Develop Reference

How to extend child classes with a parent interface class

interface Iterator {
boolean hasnext();
int next();
}
class practice5 {
public static void main(String a[]) {
Stack s = new Stack();
Queue q = new Queue();
Linkedlist l = new Linkedlist();
s.push(100);
s.push(200);
q.Enque(300);
q.Enque(400);
l.add(500);
l.add(600);
Iterator itr;
itr = s;
while (!itr.hasnext()) {
System.out.println(itr.next());
}
itr = q;
while (!itr.hasnext()) {
System.out.println(itr.next());
}
itr = l;
while (itr.hasnext()) {
System.out.println(itr.next());
}
}
}
class Stack extends Iterator {
private int stack[];
private int top;
public Stack() {
stack = new int[10];
top = -1;
}
public void push(int val) {
top++;
stack[top] = val;
}
public boolean hasnext() {
if (top >= 0) {
return false;
} else {
return true;
}
}
public int next() {
return (stack[top--]);
}
}
class Queue extends Iterator {
private int queue[];
private int front, rear;
public Queue() {
queue = new int[10];
front = 0;
rear = 0;
}
public void Enque(int val) {
queue[rear] = val;
rear++;
}
public boolean hasnext() {
if (front < rear) {
return false;
} else {
return true;
}
}
public int next() {
return (queue[front++]);
}
}
class Linkedlist extends Iterator {
private int data;
private Linkedlist nw, next, prev, first, guest;
public Linkedlist() {
nw = next = prev = first = null;
}
public void add(int val) {
nw = new Linkedlist();
nw.data = val;
if (first == null) {
prev = first = nw;
} else {
prev.next = nw;
prev = nw;
}
}
public boolean hasnext() {
if (guest != 0) {
return true;
} else {
return false;
}
}
public int next() {
int curval;
curval = first.data;
first = first.next;
return (curval);
}
}
I'm expecting that I get an output for the above code.
I need to know if I'm extending the Stack, Queue and LinkedList classes wrongly with the interface class. Whenever I'm pass the iterator class object the instance of my child class objects, I am getting an error.
Also, in the LinkedList section when I call guest != 0, I'm getting an error Bad Operand. How can I check and print whether my guest is equal to zero or not?

I'm getting a Out Of Memory Error: Java heap space Exception

I am currently trying to take in a text file and read each word in the file into a binary tree the specific error i get is:
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
the text file i am reading into the project was given to me by the professor for the assignment so i know this should not be running into any memory problems i have never felt with this type of exception before and don't know where to start please help. here is my code:
public class Tester {
public static void main(String[] args) throws FileNotFoundException {
Tester run = new Tester();
run.it();
}
public void it() throws FileNotFoundException {
BTree theTree = new BTree();
String str = this.readInFile();
String [] firstWords = this.breakIntoWords(str);
String [] finalWords = this.removeNullValues(firstWords);
for(int i = 0; i < finalWords.length; i++) {
theTree.add(finalWords[i]);
}
theTree.print();
}
public String readInFile() throws FileNotFoundException {
String myFile = "";
int numWords = 0;
Scanner myScan = new Scanner(new File("Dracula.txt"));
while(myScan.hasNext() == true) {
myFile += myScan.nextLine() + " ";
}
return myFile;
}
public String [] breakIntoWords(String myFile) {
String[] words = new String[myFile.length()];
String nextWord = "";
int position = 0;
int i = 0;
while(myFile.length() > position) {
char next = myFile.charAt(position);
next = Character.toLowerCase(next);
// First trim beginning
while (((next < 'a') || (next > 'z')) && !Character.isDigit(next)) {
position++;
next = myFile.charAt(position);
next = Character.toLowerCase(next);
}
// Now pull only letters or numbers until we hit a space
while(!Character.isWhitespace(next)) {
if (Character.isLetterOrDigit(next)) {
nextWord += myFile.charAt(position);
}
position++;
next = myFile.charAt(position);
}
words [i] = nextWord;
i++;
}
return words;
}
public String[] removeNullValues(String[] myWords) {
String[] justMyWords = new String[myWords.length];
for (int i = 0; i < myWords.length; i++) {
if (myWords[i] != null) {
justMyWords[i] = myWords[i];
}
}
return justMyWords;
}
}
Here's my B-tree class:
public class BTree {
private BTNode root;
private int nodeCount;
public boolean add(String word) {
BTNode myNode = new BTNode(word);
if(root == null) {
root = myNode;
nodeCount++;
return true;
}
if(findNode(word)) {
int tmp = myNode.getNumInstance();
tmp++;
myNode.setNumInstance(tmp);
return false;
}
BTNode temp = root;
while(temp != null) {
if(word.compareTo(temp.getMyWord()) < 0) {
if(temp.getRightChild() == null) {
temp.setLeftChild(myNode);
nodeCount++;
return true;
} else {
temp = temp.getRightChild();
}
} else {
if(temp.getLeftChild() == null) {
temp.setLeftChild(myNode);
nodeCount++;
return true;
} else {
temp = temp.getLeftChild();
}
}
}
return false;
}
public boolean findNode(String word) {
return mySearch(root, word);
}
private boolean mySearch(BTNode root, String word) {
if (root == null) {
return false;
}
if ((root.getMyWord().compareTo(word) < 0)) {
return true;
} else {
if (word.compareTo(root.getMyWord()) > 0) {
return mySearch(root.getLeftChild(), word);
} else {
return mySearch(root.getRightChild(), word);
}
}
}
public void print() {
printTree(root);
}
private void printTree(BTNode root) {
if (root == null) {
System.out.print(".");
return;
}
printTree(root.getLeftChild());
System.out.print(root.getMyWord());
printTree(root.getRightChild());
}
public int wordCount() {
return nodeCount;
}
}
And my B-tree node class:
public class BTNode {
private BTNode rightChild;
private BTNode leftChild;
private String myWord;
private int numWords;
private int numInstance;
private boolean uniqueWord;
private boolean isRoot;
private boolean isDeepest;
public BTNode(String myWord){
this.numInstance = 1;
this.myWord = myWord;
this.rightChild = null;
this.leftChild = null;
}
public String getMyWord() {
return myWord;
}
public void setMyWord(String myWord) {
this.myWord = myWord;
}
public BTNode getRightChild() {
return rightChild;
}
public void setRightChild(BTNode rightChild) {
this.rightChild = rightChild;
}
public BTNode getLeftChild() {
return leftChild;
}
public void setLeftChild(BTNode leftChild) {
this.leftChild = leftChild;
}
public int getnumWords() {
return numWords;
}
public void setnumWords(int numWords) {
this.numWords = numWords;
}
public boolean isUniqueWord() {
return uniqueWord;
}
public void setUniqueWord(boolean uniqueWord) {
this.uniqueWord = uniqueWord;
}
public boolean isRoot() {
return isRoot;
}
public void setRoot(boolean isRoot) {
this.isRoot = isRoot;
}
public boolean isDeepest() {
return isDeepest;
}
public void setDeepest(boolean isDeepest) {
this.isDeepest = isDeepest;
}
public int getNumInstance() {
return numInstance;
}
public void setNumInstance(int numInstance) {
this.numInstance = numInstance;
}
}

This little file should not be the reason for the OutOfMemory error.
Performance
That is no error, but if you want to read a whole file in the memory
don't read line per line and concatenate the strings. This slows down your programm.
You can use:
String myFile = new String(Files.readAllBytes(Paths.get("Dracula.txt")));
myFile = myFile.replaceAll("\r\n", " ");
return myFile;
That is also not superfast, but faster.
Now the Errors
word array is too large
public String[] breakIntoWords(String myFile) {
String[] words = new String[myFile.length()];
You define words as an array of lengh lenght of file . That is much too large if you
the name is mnemonic and means that you need an array of length count of words in file
nextWord is never resetted (Cause of OutOfMemory)
// Now pull only letters or numbers until we hit a space
while (!Character.isWhitespace(next)) {
if (Character.isLetterOrDigit(next)) {
nextWord += myFile.charAt(position);
}
position++;
next = myFile.charAt(position);
}
words[i] = nextWord;
i++;
because next word is never set to "" after assigning it to words[i]. So that next word grow
up word by word and your array contents looks like as:
words[0] = "Word1"
words[1] = "Word1Word2"
words[2] = "Word1Word2Word3"
As you can imagine, that will result in an very large amount of used space.

When you are building the tree, you are inserting nodes in the wrong side when you should insert the element to the right.
You should replace this code at BTree class:
while(temp != null) {
if(word.compareTo(temp.getMyWord()) < 0) {
if(temp.getRightChild() == null) {
temp.setRightChild(myNode); // <-- You were using setLeftChild()
nodeCount++;
return true;
} else {
temp = temp.getRightChild();
}
....
}
You are probably creating a huge tree with all the elements to the left side and getting the OutOfMemoryError

Add VM arguments :
-Xms<size> set initial Java heap size
-Xmx<size> set maximum Java heap size
-Xss<size> set java thread stack size
or run it using : java -Xmx256m yourclass.java

It depends on various factors.
Amount of java heap you are running with (default values differ for 32 bit and 64 bit JDK)
Size of the file you feed to the java program

You are trying to load entire contents of the file(i.e. stream object) into Java Memory. In such case, your file size limited(i.e small) Then above code will work in your limited memory but if the file size is increased(i.e. Contents of the file is increased). Then you will face issue.
You have to follow better approach to solve this problem by reading the file contents in chuck. Otherwise you will face same issue.
If you increase JVM arguments also won't work for larger files.
I feel your professor also testing the implementation of your project.

LinkedList - delete(Object) method works strange - deleting last element doesn't work properly

I have LinkedList with test program. As you can see in that program I add some Students to the list. I can delete them. If I choose s1,s2,s3 oraz s4 to delete everything runs well, and my list is printed properly and information about number of elements is proper. But if I delete last element (in this situation - s5) info about number of elements is still correct, but this element is still printed. Why is that so? Where is my mistake?
public class Lista implements List {
private Element head = new Element(null); //wartownik
private int size;
public Lista(){
clear();
}
public void clear(){
head.setNext(null);
size=0;
}
public void add(Object value){
if (head.getNext()==null) head.setNext(new Element(value));
else {
Element last = head.getNext();
//wyszukiwanie ostatniego elementu
while(last.getNext() != null)
last=last.getNext();
// i ustawianie jego referencji next na nowowstawiany Element
last.setNext(new Element(value));}
++size;
}
public Object get(int index) throws IndexOutOfBoundsException{
if(index<0 || index>size) throw new IndexOutOfBoundsException();
Element particular = head.getNext();
for(int i=0; i <= index; i++)
particular = particular.getNext();
return particular.getValue();
}
public boolean delete(Object o){
if(head.getNext() == null) return false;
if(head.getNext().getValue().equals(o)){
head.setNext(head.getNext().getNext());
size--;
return true;
}
Element delete = head.getNext();
while(delete != null && delete.getNext() != null){
if(delete.getNext().getValue().equals(o)){
delete.setNext(delete.getNext().getNext());
size--;
return true;
}
delete = delete.getNext();
}
return false;
}
public int size(){
return size;
}
public boolean isEmpty(){
return size == 0;
}
public IteratorListowy iterator() {
return new IteratorListowy();
}
public void wyswietlListe() {
IteratorListowy iterator = iterator();
for (iterator.first(); !iterator.isDone(); iterator.next())
{
System.out.println(iterator.current());
}
System.out.println();
}
public void infoOStanie() {
if (isEmpty()) {
System.out.println("Lista pusta.");
}
else
{
System.out.println("Lista zawiera " + size() + " elementow.");
}
}
private static final class Element{
private Object value;
private Element next; //Referencja do kolejnego obiektu
public Element(Object value){
setValue(value);
}
public void setValue(Object value) {
this.value = value;
}
public Object getValue() {
return value;
}
//ustawia referencję this.next na obiekt next podany w atgumencie
public void setNext(Element next) {
if (next != null)
this.next = next;
}
public Element getNext(){
return next;
}
}
private class IteratorListowy implements Iterator{
private Element current;
public IteratorListowy() {
current = head;
}
public void next() {
current = current.next;
}
public boolean isDone() {
return current == null;
}
public Object current() {
return current.value;
}
public void first() {
current = head.getNext();
}
}
}
test
public class Program {
public static void main(String[] args) {
Lista lista = new Lista();
Iterator iterator = lista.iterator();
Student s1 = new Student("Kowalski", 3523);
Student s2 = new Student("Polański", 45612);
Student s3 = new Student("Karzeł", 8795);
Student s4 = new Student("Pałka", 3218);
Student s5 = new Student("Konowałek", 8432);
Student s6 = new Student("Kłopotek", 6743);
Student s7 = new Student("Ciołek", 14124);
lista.add(s1);
lista.add(s2);
lista.add(s3);
lista.add(s4);
lista.add(s5);
lista.wyswietlListe();
lista.delete(s5);
lista.wyswietlListe();
lista.infoOStanie();
lista.clear();
lista.infoOStanie();
}
}

The problem is that your setNext(Element next) method does not set anything if next == null. And that is the case for the last element of your list.
So when you call delete.setNext(delete.getNext().getNext());, nothing is actually set because delete.getNext().getNext() is null!
Remove the if (next != null) condition in setNext and it will work.

Java: Generic Class Exception

I was designing a generic linked list to create a linked list of Strings.
However I keep getting this error :
Exception in thread "main" java.lang.NoSuchMethodError: Node.<init>(Ljava/lang/Object;)V
at LinkedList.addNode(LinkedList.java:10)
at LinkedList.<init>(LinkedList.java:22)
at Trial.main(Trial.java:7)
From the stack trace , the error is generated at LinkedList's addNode() method. Im including the definition to this method as well as the definition of the Node class.
LinkedList addNode()
public void addNode(T n) {
Node<T> temp = new Node<T>(n);
if(start==null) {
start = temp;
current = start;
} else {
end.setNext(temp);
}
end =temp;
}
Node.java
public class Node<T>{
private T n;
Node next;
Node(T n) {
this.n = n;
next = null;
}
public void setNext(Node nextNode) {
next = nextNode;
}
public Node getNext() {
return next;
}
public T getN() {
return n;
}
#Override
public String toString() {
if(n instanceof String)
return n.toString();
else {
return T.toString();
}
}
}
LinkedList.java
public class LinkedList<T>{
Node start;
Node end;
Node current;
private static final long serialVersionUID = 901L;
LinkedList(T n) {
addNode(n);
}
public void addNode(T n) {
Node<T> temp = new Node<>(n);
if(start==null) {
start = temp;
current = start;
} else {
end.setNext(temp);
}
end =temp;
}
LinkedList(T[] n) {
for(T print : n)
addNode(print);
}
public void addNode(T[] n) {
if(n!=null) {
for (T values : n) {
addNode(values);
}
}
}
public void incC() {
current = current.getNext();
}
public void insert(T n) {
Node newNode = new Node(n);
if(current==start){
newNode.setNext(current);
start = newNode;
}else {
Node tempstart = start;
Node prevAdd=null;
while(tempstart!=current){
prevAdd = tempstart;
tempstart = tempstart.getNext();
}
prevAdd.setNext(newNode);
newNode.setNext(current);
}
}
public void find(T x) {
Node tempstart;
tempstart = start;
while (tempstart!=null) {
if(tempstart.getN()==x) {
System.out.println("Element found");
tempstart = tempstart.getNext();
} else {
tempstart = tempstart.getNext();
}
}
}
public void delete(T x) {
Node previous=null;
Node tempstart = start;
while(tempstart!=null) {
if(tempstart.getN()==x) {
if(previous ==null) {
previous = tempstart;
tempstart = tempstart.getNext();
start = tempstart;
previous.setNext(null);
previous = null;
} else {
tempstart = tempstart.getNext();
previous.setNext(tempstart);
}
}else {
previous = tempstart;
tempstart = tempstart.getNext();
}
}
}
#Override
public String toString() {
Node tempNode = start;
String str = "Values: ";
while (tempNode!=null) {
str = str + " " + tempNode.toString();
tempNode = tempNode.getNext();
}
return str;
}
}
Trial.java
public class Trial {
public static void main(String[] args) {
String[] para = {"Hollo","this","is","me"};
LinkedList<String> L1;
L1 = new LinkedList<String>(para);
System.out.println(L1);
}

return T.toString();
this doesn't work. T is a type variable and only available at compile time due to type erasure.
But apart from that, I can't see what's wrong, you need to poost more code from your LinkedList class.

You should declare the start, end, and current fields in LinkedList< T > and the next field in Node< T > as type Node< T >, not Node. Don't use raw types anywhere in the code, because they translate into Node< Object >.

Your class Nodedoes not compile, so it is likely that you should first fix that issue before continuing:
return T.toString();
does not make sense. Probably that just writing this:
return n.toString();
is enough for now.

In Node.java, in the method
#Override
public String toString() {
if(n instanceof String)
return n.toString();
else {
return T.toString();
}
}
// the below statement thows compilation error.
return T.toString();

Your Node Constructor is not public, so it will not work if you call it from a class in another package.

Creating Minimum number and Addition Function of LinkedList

I'm having a problem with trying the logic and trying to write a min and additionMerge function and their recursive versions of the function that takes at least one list as an argument (the first node of the list). This will be a private helper function that is called by a wrapper function that is a member function of the LinkedList class.
public class LinkedList {
private static class ListNode {
public int firstItem;
public ListNode restOfList;
}
private ListNode first;
/**
* Create an empty list.
*/
public LinkedList() {
first = null;
}
public LinkedList(int n) {
first = countDown(n);
}
public LinkedList(String s) {
String[] temp = s.split(",");
for (int i = temp.length-1; i >= 0; i--) {
first = insertAtFront(first, Integer.parseInt(temp[i]));
}
}
public int length() {
return length(first);
}
private static int length(ListNode list) {
if (list == null) {
return 0;
}
int temp = length(list.restOfList);
return temp + 1;
}
public boolean contains(int value) {
return contains(first, value);
}
private static boolean contains(ListNode list, int value) {
if (list == null) {
return false;
}
if (list.firstItem == value) {
return true;
}
return contains(list.restOfList, value);
}
public int sum() {
return sum(first);
}
private static int sum(ListNode list) {
if (list == null) {
return 0;
}
return sum(list.restOfList) + list.firstItem;
}
public int count(int target) {
return count(first, target);
}
private static int count(ListNode list, int target) {
if (list == null) {
return 0;
}
int temp = count(list.restOfList, target);
if (list.firstItem == target) {
temp++;
}
return temp;
}
public void replace(int oldValue, int newValue) {
replace(first, oldValue, newValue);
}
private static void replace(ListNode list, int oldValue, int newValue) {
if (list == null) {
return;
}
replace(list.restOfList, oldValue, newValue);
if (list.firstItem == oldValue) {
list.firstItem = newValue;
}
}
public void insertAtFront(int n) {
first = insertAtFront(first, n);
}
private static ListNode insertAtFront(ListNode list, int n) {
ListNode answer = new ListNode();
answer.firstItem = n;
answer.restOfList = list;
return answer;
}
private static ListNode countDown(int n) {
if (n == 1) {
ListNode answer = new ListNode();
answer.firstItem = 1;
answer.restOfList = null;
return answer;
}
ListNode temp = countDown(n - 1);
ListNode answer = insertAtFront(temp, n);
return answer;
}
public void insertAtBack(int item) {
first = insertAtBack(first, item);
}
private static ListNode insertAtBack(ListNode list, int item) {
if (list == null) {
ListNode answer = new ListNode();
answer.firstItem = item;
answer.restOfList = null;
return answer;
}
//List answer = new ListNode();
//answer.firstItem = list.firstItem;
ListNode temp = insertAtBack(list.restOfList, item);
//answer.restOfList = temp;
list.restOfList = temp;
return list;
}
public void concatenate(LinkedList otherList) {
this.first = concatenate(this.first, otherList.first);
}
private static ListNode concatenate(ListNode list1, ListNode list2) {
if (list1 == null) {
return list2;
}
ListNode temp = concatenate(list1.restOfList, list2);
list1.restOfList = temp;
return list1;
}
public void filter(int item) {
first = filter(first, item);
}
#Override
public String toString() {
if (first == null) {
return "";
}
StringBuilder sb = new StringBuilder(256);
sb.append(first.firstItem);
for (ListNode current = first.restOfList;
current != null;
current = current.restOfList) {
sb.append(',');
sb.append(current.firstItem);
}
return sb.toString();
}
private static ListNode filter(ListNode list, int item) {
if (list == null) {
return null;
}
ListNode temp = filter(list.restOfList, item);
if (list.firstItem == item) {
return temp;
}
list.restOfList = temp;
return list;
}
public int min() throws RuntimeException {
if (first == null)
throw new RuntimeException("List is Empty");
else
return min();
}
// * A private recursive helper function that returns the minimum item in a
* list whose first node is the argument list.
private static int min(ListNode list) throws RuntimeException {
if (list == null) {
return 0;
}
}
public void additionMerge(LinkedList l2) {
}
* Every node in the list that begins with node
* node1 is increased by the ammount of the corresponding
* node in the list that begins with node node2.
* If one list is longer than the other, the missing nodes
* in the shorter list are assumed to be 0.
private static ListNode additionMerge(ListNode node1, ListNode node2) {
if (list == null) {
return null;
}
}
}

If this is not homework, then my advice is:
Don't write your own LinkedList class. Use the existing out, and add the extra functionality either as a helper class or by extending the existing class.
If you do decide to implement your own linked list class, then you should beware of using recursion. Recursion gives a neat soltion, but there is a major drawback with recursion in Java. The JVM does not do tail call optimization, so a recusive algorithm that recurses deeply (e.g. recursively traversing a long list) is liable to cause a StackOverflowError.