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How to generate random ID numbers that can't be reused in java?

For my current java project, I am trying to generate random ID's for registered users. So far I have been using min +(int) (Math.random()*((max-min)+1)) as my formula to generate the random number. The problem that I am facing is that sometimes the numbers repeat themselves and my application wouldn't work with them.
int min = 1001;
int max = 1050;
for (int i=1; i<=1; i++)
{
int a = min +(int) (Math.random()*((max-min)+1));
}
I have tried using and incorporating
Integer[] arr = new Integer[100];
for (int i = 1; i < arr.length; i++) {
arr[i] = i;
}
Collections.shuffle(Arrays.asList(arr));
but numbers generated would constantly come out as "null" and it would repeat the loop a few hundred times and flood my txt file.

In general, random generators Random or Math.random() are not the correct ways to generate a unique id. As you mentioned, it could be repeated (and it will definitely be).
I would recommend two ways of generating ID.
The first one is to use AtomicInteger. This is good when your ID should be unique but not random.
private static final AtomicInteger ID = new AtomicInteger(0);
public static String generateUniqueId() {
return String.valueOf(ID.incrementAndGet());
}
The second one, which is preferable to me, is to use UUID. This is good when your ID should be as unique as random.
public static String generateUniqueId() {
return String.valueOf(UUID.randomUUID());
}
Another one, I can mention is to use System.nanoTime().
public static String generateUniqueId() {
return String.valueOf(System.nanoTime());
}
Long ago I had some investigation and find out that this is pretty stable for normal payload. But in general, it could retrieve the same value if you build such a system, that should generate ID so often.

Instead of generating numbers I would recommend to generate UUID. The chance of a is collision is close to impossible.
UUID id = UUID.randomUUID();
Otherwise if you want to stick with numbers I would recommend you to implement yourself some Sequence service within your application.
import java.util.concurrent.atomic.AtomicLong;
public class SequenceService {
private final AtomicLong ids;
public SequenceService() {
long initialValue = getInitialValue();
this.ids = new AtomicLong(initialValue);
}
public long generateNextId() {
return ids.incrementAndGet();
}
private long getInitialValue() {
// this methods reads the last known leased id (e.g. from the file system)
}
}

What's the fastest way to initialize a large list of integers?

I need to pre-populate a List with a large number of integer values.
Is there are faster way to do this other than iteration?
Current Code:
class VlanManager {
Queue<Integer> queue = Lists.newLinkedList();
public VlanManager(){
for (int i = 1; i < 4094; i++) {
queue.add(i);
}
}
This code is in the constructor of a class that is created pretty frequently so I'd like this to be as efficient (read:performance not lines of code) as possible

4094 isnt to many items to loop but if it is getting called very frequently you might look at doing something with a static variable.
private static Integer[] theList;
static {
theList = new Integer[4094];
for (int i = 1; i < 4094; i++) {
theList[i-1] = i;
}
}
then make that list a List
Queue<Integer> intQue = new LinkedList(Arrays.asList(theList));
There is a danger of using this method if you have a list of mutable objects. Heres an example of what can happen. Integers are immutable so this doesnt actually apply to your question as it stands
class MyMutableObject {
public int theValue;
}
class Test {
private static MyMutableObject[] theList;
static {
theList = new MyMutableObject[4094];
for (int i = 1; i <= 4094; i++) {
theList[i-1] = new MyMutableObject();
theList[i-1].theValue = i;
}
}
public static void main(String [] args) {
Queue<MyMutableObject> que = new LinkedList(Arrays.asList(theList));
System.out.println(que.peek().theValue); // 1
// your actually modifing the same object as the one in your static list
que.peek().theValue = -100;
Queue<MyMutableObject> que2 = new LinkedList(Arrays.asList(theList));
System.out.println(que2.peek().theValue); // -100
}
}
#Bohemian Has some good points on using a static List instead of an array, while the performance gains are very small they are none the less performance gains. Also because the 'array' is actually only ever being used as a List not an array it should be declared as such.
private static List<Integer> theList;
static {
theList = new ArrayList(4094);
for (Integer i = 0; i < 4094; i++) {
theList.add(i+1);
}
}

The fastest way would be to create a reference list (initialized using an instance block - neatly wrapping it all up in one statement):
private static final List<Integer> LIST = new ArrayList<Integer>(4094) {{
for (int i = 1; i < 4094; i++)
LIST.add(i);
}};
Then in your constructor, initialize the queue using the copy constructor:
Queue<Integer> queue;
public VlanManager(){
queue = new LinkedList<Integer>(LIST);
}
You will not write a faster implementation than what's in the JDK.

I realize this question has already been answered. But I think one important answer is missing: The fastest way to initialize a LinkedList with the values 0..4093 is .. DON'T DO IT AT ALL. Especially if speed is an issue.
What you basically are doing is creating a structure consisting of 4093 Node elements each consiting of two pointers to prev/next element and one pointer to an Integer object. Each of this Nodes must be created (and free). In addition nearly each contained Integer must be created (and freed). 'Nearly' because Java uses a cache for Integer but normally (you can change this with system properties) in the range of -127..127.
This is a lot to do in order to get a simple list of integer and if used intensively gives the GC a lot to do afterwards.
That being said there are numerous possible ways of doing this in a more efficient way. But they depend on what your concrete usage pattern is. Just to name a few:
Use an Array: boolean [] inUse' and set the taken vlan-id totrue` if it's taken
Even better use a BitSet instead of the array
Don't store which vlan is free, but which vlan is taken. I think they tend to be free and so there are much more free as there are taken ones. (this means much less to keep track of).
If you insist on using a LinkedList don't initialize it with your class but have it already initialized. This depends on how much of them you would need. You could keep a pool of them. Or perhaps your codes allows reusage of old lists. (yes, you could sort them after usage.)
Surely there are more...
All of this methods require you to build your own 'Queue' interface. But perhaps this has not to be as rich as Java's. And it really isn't that difficult. If you really use this intensively you could reach perfomance improvement factor 10x-1000x++.
A possible implementation using BitSet with an instantiation cost of nearly nothing could be:
import java.util.BitSet;
import org.testng.annotations.Test;
public class BitSetQueue {
// Represents the values 0..size-1
private final BitSet bitset;
private final int size;
private int current = 0;
private int taken = 0;
public BitSetQueue( int size ){
this.bitset = new BitSet( size );
this.size = size;
this.current = size-1;
}
public int poll(){
// prevent endless loop
if( taken == size ) return -1;
// seek for next free value.
// can be changed according to policy
while( true ){
current = (current+1)%size;
if( ! bitset.get( current ) ){
bitset.set( current );
taken++;
return current;
}
}
}
public boolean free( int num ){
if( bitset.get( num ) ){
bitset.clear( num );
taken--;
return true;
}
return false;
}
#Test
public static void usage(){
BitSetQueue q = new BitSetQueue( 4094 );
for( int i = 0; i < 4094; i++ ){
assertEquals( q.poll(), i );
}
assertEquals( q.poll(), -1 ); // No more available
assertTrue( q.free( 20 ) );
assertTrue( q.free( 51 ) );
assertEquals( q.poll(), 20 );
assertEquals( q.poll(), 51 );
}
}

Find and print the product of all the entries in array

Okay I have tried to write a simple Java code in BlueJ, that finds and prints the product of all the entries in data such as if data is {1,2,3,4} then the result will be 24.
And my code is below:
public class Product {
public static int[] product(int[] a) {
int [] s = new int[a.length];
for (int i =0; i< a.length; i++)
s[i] = a[i]*a[i];
return s; //the definition of your method...
}
public static void main(String[] args) {
//calling the method to seek if compiles
int[] results = Product.product(new int[] { 1,2,3,4 });
//printing the results
System.out.println(java.util.Arrays.toString(results));
}
}
The above code is giving me the square of each number, which is not what I want to have, somehow I have modify the code that the result will be 24 but I couldn't figure it out, anyone knows how to do it?

First of all, if you are first writing Java it is important to know that variable, function and class names are quite important. Please note that having Product.product() is not a good idea, since the function name is almost the same as the class name. Anyway, regarding your code. Your code is indeed returning the square of your input, what you would want is the following:
public class Product {
public static int getProduct(int[] input) {
int total = 1;
for (int v : input) {
total *= v;
}
return total;
}
}
This will return an integer value with the product of your input array. This also uses a for-each loop instead of a regular for-loop for readability. Also you don't need the index in this case. Good luck with it!

First, your product method needs to return an int rather than an int [].
You need to maintain the product as a variable. You can set it to 1 initially, and then multiply it by each element of the a array in turn; then you just return this value.

How can I reset all object values with only one method call?

Basically, I want to create Counter objects, all they have to do is hold number values. And in my resetCounters method, I would like to reset each object's values. This is probably very easy, but I'm a newb.
public class Counter
{
Random number = new Random();
Counter()
{
Random number = new Random();
}
public Random getNumber()
{
return number;
}
public void setNumber(Random number)
{
this.number = number;
}
public static void main(String[] args)
{
Counter counter1 = new Counter();
Counter counter2 = new Counter();
Counter counter3 = new Counter();
Counter counter4 = new Counter();
Counter counter5 = new Counter();
}
public static void resetCounters()
{
}
}

First option: Memorize each instance of Counter.
Collect each instance of Counter in some static collection.
To reset all, simply iterate over all items in the collection.
But strong references are too strong for this -- make sure it's a collection of weak references.
Remarks:
Using weak references will avoid the issue that the Counter objects exist indefinitely only because of their reference from within the static collection. Objects that are referred to only by weak references are eventually collected by the garbage collector.
The collection of every instance can be achieved by declaring the Counter constructor private and allowing only construction through a static member function which will also do the registration. (Or use some other incarnation of the Factory pattern.) I believe a factory is the way to go here, since each construction of an object has to carry out also a side effect. But perhaps it will make do to have the Counter constructor register this with the static collection.
Second option: Generation counter
Keep a static generation counter of type long, and also a copy of this counter in each instance. When resetting all counters, just increase the static generation counter. The getNumber() method will then check the static generation counter against its own copy and reset the counter if the static generation counter has changed.
(I don't really know the "official" name for this trick. How to zero out array in O(1)?)

Since we have no idea what the context is, we can't narrow down the specific thing you should do is, but the options that occur to me immediately are...
1: If the counters have distinct meanings beyond "counter1, counter2, counter3," then they could be static class variables (with more useful names).
public class Counter {
static Counter counter1 = new Counter();
...
public void resetCounters() {
counter1.clear();
counter2.clear();
...
}
}
2: If you just want several distinct counters, and they have no particular meaning by themselves, and you know that there will only ever be five of them, then you should use an array:
public class Counter {
public static void main(String[] args) {
Counter[] counters = {new Counter(), new Counter(), new Counter(), new Counter(), new Counter()};
...
}
static void resetCounters(Counter[] counters) {
for (Counter c : counters) {
c.reset();
}
}
}
Or, if you're planning to have an arbitrary number of them, you might try one of the fancier factory patterns. It really depends on what the context is, what you're actually trying to do, and what the point of the exercise is.

Since you're working with a large number of objects, you would be well served placing them in some sort of collection, like an ArrayList.
List<Counter> counters = new ArrayList<Counter>();
Insert all of your counters into there using the .add() method. Then, you can author your resetCounters() method in this manner:
public static void resetCounters(List<Counter> counters) {
for(Counter c: counters) {
// perform some action to reset the counters, as described by you
}
}

1. First of all there is not need to Initialize a Random nos as an instance variable,just have a Object Reference Varaible, as you are already initializing it in the Constructor.
eg:
Random number;
2. Create an ArrayList and store all the Counter objects.
ArrayList<Counter> arr = new ArrayList<Counter>();
3. Add each counter object in to the ArrayList.
4. Make reset function Non-static..there is no need for it to be static.
5. Iterate and reset...
for (Counter c : arr){
c.reset();
}
6. In reset() do the following..
public void reset(){
this.number = 0;
}

The easiest and elegant way of achieving what you want is keeping a reference to all created objects somewhere, in a factory for example and resetting them when needed.
public class CounterFactory{
private List<Counter> counters = new ArrayList<Counter>();
public Counter createCounter(){
Counter c = new Counter();
counters.add(c);
return c;
}
public void resetCounters(){
for(Counter c : counters) c.setNumber(new Random());
}
}
And in the main method, use it this way:
public static void main(String[] args)
{
CounterFactory f = new CounterFactory();
Counter counter1 = f.createCounter();
Counter counter2 = f.createCounter();
Counter counter3 = f.createCounter();
Counter counter4 = f.createCounter();
Counter counter5 = f.createCounter();
// Reset all counters
f.resetCounters();
}

Having trouble understanding how to maintain state using classes

I'm new to using OOP, I typically just put all my code in a single class and use methods. But I want to maintain state information and think classes are the best fit but I'm having trouble wrapping my head around it.
Say I have a list of items and I want to stop when the total sum of all previous items in the list equals X(in this case 10 so it takes item 1 + 2, then 2+3.etc..until it hits the threshold 10), I can use a method to calculate it but it involves me doing the entire process all over again when all I really need to do is increment by the last item and then see if my data exceeds the threshold. Here's my code so far but I know its not good because although it works its really just using the class as an independent method and recalculating on every loop. My goal is to,using this structure, reduce loops if not necessary to check thresholds.
Any suggestions?
Code:
public class LearningClassesCounter {
public static void main(String[] args) {
int[] list = new int[]{1,2,3,4,5,6,7,8,9,10};
int[] data_list = new int[list.length];
for (int current_location = 0; current_location<list.length;current_location++) {
//can only put commands in here. Nothing above.
Counter checker = new Counter(data_list);
System.out.println(checker.check_data(current_location));
for (int i =0; i<100; i++){
if (checker.check_data(current_location) == false) {
break;
}
data_list[current_location] = (list[current_location]+1); //this is just a random function, it could be any math function I just put it in here to show that some work is being done.
}
}
//its done now lets print the results
for (Integer item : data_list) {
System.out.println(item);
}
}
}
class Counter {
private int[] data_list;
private int total_so_far;
// create a new counter with the given parameters
public Counter(int[] data_list) {
this.data_list = data_list;
this.total_so_far = 0;
}
public boolean check_data(int current_location) {
// TODO Auto-generated method stub
int total_so_far = 0;
//System.out.println(total_so_far);
for (int item : data_list) {
total_so_far = item + total_so_far;
if (total_so_far >= 10) {
break;
}
}
if (total_so_far>=10) {
return false;
} else {
return true;
}
}
}
I don't need anyone to fix my code or anything(I want to do it myself, the code is just to give an idea of what I'm doing). I'm more interested in the flaw in my logic and maybe a way for me to better think about designing classes so I can apply them to my own situations better.

So the solution is that you do not update the data_list directly. Instead have a setter method in the Counter class that takes the index and value to update. It updates the value in the array and also updates a count value.
Something like this:
class Counter{
private final int[] list;
private count = 0;
private final maxCount = 10;
public Counter(int[] list){
this.list = list;
}
public boolean updateValueAndCheckPastMax(int index, int value){
list[index] = value;
count += value;
return count >= maxCount;
}
}

You are way over thinking this, and a counter class is not really necessary in this case.
I'm also interested as to why you'd be doing this line:
data_list[current_location] = (list[current_location]+1);
Do you want your data_list to be the same as list, but each value is incremented by 1?
If you are merely trying to return a sub-array of the values that are < 10, i would suggest just doing this in a for loop, and using an int as a counter.