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Generic Linear List based on Arrays

I'm trying to write a Linear List based on arrays, but make the list be able to store any value by using Java Generics. This way I can create other programs that utilize it, but pass in different data types. I'm not entirely sure how to do this, any help would be appreciated.
I guess Im struggling trying to set it up and create the functions. The generic type really messes me up.
For example, trying to add a removeFirst() function, I cant use a loop like this:
for (int i = 0; i < n - 1; i++)
newList[i] = newList[i + 1];
— as it says The type of the expression must be an array type but it resolved to ArrayList.
Fair warning, I'm still learning data structures. This is what I have so far:
import java.util.ArrayList;
public class LinearList<T> {
private static int SIZE = 10;
private int n = 0;
private final ArrayList<T> newList = new ArrayList<T>(SIZE);
private T t;
public void set(T t) {
this.t = t;
}
public T get() {
return t;
}
public void add(T value, int position) {
newList.add(position, value);
n++;
}
public void addFirst(T value) {
newList.add(0, value);
n++;
}
public void removeLast() {
T value = null;
for (int i = 0; i < newList.size(); i++)
value = newList.get(i);
newList.remove(value);
n--;
}
public void removeFirst() {
newList.remove(0);
n--;
}
public T first() {
return newList.get(0);
}
public T last() {
int value = 0;
for (int i = 0; i < newList.size() - 1; i++)
value++;
return newList.get(value);
}
public int count() {
return n;
}
public boolean isFull() {
return (n >= SIZE);
}
public boolean isEmpty() {
return (n <= 0);
}
//part 4
public void Grow() {
int grow = SIZE / 2;
SIZE = SIZE + grow;
}
public void Shrink() {
int grow = SIZE / 2;
SIZE = SIZE - grow;
}
public String toString() {
String outStr = "" + newList;
return outStr;
}
}

A good start would be to make it non-generic with a class you are comfortable with, such as an Integer.
Once you have it set up, you can then make it generic by adding <T> to the class name, then replacing all references of Integer with T.
public class MyArray{ becomes public class MyArray<T>{
public Integer add(Integer value){ becomes public T add(T value){
See What are Generics in Java? for more help

Copy Constructor for a String Array in Java

So I'm currently working on a project that is recreating methods for Array String Lists and Linked String Lists. There is a StringList interface, that both ArrayStringList and LinkedStringList implement. We are not allowed to see the source code for the interface - only the API documentation. For each class, we have to create a default constructor and copy constructor for both classes. I've ran tests, and the default constructors both pass but the ArrayStringList copy constructor does not work and has been throwing the error message of "null" or "-1". I am pretty new to inheritance and interfaces, and I think the object parameters vs string array data types are throwing me off a bit.
Here is the code I have so far, and the methods used in the constructor:
My Copy Constructor:
private String[] stringArray;
private int size;
public ArrayStringList(StringList sl) {
size = sl.size();
ArrayStringList asl = new ArrayStringList();
for(int i = 0; i < size-1; i++) {
if(sl.get(i) != null) {
asl.set(i,sl.get(i).toString());
} //if
} // for
} // copy constructor
Size Method:
public int size() {
return stringArray.length;
} // size
Get Method:
public String get(int index) {
if(index < 0 || index >= size) {
throw new IndexOutOfBoundsException("out of bounds");
} else {
return stringArray[index];
}
} //get
Set Method:
public String set(int index, String s) {
String old = stringArray[index];
stringArray[index] = s;
return old;
} // set
In the project, the description of the copy constructor was as follows:
The implementing class must explicitly define a copy constructor. The copy constructor should take exactly one parameter of the interface type StringList. It should make the newly constructed list object a deep copy of the list referred to by the constructor's parameter. Therefore, the initial size and string elements of the new list object will be the same as the other list. To be clear, the other list can be an object of any implementation of the StringList interface. No other assumptions about the type of the object should be made.

public class ArrayStringList implements StringList {
private static final int INITIAL_CAPACITY = 10;
private String[] stringArray;
private int size;
public ArrayStringList(StringList sl) {
stringArray = sl.toArray();
size = stringArray.length;
}
public ArrayStringList() {
stringArray = new String[INITIAL_CAPACITY];
size = 0;
}
// TODO: Extract 'if-cascade' to an validate(..) method
#Override
public String set(int index, String s) {
if (index >= size) {
throw new IndexOutOfBoundsException("")
} else if (s == null) {
throw new NullPointerException("the specified string is null");
} else if (s.isEmpty()) {
throw new IllegalArgumentException("specified string is empty");
}
String old = stringArray[index];
stringArray[index] = s;
return old;
}
// TODO: Check if posible to extend the stringArray
#Override
public boolean add(String s) {
if (s == null) {
throw new NullPointerException("the specified string is null");
} else if (s.isEmpty()) {
throw new IllegalArgumentException("specified string is empty");
}
if (size == stringArray.length) {
int newListCapacity = stringArray.length * 2;
stringArray = Arrays.copyOf(stringArray, newListCapacity);
}
stringArray[++size] = s;
return true;
}
// TODO: implement other methods ...
}
Keep in mind that this implementation is still buggy, but you can use it as a starting point

public void ArrayStringList(StringList sl) {
size = sl.size();
ArrayStringList asl = new ArrayStringList();
for(int i = 0; i < size; i++) {
if(sl.get(i) != null) {
String s = asl.set(i,sl.get(i).toString());
System.out.println(s);
} //if
} // for
}
Change set method like below. And call it by the help of class object. it will set value in global static list.
//Change set method like this
public String set(int index, String s) {
stringArray[index] = s;
return stringArray[index];
}

I would initialise the internal array to the value of size and also make use of the fact that the String class also has a copy-constructor
public ArrayStringList(StringList sl) {
this.size = sl.size();
this.stringArray = new String[size];
for(int j = 0; j < size; j++) {
this.stringArray[j] = new String(sl.get(i));
}
}

Is it not possible to have an array based Queue with generics ? [duplicate]

This question already has answers here:
How to create a generic array in Java?
(32 answers)
Closed 7 years ago.
This is an array based Queue , for int :
/**
* Array based
* #author X220
*
*/
public class MyQueue {
private int[] _data;
private int MAX_SIZE;
private int front = -1;
private int back = 0;
private int elementsCount = 0;
public void printQueue()
{
int j = this.front + 1;
int i = 0;
while (i < this._data.length && i < elementsCount)
{
System.out.println("At location " + j % MAX_SIZE + " element :" + this._data[j % MAX_SIZE]);
j++;
i++;
}
}
public MyQueue(int _size)
{
MAX_SIZE = _size > 0 ? _size : 10;
this._data = new int[MAX_SIZE];
}
public boolean IsEmpty()
{
return this.elementsCount == 0;
}
public boolean IsFull()
{
return this.elementsCount == MAX_SIZE;
}
public void Push(int pushMe) throws QueueIsFullException
{
if (IsFull())
{
throw new QueueIsFullException("Queue is full");
}
this.elementsCount++;
_data[back++ % MAX_SIZE] = pushMe;
}
public int Pop() throws QueueIsEmptyException
{
if (IsEmpty())
{
throw new QueueIsEmptyException("Queue is full");
}
elementsCount--;
return _data[++front % MAX_SIZE];
}
public static void main(String args[])
{
try
{
MyQueue q1 = new MyQueue(15);
q1.Push(1);
q1.Push(2);
q1.Push(3);
q1.Push(4);
q1.Push(5);
q1.Pop();
q1.Pop();
q1.Pop();
q1.Pop();
q1.Pop();
q1.Push(6);
q1.Pop();
q1.Push(7);
q1.Push(8);
q1.Push(9);
q1.Push(10);
q1.Push(11);
q1.Push(12);
// q1.Push(1);
// q1.Push(2);
// q1.Push(3);
// q1.Push(4);
// q1.Push(5);
// q1.Push(7);
// q1.Push(8);
// q1.Push(9);
// q1.Push(10);
// q1.Push(11);
// q1.Push(12);
// q1.Push(40);
// q1.Push(50);
q1.printQueue();
}
catch (Exception e)
{
System.out.println(e);
}
}
#SuppressWarnings("serial")
class QueueIsFullException extends Exception
{
public QueueIsFullException(String message){
super(message);
}
}
#SuppressWarnings("serial")
class QueueIsEmptyException extends Exception
{
public QueueIsEmptyException(String message){
super(message);
}
}
}
I wanted to use generics so I changed the int to T but then I got for this :
public class MyQueue <T>{
private T[] _data;
private int MAX_SIZE;
private int front = -1;
private int back = 0;
private int elementsCount = 0;
public void printQueue()
{
int j = this.front + 1;
int i = 0;
while (i < this._data.length && i < elementsCount)
{
System.out.println("At location " + j % MAX_SIZE + " element :" + this._data[j % MAX_SIZE]);
j++;
i++;
}
}
public MyQueue(int _size)
{
MAX_SIZE = _size > 0 ? _size : 10;
this._data = new T[MAX_SIZE];
}
....
}
That :
Cannot create a generic array of T
And from the answers to this post I see that I can't use generics with arrays .
Does this mean that there is no work around for a generics Queue based on array ? Must I switch to some other data structure ?

The root cause of your problem is not with your MyQueue class, I think you misunderstand the way Java handles generics. Generic types exist only at compile time, after that they are simply erased from the byte code and at runtime only real Java types exist behind the scenes.
This is why you cannot instantiate a generic type, because at runtime this parameterized type simply doesn't exist.
What you can do is to provide a real class (extending T) as a parameter in your MyQueue class an instantiate this class type, since this is a first-class Java type.
Here is a very similar StackOverflow question and a solution:
Instantiating a generic class in Java
It is also recommended to read the Java reference about generics, like the answer for you original question is here:
https://docs.oracle.com/javase/tutorial/java/generics/restrictions.html#createObjects

No there is a work around for this the ugly cast, change your array generic creation to:
this._data = (T[])new Object[MAX_SIZE];
Due to the implementation of Java generics, you can't have code like this:
this._data = new T[MAX_SIZE];
Have a look at this How to create a generic array in Java?

The method I prefer is using
#SuppressWarnings("unchecked")
T[] arr = (T[]) Array.newInstance(clazz,length);
where clazz is the Class<T> object corresponding to the generic type. Note that the cast is unchecked, but Array.newInstance ensures you that you won't be able to insert invalid types into your array.
To me this is the best solution because :
you handle the type consistency to the Array class by passing a Class<T> instance which will be used to cast all the objects inserted in the array. This is thus type-safe without needing you to do anything.
this is relatively small and self-contained, it won't force you to manually cast objects over and over everytime you use the array. This would be the case if you were using an Object[] under the hood.

ClassCastException when casting Object[] array to generic type array in Java

Hi I'm very new to Java and in this code, I think I'm not creating the Bag correctly in the Main? Please help thanks!
Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;
at mid.Bag.(Bag.java:12)
at mid.Bag.main(Bag.java:91)
public class Bag<T extends Comparable<T>> implements Iterable<T> {
private int MAX_ITEMS = 10; // initial array size
private int size;
private T[] data;
public Bag( ) {
data = (T []) new Object[MAX_ITEMS];
size = 0;
}
public void add(T newItem) {
// check if it's full, then extend (array resizing)
if (size == data.length) {
T[ ] temp = (T [ ] ) new Object[data.length*2];
for (int i = 0; i < size; ++i)
temp[i] = data[i];
// reassign data to point to temp
data = temp;
}
// then do the assignment
data[size++] = newItem; // assign newItem in the next-available slot
}
public Iterator<T> iterator() {
return new BagIterator();
}
/***************************
* nested class BagIterator
***************************/
class BagIterator implements Iterator<T> {
// instance member
private int index;
// (0) constructor
public BagIterator() {
index = 0;
}
// (1)
public boolean hasNext() {
return (index < size); // size in the outer Bag<E>
}
// (2)
public T next() {
/*
T temp = data[index]; // save the element value
index++; // increment index
return temp;
*/
return data[index++];
}
public static void main(String[ ] args) {
Bag<String> bag1=new Bag<String>();
bag1.add("good");
bag1.add("fortune");
bag1.add("billionarie");
for (String x: bag1)
System.out.println(x);
}

Yes, you're creating an Object[] and then trying to cast it to T[], which the compiler is converting to a cast to Comparable[] (using the raw Comparable type) due to your constraint on T.
Arrays and generics don't work terribly nicely together, basically.
It would probably be simpler to make your data field just an Object[] and cast individual values where necessary.

Here:
data = (T []) new Object[MAX_ITEMS];
you are constructing an Object array and trying to cast it to T[]. But you have declared that T inherits from Comparable. So use:
data = (T []) new Comparable[MAX_ITEMS];

You can probably rewrite your constructor as well:
public Bag(Class<T> c, int s) {
// Use Array native method to create array of a type only known at run time
#SuppressWarnings("unchecked")
final T[] dataArray = (T[]) Array.newInstance(c, s);
this.data = dataArray;
}
Then you can use it like:
Bag<String> bag1 = new Bag<>(String.class,10);
That should also work, IMO. The instances of T must be comparable in any case.

java null pointer exception with static array

I got a null pointer exception when accessing a static array from a static member method.
The exception is thrown when i call setData(x, y, z) from a thread. When I debugged it I found out data[0] is null when i try to write to it. I just don't understand how it can be null
public class dataContainer
{
private static final short nrOfDataElements = ids.total_ids;
private static regularDataElement[] data = new regularDataElement[nrOfDataElements];
public static synchronized void getData(final short i, regularDataElement r)
{
if ( (i >= 0) && (i < nrOfDataElements) )
r.set(data[i].getTimestamp(), data[i].getValue());
}
public static synchronized void setData(short i, double ts, long val)
{
if ( (i >= 0) && (i < nrOfDataElements) )
data[i].set(ts, val); //<<-------null pointer exception, debugging showed data[i] == null, (with i = 0 and nrOfDataElements = 12)
}
}
and
public class regularDataElement
{
regularDataElement()
{
set(0, 0);
}
public void set(double _ts, long _val)
{
System.out.println(this.ts + " " + _ts + " " + this.val + " " + _val); System.out.flush();
this.ts = _ts;
this.val = _val;
}
public double getTimestamp()
{
return this.ts;
}
public long getValue()
{
return this.val;
}
private double ts;
private long val;
}

The statement private static regularDataElement[] data = new regularDataElement[nrOfDataElements]; initializes data with an array the size of nrOfDataElements. It does not initialize each element in this array. I.e., all the elements are null.
If you wish to initialize the elements, you should do so yourself. E.g., you could add a static block to handle this:
static regularDataElement[] data = new regularDataElement[nrOfDataElements];
static {
for (int i = 0; i < nrOfDataElements; ++i) {
data[i] = new regularDataElement();
}
}

Did you ever initialize the data array?
private static regularDataElement[] data =
new regularDataElement[nrOfDataElements];
will create an array full of null objects of size nrOfDataElements. It won't actually initialize any elements in the array.

You don't appear to be allocating memory for data[i], which is why you're getting the NPE.
Allocating memory for the array itself is not enough. You need to allocate memory for each element:
for (int i = 0; i < nrOfDataElements; ++i) {
data[i] = new regularDataElement(...);
}
(Replace ... with the actual arguments.)

You never actually create any objects. You must add somewhere:
for (int i = 0; i < data.length; i++) {
data[i] = new regularDataElement();
}

When you initialize an Object array in Java, like data in your code, all elements are by default set to null.
So, you need to populate the data array first, before being able to call any methods against its elements.
Assuming that the regularDataElement class has a no-args (i.e., no parameters) constructor, you could do
static regularDataElement[] data = new regularDataElement[nrOfDataElements];
static
{
for (int i=0; i<nrOfDataElements; i++)
{
data[i] = new regularDataElement();
}
}
Of course, you could have a separate method to initialize the array, e.g.
static regularDataElement[] initialize(int nrOfDataElements)
{
regularDataElement[] elements = new regularDataElement[nrOfDataElements];
for (int i=0; i<nrOfDataElements; i++)
{
elements[i] = new regularDataElement();
}
return elements;
}
and then call that method to create and initialize the data array, replacing the statement
static regularDataElement[] data = new regularDataElement[nrOfDataElements];
with
static regularDataElement[] data = initialize(nrOfDataElements);
Also, as a matter of following established coding conventions, you should name your classes starting with a capital letter, i.e. use RegularDataElement instead of regularDataElement.