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storing the value of static variables in java

I have used a static variable in a Class and I want that the value of this variable is kept unchanged after restart of the jvm.
The idea is to store this value.
Can someone help me ?

If you want a variable whose value can change during the execution of the code, to be restored to whatever it was when you stopped the JVM, the next time you start the JVM, then you need to persist the value to an external medium. This is typically done by storing the value in a file or database, and retrieving it the next time the program starts. A key question when solving this problem is how to represent the value outside the executing program. For simple types like numbers and strings, this is not much of an issue. For values that are objects of non-trivial classes, the problem becomes more interesting. This is known as object serialization.
With the disclaimer that there are many different ways to persist data, here is a minimal example using Java Serialization to make the point more concrete.
class MyClassData implements Serializable {
private String myString = "A string";
private int myNumber = 5;
// To make the point that the state of the object stored in the
// variable can change at run-time.
public void changeThings( String myString, int myNumber) {
this.myString = myString;
this.myNumber = myNumber;
}
}
public class MyClass {
private static MyClassData data = restore();
// Call before exiting the program
private static void store() {
try( ObjectOutputStream out =
new ObjectOutputStream(new FileOutputStream("data.dat"))) {
out.writeObject(data);
}
catch( IOException e ) {
// Deal with this
}
}
private static MyClassData restore() {
try( ObjectInputStream in =
new ObjectInputStream(new FileInputStream("data.dat"))) {
return (MyClassData) in.readObject();
}
catch( IOException | ClassNotFoundException e ) {
return new MyClassData();
}
}
}

You restart the jvm every thing will be clear.So you can't get the values from static variables. If you use database then only you get the values without failing.

Loop through a LinkedList

I'm trying to get a LinkedList from my buffer class (BufferCharacter) and then loop through every element in the LinkedList in the Reader class. But when I try return the LinkedList in the get() method in BufferCharacter class I then cannot loop through it in the Reader class. I've tried to loop through the List in the Buffer class and then return each element from there but that doesn't work either.
Any help is highly appreciated!
public class CharacterBuffer {
private char ch;
private LinkedList buffer = new LinkedList();
private boolean filled;
public void put(char ch) {
buffer.addLast(ch);
}
public void filled() {
filled = true;
}
public Object get() throws InterruptedException {
while (buffer.isEmpty()) {
// wait();
return "Waiting";
}
return buffer;
}
public synchronized void putSync(char ch) {
buffer.addLast(ch);
}
public synchronized Object getSync() throws InterruptedException {
while (buffer.isEmpty()) {
// wait();
return "---------";
}
for(int i = 0; i<buffer.size(); i++){
System.out.println(buffer.get(i));
}
return buffer;
}
public int size(){
return buffer.size();
}
}
public class Reader extends Thread {
private GUIMutex gui;
private CharacterBuffer buffer;
private boolean isSynced;
public Reader(GUIMutex gui, CharacterBuffer buffer, boolean isSynced) {
this.gui = gui;
this.buffer = buffer;
this.isSynced = isSynced;
}
public void run() {
String data = "test";
while (true) {
try {
// data = buffer.get();
if (isSynced) {
gui.setReaderText(buffer.getSync() + "\n");
} else {
for(int i = 0; i<buffer.get().size(); i++){
gui.setReaderText(i);
}
gui.setReaderText(buffer.get() + "\n");
}
Thread.sleep(700);
} catch (InterruptedException e) {
}
}
}
}

I think you don't understand what you are talking about; so lets try to shed some light here.
In the end, you are talking about some kind of "collection" class that contains multiple elements; in your case a LinkedList. The thing is: in order to make use of such a class, you need a clear understanding of the APIs you intend to provide.
You figured that you want to use that buffer to store individual char values, that you add using putSync().
But then ... what exactly is getSync() supposed to do?
In your case, you are simply returning the buffer, and that is probably wrong.
Instead, you want to have methods like:
synchronized boolean hasNext()
and
synchronized char getNext()
A user of your class can call the first method to figure: are there other chars; and if so, the second method returns those values.
That would a first, simple way to improve your code. A more reasonable way would be that you implement a method getIterator() that would return an object implementing the Iterator interface.
Other things to note: if you are using the "built-in" LinkedList; please understand that this class supports generics!
Thus you should be using it like:
private final List<Character> buffer = new LinkedList<>();
to get all the benefits from using strongly typed collections!
EDIT: upon your comments, I think using a LinkedList is simply the wrong approach here.
Instead of using a List, you want to use a Queue, like:
private final Queue<Character> buffer = new ConcurrentLinkedQueue<>();
That class gives you that functionality that one party can add elements at the queue tail; whereas another party removes elements from the queue head.
Extra bonus: that class is doing the synchronisation work for you already, so you don't need to care about that!

Use StringBuilder instead
StringBuilder sb = new StringBuilder(128);
// add chars using sb.append(char)
for (int i = 0, n = sb.length(); i < n; i++)
{
char c = sb.charAt(i);
}
or
String s = sb.toString();

When I create a 2D array outside of my constructor, I'm not able to resize it, but when I create one inside of it, I'm not able to access it

I'm trying to create a method which will create an object which contains a 2D boolean array, with int parameters as the number of rows and columns. Then inside the class, I have methods that try to grab the length and width of that array. The two ways I tried to solve this problem were:
public GameOfLife(int rows, int cols) {
boolean[][] society = new boolean[rows][cols];
}
public int numberOfRows() {
return society.length;
}
In my tests, this attempt was giving me the error that society cannot be resolved to a variable. Then I tried:
private boolean[][] society;
public GameOfLife(int rows, int cols) {
boolean[][] temp = new boolean[rows][cols];
society = temp;
}
EDIT: Oops, forgot to add my method for numberOfColumns:
public int numberOfColumns() {
return cols;
}
But the issue with this one was that it was returning 0 instead of 4 when I tried:
#Test
public void FailedTestingRowsAndCols(){
GameOfLife g1 = new GameOfLife(4,4);
assertEquals(4, g1.numberOfColumns());
}
I'm rather new to this, so I apologize if this is a dumb question, but I'm not really sure about all the details of where and when variables expire, which is giving me a lot of difficulties. THank you for any help!

When I create a 2D array outside of my constructor, I'm not able to resize it, but when I create one inside of it, I'm not able to access it
Take note that you will never be able to resize an array. An array once created has its size fixed. You are merely assigning your current array to reference to another newly created array (which gives you the illusion that you successfully resized it).
As for your question of not being able to access it is highly likely the variable you created exist within different scope.
You can use the following codes (which is very similar to yours), it works fine for me. Hence I am guessing your error does not actually comes from the code snippet you showed.
class TestRunner
{
public static void main(String[] args){
GameOfLife gol = new GameOfLife(5, 3);
System.out.println(gol.getColumns());
System.out.println(gol.getRows());
}
}
class GameOfLife
{
private boolean[][] society;
public GameOfLife(int rows, int cols){
society = new boolean[rows][cols];
}
public int getColumns(){
return society[0].length;
}
public int getRows(){
return society.length;
}
}
Output:
5
3

I don't see any problems with what you have posted so far. The below example works fine for me:
public class GameOfLife {
public static void main(String[] args) {
GameOfLife g1 = new GameOfLife(4,4);
System.out.println(g1);
}
private boolean[][] society;
#Override
public String toString() {
final StringBuffer sb = new StringBuffer("GameOfLife{");
sb.append("society=").append(society == null ? "null" : Arrays.deepToString(society));
sb.append('}');
return sb.toString();
}
public GameOfLife(int rows, int cols) {
boolean[][] temp = new boolean[rows][cols];
society = temp;
}
}

Calculate size of Object in Java [duplicate]

This question already has answers here:
How to determine the size of an object in Java
(28 answers)
Closed 9 years ago.
I want to record how much memory (in bytes, hopefully) an object takes up for a project (I'm comparing sizes of data structures) and it seems like there is no method to do this in Java. Supposedly, C/C++ has sizeOf() method, but this is nonexistant in Java. I tried recording the free memory in the JVM with Runtime.getRuntime().freeMemory() before and after creating the object and then recording the difference, but it would only give 0 or 131304, and nothing in between, regardless of the number of elements in the structure. Help please!

You can use the java.lang.instrumentation package.
It has a method that can be used to get the implementation specific approximation of object size, as well as overhead associated with the object.
The answer that Sergey linked has a great example, which I'll repost here, but you should have already looked at from his comment:
import java.lang.instrument.Instrumentation;
public class ObjectSizeFetcher {
private static Instrumentation instrumentation;
public static void premain(String args, Instrumentation inst) {
instrumentation = inst;
}
public static long getObjectSize(Object o) {
return instrumentation.getObjectSize(o);
}
}
Use getObjectSize:
public class C {
private int x;
private int y;
public static void main(String [] args) {
System.out.println(ObjectSizeFetcher.getObjectSize(new C()));
}
}
Source

Look into https://github.com/DimitrisAndreou/memory-measurer.
Guava uses it internally, and ObjectGraphMeasurer is especially straightforward to use out-of-the-box, without any special command-line arguments.
import objectexplorer.ObjectGraphMeasurer;
public class Measurer {
public static void main(String[] args) {
Set<Integer> hashset = new HashSet<Integer>();
Random random = new Random();
int n = 10000;
for (int i = 1; i <= n; i++) {
hashset.add(random.nextInt());
}
System.out.println(ObjectGraphMeasurer.measure(hashset));
}
}

The java.lang.instrument.Instrumentation class provides a nice way to get the size of a Java Object, but it requires you to define a premain and run your program with a java agent. This is very boring when you do not need any agent and then you have to provide a dummy Jar agent to your application.
So I got an alternative solution using the Unsafe class from the sun.misc. So, considering the objects heap alignment according to the processor architecture and calculating the maximum field offset, you can measure the size of a Java Object. In the example below I use an auxiliary class UtilUnsafe to get a reference to the sun.misc.Unsafe object.
private static final int NR_BITS = Integer.valueOf(System.getProperty("sun.arch.data.model"));
private static final int BYTE = 8;
private static final int WORD = NR_BITS/BYTE;
private static final int MIN_SIZE = 16;
public static int sizeOf(Class src){
//
// Get the instance fields of src class
//
List<Field> instanceFields = new LinkedList<Field>();
do{
if(src == Object.class) return MIN_SIZE;
for (Field f : src.getDeclaredFields()) {
if((f.getModifiers() & Modifier.STATIC) == 0){
instanceFields.add(f);
}
}
src = src.getSuperclass();
}while(instanceFields.isEmpty());
//
// Get the field with the maximum offset
//
long maxOffset = 0;
for (Field f : instanceFields) {
long offset = UtilUnsafe.UNSAFE.objectFieldOffset(f);
if(offset > maxOffset) maxOffset = offset;
}
return (((int)maxOffset/WORD) + 1)*WORD;
}
class UtilUnsafe {
public static final sun.misc.Unsafe UNSAFE;
static {
Object theUnsafe = null;
Exception exception = null;
try {
Class<?> uc = Class.forName("sun.misc.Unsafe");
Field f = uc.getDeclaredField("theUnsafe");
f.setAccessible(true);
theUnsafe = f.get(uc);
} catch (Exception e) { exception = e; }
UNSAFE = (sun.misc.Unsafe) theUnsafe;
if (UNSAFE == null) throw new Error("Could not obtain access to sun.misc.Unsafe", exception);
}
private UtilUnsafe() { }
}

Javolution ByteBuffer question

I have the following implementation with Javolution:
public class RunScan extends Struct
{
public final Signed32 numOfClusters = new Signed32();
public final ClusterData[] clusters;
public final Signed32 numOfRecons = new Signed32();
public final ReconData[] recons ;
public RunScan (int numOfClusters, int numOfRecons)
{
this.numOfClusters.set(numOfClusters);
this.numOfRecons.set(numOfRecons);
clusters = array(new ClusterData[numOfClusters]);
recons = array(new ReconData[numOfRecons]);
}
}
public class ClusterData extends Struct
{
public final UTF8String scanType = new UTF8String(CommInterfaceFieldConstants.SCAN_TYPE_SIZE);
public final UTF8String patientId = new UTF8String(CommInterfaceFieldConstants.PATIENT_ID_SIZE);
.
.
.
}
public class ReconData extends Struct
{
public final UTF8String patientId = new UTF8String(CommInterfaceFieldConstants.PATIENT_ID_SIZE);
public final UTF8String scanSeriesId = new UTF8String(CommInterfaceFieldConstants.SCAN_SERIES_ID_SIZE);
.
.
.
}
In our communication class, before we put data onto socket, we need to get the bytes[] of the RunScan object but we get BufferUnderflowException in the line with "//<<<<<<<":
private byte[] getCmdBytes(Struct scCmd)
{
ByteBuffer cmdBuffer = scCmd.getByteBuffer();
int cmdSize = scCmd.size();
byte[] cmdBytes = new byte[cmdSize];
if (cmdBuffer.hasArray())
{
int offset = cmdBuffer.arrayOffset() + scCmd.getByteBufferPosition();
System.arraycopy(cmdBuffer.array(), offset, cmdBytes, 0, cmdSize);
}
else
{
String msg = "\n\ncmdBufferRemaining=" + cmdBuffer.remaining() + ", cmdBytesSize=" + cmdBytes.length + "\n\n";
System.out.println(msg);
cmdBuffer.position(scCmd.getByteBufferPosition());
cmdBuffer.get(cmdBytes); //<<<<<<<<<< underFlowException
}
return cmdBytes;
}
This method works in other cases. The exception happens because this line,
ByteBuffer cmdBuffer = scCmd.getByteBuffer();
only returns a 8 bytes (from the remaining() method) ByteBuffer of the RunScan object which are those two Signed32 fields, I think. But this line,
int cmdSize = scCmd.size();
returns a right length of the RunScan object which includes the size of those two arrays.
If I create those two array at the time I declare them (not "new" them in the constructor) with hard coded length, it works fine without any exception.
Anybody can help me figure out what's wrong with our implementation?

I ran into a similar situation with my code. Generally, with the current Struct object, you cannot have a variable length array defined in the same struct as the member that contains the number of elements in the array.
Try something like this:
public class RunScanHeader extends Struct
{
public final Signed32 numOfClusters = new Signed32();
public final Signed32 numOfRecons = new Signed32();
}
public class RunScanBody extends Struct
{
public final ClusterData[] clusters;
public final ReconData[] recons ;
public RunScan (int numOfClusters, int numOfRecons)
{
clusters = array(new ClusterData[numOfClusters]);
recons = array(new ReconData[numOfRecons]);
}
}
You'll then need a two phase approach to read and write, first read/write the header data, then read/write the body data.
Sorry I don't have more details at this time, if you can't solve this, let me know and I'll dig back through my code.

The initialization order is important has it defines the position of each field. Either your initialization is done when the field is declared (most common case). Or if you do it in the constructor you have to remember that the constructor is called after the member initialization. Here is an example with initialization done in the constructor:
public class RunScan extends Struct {
public final Signed32 numOfClusters;
public final ClusterData[] clusters;
public final Signed32 numOfRecons;
public final ReconData[] recons ;
public RunScan (int numOfClusters, int numOfRecons) {
// Initialization done in the constructor for all members
// Order is important, it should match the declarative order to ensure proper positioning.
this.numOfClusters = new Signed32();
this.clusters = array(new ClusterData[numOfClusters]);
this.numOfRecons = new Signed32();
this.recons = array(new ReconData[numOfRecons]);
// Only after all the members have been initialized the set method can be used.
this.numOfClusters.set(numOfClusters);
this.numOfRecons.set(numOfRecons);
}
}

get() will move the position of the ByteBuffer.
scCmd.getByteBuffer().slice().get(dest) might solve your issue with moving the position and unintended side effects.
scCmd.getByteBuffer().duplicate().get(dest) might also solve your issue if slice() produces the wrong picture of the origin buffer.
Additionally, it appears as though scCmd.getByteBuffer() creates a redundant reference and you are calling the source and child reference in the same method.
If scCmd.getByteBuffer() is already passing you a slice(), your redundant access to these methods is certainly going to do something other than what you planned.