLine();
b_List.remove(toRemove);
}
}
System.out.println("The contestants were: ");
list.print(list.getHead().getNext());
}
}
node
public class Node {
// Fields
private String data;
private Node next;
// constructor
public Node(String data) {
this(data,null);
}
// constructor two with Node parameter
public Node(String data, Node node) {
this.data = data;
next = node;
}
/**
* Methods below return information about fields within class
* */
// #return the data
public String getData() {
return data;
}
// #param String data to this.data
public void setData(String data) {
this.data = data;
}
// #return next
public Node getNext() {
return next;
}
// #param Node next set to this.next
public void setNext(Node next) {
this.next = next;
}
}

First of all, your head is just a before-first marker so you shouldn't start the remove check from it.
Second, your remove method fails if node data is null
Third - your implementation is broken anyway because of last.getNext().setNext(current) - it won't link previous node with next, it will link current to next (i.e. will do nothing)
Fourth - it still fails to remove first element because of mysterious operations with last...
Correct implementation of remove would be something like this:
public boolean remove(String data){
Node previous = head;
Node current = head.getNext();
while (current != null) {
String dataOld = current.getData();
if ((dataOld == null && data == null) || (dataOld != null && dataOld.equals(data))) {
Node afterRemoved = current.getNext();
previous.setNext(afterRemoved);
if (afterRemoved == null) { // i.e. removing last element
last = previous;
}
size--;
return true;
} else {
previous = current;
current = current.getNext();
}
}
return false;
}

Here we can see the simple implementation of LinkedList with iterator
class LinkedList implements Iterable{
private Node node;
public void add(Object data){
if(!Optional.ofNullable(node).isPresent()){
node = new Node();
node.setData(data);
}else{
Node node = new Node();
node.setData(data);
Node lastNode = getLastNode(this.node);
lastNode.setNext(node);
}
}
private Node getLastNode(Node node){
if(node.getNext()==null){
return node;
}else{
return getLastNode(node.getNext());
}
}
class Node{
private Object data;
private Node next;
public Object getData() {
return data;
}
public void setData(Object data) {
this.data = data;
}
public Node getNext() {
return next;
}
public void setNext(Node next) {
this.next = next;
}
}
public Iterator iterator() {
return new NodeIterator();
}
class NodeIterator implements Iterator{
private Node current;
public boolean hasNext() {
if(current == null){
current = node;
return Optional.ofNullable(current).isPresent();
}else{
current = current.next;
return Optional.ofNullable(current).isPresent();
}
}
public Node next() {
return current;
}
}
}
public class LinkedListImpl {
public static void main(String[] args) {
LinkedList linkedList = new LinkedList();
linkedList.add("data1");
linkedList.add("data2");
linkedList.add("data3");
for(LinkedList.Node node: linkedList){
System.out.println(node.getData());
}
}
}

Here is Full Implementaion of Linked List
including insertion,deletion,searching,reversing,swaping,size,display and various important operations of linked list
import java.util.NoSuchElementException;
import java.util.Scanner;
class Node<T> {
public Node<T> next;
public T data;
public Node() {
}
public Node(T data, Node<T> next) {
this.data = data;
this.next = next;
}
#Override
public String toString() {
return "Node [next=" + next + ", data=" + data + "]";
}
}
class LinkedList<T> {
Node<T> start = null;
Node<T> end = null;
public void insertAtStart(T data) {
Node<T> nptr = new Node<T>(data, null);
if (empty()) {
start = nptr;
end = start;
} else {
nptr.next = start;
start = nptr;
}
display();
}
public void insertAtEnd(T data) {
Node<T> nptr = new Node<T>(data, null);
if (empty()) {
insertAtStart(data);
return;
} else {
end.next = nptr;
end = nptr;
}
display();
}
public void insertAtPosition(int position, T data) {
if (position != 1 && empty())
throw new IllegalArgumentException("Empty");
if (position == 1) {
insertAtStart(data);
return;
}
Node<T> nptr = new Node<T>(data, null);
if (position == size()) {
Node<T> startPtr = start;
Node<T> endPtr = startPtr;
while (startPtr.next != null) {
endPtr = startPtr;
startPtr = startPtr.next;
}
endPtr.next = nptr;
nptr.next = end;
} else {
position -= 1;
Node<T> startPtr = start;
for (int i = 1; i < size(); i++) {
if (i == position) {
Node<T> temp = startPtr.next;
startPtr.next = nptr;
nptr.next = temp;
}
startPtr = startPtr.next;
}
}
display();
}
public void delete(int position) {
if (empty())
throw new IllegalArgumentException("Empty");
if (position == 1) {
start = start.next;
} else if (position == size()) {
Node<T> startPtr = start;
Node<T> endPtr = start;
while (startPtr.next != null) {
endPtr = startPtr;
startPtr = startPtr.next;
}
endPtr.next = null;
end = endPtr;
} else {
position -= 1;
Node<T> startPtr = start;
for (int i = 1; i <= position; i++) {
if (i == position) {
Node<T> temp = startPtr.next.next;
startPtr.next = temp;
}
startPtr = startPtr.next;
}
}
display();
}
public int index(T data) {
if (empty())
throw new IllegalArgumentException("Empty");
return index(start, data, 0);
}
private int index(Node<T> link, T data, int index) {
if (link != null) {
if (link.data == data) {
return index;
}
return index(link.next, data, ++index);
}
return -1;
}
public void replace(int position, T data) {
if (empty())
throw new IllegalArgumentException("Empty");
if (position == 1)
start.data = data;
else if (position == size())
end.data = data;
else {
Node<T> startPtr = start;
for (int i = 1; i <= position; i++) {
if (i == position)
startPtr.data = data;
startPtr = startPtr.next;
}
}
display();
}
public void replaceRecursively(int position, T data) {
replaceRecursively(start, position, data, 1);
display();
}
private void replaceRecursively(Node<T> link, int position, T data, int count) {
if (link != null) {
if (count == position) {
link.data = data;
return;
}
replaceRecursively(link.next, position, data, ++count);
}
}
public T middle() {
if (empty())
throw new NoSuchElementException("Empty");
Node<T> slowPtr = start;
Node<T> fastPtr = start;
while (fastPtr != null && fastPtr.next != null) {
slowPtr = slowPtr.next;
fastPtr = fastPtr.next.next;
}
return slowPtr.data;
}
public int occurence(T data) {
if (empty())
throw new NoSuchElementException("Empty");
return occurence(start, data, 0);
}
private int occurence(Node<T> link, T data, int occurence) {
if (link != null) {
if (link.data == data)
++occurence;
return occurence(link.next, data, occurence);
}
return occurence;
}
public void reverseRecusively() {
reverseRecusively(start);
swapLink();
display();
}
private Node<T> reverseRecusively(Node<T> link) {
if (link == null || link.next == null)
return link;
Node<T> nextLink = link.next;
link.next = null;
Node<T> revrseList = reverseRecusively(nextLink);
nextLink.next = link;
return revrseList;
}
public void reverse() {
if (empty())
throw new NoSuchElementException("Empty");
Node<T> prevLink = null;
Node<T> currentLink = start;
Node<T> nextLink = null;
while (currentLink != null) {
nextLink = currentLink.next;
currentLink.next = prevLink;
prevLink = currentLink;
currentLink = nextLink;
}
swapLink();
display();
}
private void swapLink() {
Node<T> temp = start;
start = end;
end = temp;
}
public void swapNode(T dataOne, T dataTwo) {
if (dataOne == dataTwo)
throw new IllegalArgumentException("Can't swap " + dataOne + " and " + dataTwo + " both are same");
boolean foundDataOne = false;
boolean foundDataTwo = false;
Node<T> dataOnePtr = start;
Node<T> dataOnePrevPtr = start;
while (dataOnePtr.next != null && dataOnePtr.data != dataOne) {
dataOnePrevPtr = dataOnePtr;
dataOnePtr = dataOnePtr.next;
}
Node<T> dataTwoPtr = start;
Node<T> dataTwoPrevPtr = start;
while (dataTwoPtr.next != null && dataTwoPtr.data != dataTwo) {
dataTwoPrevPtr = dataTwoPtr;
dataTwoPtr = dataTwoPtr.next;
}
if (dataOnePtr != null && dataOnePtr.data == dataOne)
foundDataOne = true;
if (dataTwoPtr != null && dataTwoPtr.data == dataTwo)
foundDataTwo = true;
if (foundDataOne && foundDataTwo) {
if (dataOnePtr == start)
start = dataTwoPtr;
else if (dataTwoPtr == start)
start = dataOnePtr;
if (dataTwoPtr == end)
end = dataOnePtr;
else if (dataOnePtr == end)
end = dataTwoPtr;
Node<T> tempDataOnePtr = dataOnePtr.next;
Node<T> tempDataTwoPtr = dataTwoPtr.next;
dataOnePrevPtr.next = dataTwoPtr;
dataTwoPtr.next = tempDataOnePtr;
dataTwoPrevPtr.next = dataOnePtr;
dataOnePtr.next = tempDataTwoPtr;
if (dataOnePtr == dataTwoPrevPtr) {
dataTwoPtr.next = dataOnePtr;
dataOnePtr.next = tempDataTwoPtr;
} else if (dataTwoPtr == dataOnePrevPtr) {
dataOnePtr.next = dataTwoPtr;
dataTwoPtr.next = tempDataOnePtr;
}
} else
throw new NoSuchElementException("Either " + dataOne + " or " + dataTwo + " not in the list");
display();
}
public int size() {
return size(start, 0);
}
private int size(Node<T> link, int i) {
if (link == null)
return 0;
else {
int count = 1;
count += size(link.next, 0);
return count;
}
}
public void printNthNodeFromLast(int n) {
if (empty())
throw new NoSuchElementException("Empty");
Node<T> main_ptr = start;
Node<T> ref_ptr = start;
int count = 0;
while (count < n) {
if (ref_ptr == null) {
System.out.println(n + " is greater than the no of nodes in the list");
return;
}
ref_ptr = ref_ptr.next;
count++;
}
while (ref_ptr != null) {
main_ptr = main_ptr.next;
ref_ptr = ref_ptr.next;
}
System.out.println("Node no " + n + " from the last is " + main_ptr.data);
}
public void display() {
if (empty())
throw new NoSuchElementException("Empty");
display(start);
}
private void display(Node<T> link) {
if (link != null) {
System.out.print(link.data + " ");
display(link.next);
}
}
public boolean empty() {
return start == null;
}
}
public class LinkedListTest {
public static void main(String[] args) {
LinkedList<Integer> linkedList = new LinkedList<Integer>();
boolean yes = true;
Scanner scanner = new Scanner(System.in);
do {
System.out.println("\n1. Insert At Start");
System.out.println("2. Insert At End");
System.out.println("3. Insert at Position");
System.out.println("4. Delete");
System.out.println("5. Display");
System.out.println("6. Empty status");
System.out.println("7. Get Size");
System.out.println("8. Get Index of the Item");
System.out.println("9. Replace data at given position");
System.out.println("10. Replace data at given position recusively");
System.out.println("11. Get Middle Element");
System.out.println("12. Get Occurence");
System.out.println("13. Reverse Recusively");
System.out.println("14. Reverse");
System.out.println("15. Swap the nodes");
System.out.println("16. Nth Node from last");
System.out.println("\nEnter your choice");
int choice = scanner.nextInt();
switch (choice) {
case 1:
try {
System.out.println("Enter the item");
linkedList.insertAtStart(scanner.nextInt());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 2:
try {
System.out.println("Enter the item");
linkedList.insertAtEnd(scanner.nextInt());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 3:
try {
System.out.println("Enter the position");
int position = scanner.nextInt();
if (position < 1 || position > linkedList.size()) {
System.out.println("Invalid Position");
break;
}
System.out.println("Enter the Item");
linkedList.insertAtPosition(position, scanner.nextInt());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 4:
try {
System.out.println("Enter the position");
int position = scanner.nextInt();
if (position < 1 || position > linkedList.size()) {
System.out.println("Invalid Position");
break;
}
linkedList.delete(position);
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 5:
try {
linkedList.display();
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 6:
System.out.println(linkedList.empty());
break;
case 7:
try {
System.out.println(linkedList.size());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 8:
try {
System.out.println(linkedList.index(scanner.nextInt()));
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 9:
try {
System.out.println("Enter the position");
int position = scanner.nextInt();
if (position < 1 || position > linkedList.size()) {
System.out.println("Invalid Position");
break;
}
linkedList.replace(position, scanner.nextInt());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 10:
try {
System.out.println("Enter the position");
int position = scanner.nextInt();
if (position < 1 || position > linkedList.size()) {
System.out.println("Invalid Position");
break;
}
System.out.println("Enter the item");
linkedList.replaceRecursively(position, scanner.nextInt());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 11:
try {
System.out.println(linkedList.middle());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 12:
try {
System.out.println("Enter the item");
System.out.println(linkedList.occurence(scanner.nextInt()));
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 13:
try {
linkedList.reverseRecusively();
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 14:
try {
linkedList.reverse();
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 15:
try {
System.out.println("Enter the nodes");
linkedList.swapNode(scanner.nextInt(), scanner.nextInt());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
case 16:
try {
System.out.println("Enter which node do you want from last");
linkedList.printNthNodeFromLast(scanner.nextInt());
} catch (Exception e) {
System.out.println(e.getMessage());
}
break;
default:
System.out.println("Invalid Choice");
break;
}
} while (yes);
scanner.close();
}
}

Consider another possible implementation of a working non-recursive Linked List with generic T placeholder. It works out of the box and the code is a more simple one:
import java.util.Iterator;
import java.util.NoSuchElementException;
public class LinkedList<T> implements Iterable<T> {
private Node first;
private Node last;
private int N;
public LinkedList() {
first = null;
last = null;
N = 0;
}
public void add(T item) {
if (item == null) { throw new NullPointerException("Null object!"); }
if (!isEmpty()) {
Node prev = last;
last = new Node(item, null);
prev.next = last;
}
else {
last = new Node(item, null);
first = last;
}
N++;
}
public boolean remove(T item) {
if (isEmpty()) { throw new IllegalStateException("Empty list!"); }
boolean result = false;
Node prev = first;
Node curr = first;
while (curr.next != null || curr == last) {
if (curr.data.equals(item)) {
// remove the last remaining element
if (N == 1) { first = null; last = null; }
// remove first element
else if (curr.equals(first)) { first = first.next; }
// remove last element
else if (curr.equals(last)) { last = prev; last.next = null; }
// remove element
else { prev.next = curr.next; }
N--;
result = true;
break;
}
prev = curr;
curr = prev.next;
}
return result;
}
public int size() {
return N;
}
public boolean isEmpty() {
return N == 0;
}
private class Node {
private T data;
private Node next;
public Node(T data, Node next) {
this.data = data;
this.next = next;
}
}
public Iterator<T> iterator() { return new LinkedListIterator(); }
private class LinkedListIterator implements Iterator<T> {
private Node current = first;
public T next() {
if (!hasNext()) { throw new NoSuchElementException(); }
T item = current.data;
current = current.next;
return item;
}
public boolean hasNext() { return current != null; }
public void remove() { throw new UnsupportedOperationException(); }
}
#Override public String toString() {
StringBuilder s = new StringBuilder();
for (T item : this)
s.append(item + " ");
return s.toString();
}
public static void main(String[] args) {
LinkedList<String> list = new LinkedList<>();
while(!StdIn.isEmpty()) {
String input = StdIn.readString();
if (input.equals("print")) { StdOut.println(list.toString()); continue; }
if (input.charAt(0) == ('+')) { list.add(input.substring(1)); continue; }
if (input.charAt(0) == ('-')) { list.remove(input.substring(1)); continue; }
break;
}
}
}
For more LinkedList examples, your can check out the article.