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What is JavaScript’s highest integer value that a number can go to without losing precision?

Is this defined by the language? Is there a defined maximum? Is it different in different browsers?

Answer

JavaScript has two number types: Number and BigInt.

The most frequently-used number type, Number, is a 64-bit floating point IEEE 754 number.

The largest exact integral value of this type is Number.MAX_SAFE_INTEGER, which is:

  • 253-1, or
  • +/- 9,007,199,254,740,991, or
  • nine quadrillion seven trillion one hundred ninety-nine billion two hundred fifty-four million seven hundred forty thousand nine hundred ninety-one

To put this in perspective: one quadrillion bytes is a petabyte (or one thousand terabytes).

“Safe” in this context refers to the ability to represent integers exactly and to correctly compare them.

From the spec:

Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in the Number type (indeed, the integer 0 has two representations, +0 and -0).

To safely use integers larger than this, you need to use BigInt, which has no upper bound.

Note that the bitwise operators and shift operators operate on 32-bit integers, so in that case, the max safe integer is 231-1, or 2,147,483,647.

const log = console.log
var x = 9007199254740992
var y = -x
log(x == x + 1) // true !
log(y == y - 1) // also true !

// Arithmetic operators work, but bitwise/shifts only operate on int32:
log(x / 2)      // 4503599627370496
log(x >> 1)     // 0
log(x | 1)      // 1

Technical note on the subject of the number 9,007,199,254,740,992: There is an exact IEEE-754 representation of this value, and you can assign and read this value from a variable, so for very carefully chosen applications in the domain of integers less than or equal to this value, you could treat this as a maximum value.

In the general case, you must treat this IEEE-754 value as inexact, because it is ambiguous whether it is encoding the logical value 9,007,199,254,740,992 or 9,007,199,254,740,993.

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discuss

Native Swing Menu Bar Support For MacOS X In Java

A link that stands out is http://www.devdaily.com/blog/post/jfc-swing/handling-main-mac-menu-in-swing-application/ however the menu bar under Mac OS X displays as the package name as opposed to the application name. I’m using the code in the above link without any luck, so I’m unsure if anything’s changed in recent Mac OS versions.

Here’s an extract:

public RootGUI() {
    super("Hello");
    JMenuBar menuBar = new JMenuBar();
    JMenu file = new JMenu("File");
    JMenuItem item = new JMenuItem("Woah");
    file.add(item);
    menuBar.add(file);
    setJMenuBar(menuBar);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(100, 100);
    pack();
    setVisible(true);
}
public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            try {
                System.setProperty("apple.laf.useScreenMenuBar", "true");
                System.setProperty("com.apple.mrj.application.apple.menu.about.name", "Test");
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                new RootGUI();
            }
            catch(ClassNotFoundException e) {
                System.out.println("ClassNotFoundException: " + e.getMessage());
            }
            catch(InstantiationException e) {
                System.out.println("InstantiationException: " + e.getMessage());
            }
            catch(IllegalAccessException e) {
                System.out.println("IllegalAccessException: " + e.getMessage());
            }
            catch(UnsupportedLookAndFeelException e) {
                System.out.println("UnsupportedLookAndFeelException: " + e.getMessage());
            }

        }
    });
}

The first menu item on the menu bar should display as “test”, unfortunately this isn’t the case. The file menu works fine, on the other hand. Any ideas?

Answer

@Kezzer

I think I see what’s going on. If you put the main() method in a different class, then everything works. So you need something like:

public class RootGUILauncher {
  public static void main(String[] args) {
    try {
                System.setProperty("apple.laf.useScreenMenuBar", "true");
                System.setProperty("com.apple.mrj.application.apple.menu.about.name", "Test");
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        }
        catch(ClassNotFoundException e) {
                System.out.println("ClassNotFoundException: " + e.getMessage());
        }
        catch(InstantiationException e) {
                System.out.println("InstantiationException: " + e.getMessage());
        }
        catch(IllegalAccessException e) {
                System.out.println("IllegalAccessException: " + e.getMessage());
        }
        catch(UnsupportedLookAndFeelException e) {
                System.out.println("UnsupportedLookAndFeelException: " + e.getMessage());
        }
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new RootGUI();
        }
    });
}

And then put your RootGUI class in a different file.

Categories
discuss

Does using final for variables in Java improve garbage collection?

Today my colleagues and me have a discussion about the usage of the final keyword in Java to improve the garbage collection.

For example, if you write a method like:

public Double doCalc(final Double value)
{
   final Double maxWeight = 1000.0;
   final Double totalWeight = maxWeight * value;
   return totalWeight;  
}

Declaring the variables in the method final would help the garbage collection to clean up the memory from the unused variables in the method after the method exits.

Is this true?

Answer

Here’s a slightly different example, one with final reference-type fields rather than final value-type local variables:

public class MyClass {

   public final MyOtherObject obj;

}

Every time you create an instance of MyClass, you’ll be creating an outgoing reference to a MyOtherObject instance, and the GC will have to follow that link to look for live objects.

The JVM uses a mark-sweep GC algorithm, which has to examine all the live refereces in the GC “root” locations (like all the objects in the current call stack). Each live object is “marked” as being alive, and any object referred to by a live object is also marked as being alive.

After the completion of the mark phase, the GC sweeps through the heap, freeing memory for all unmarked objects (and compacting the memory for the remaining live objects).

Also, it’s important to recognize that the Java heap memory is partitioned into a “young generation” and an “old generation”. All objects are initially allocated in the young generation (sometimes referred to as “the nursery”). Since most objects are short-lived, the GC is more aggressive about freeing recent garbage from the young generation. If an object survives a collection cycle of the young generation, it gets moved into the old generation (sometimes referred to as the “tenured generation”), which is processed less frequently.

So, off the top of my head, I’m going to say “no, the ‘final’ modifer doesn’t help the GC reduce its workload”.

In my opinion, the best strategy for optimizing your memory-management in Java is to eliminate spurious references as quickly as possible. You could do that by assigning “null” to an object reference as soon as you’re done using it.

Or, better yet, minimize the size of each declaration scope. For example, if you declare an object at the beginning of a 1000-line method, and if the object stays alive until the close of that method’s scope (the last closing curly brace), then the object might stay alive for much longer that actually necessary.

If you use small methods, with only a dozen or so lines of code, then the objects declared within that method will fall out of scope more quickly, and the GC will be able to do most of its work within the much-more-efficient young generation. You don’t want objects being moved into the older generation unless absolutely necessary.

Categories
discuss

Hibernate: Automatically creating/updating the db tables based on entity classes

I have the following entity class (in Groovy):

import javax.persistence.Entity
import javax.persistence.Id
import javax.persistence.GeneratedValue
import javax.persistence.GenerationType

@Entity
public class ServerNode {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  Long id

  String firstName
  String lastName

}

and my persistence.xml:

<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0">
    <persistence-unit name="NewPersistenceUnit">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <properties>
            <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/Icarus"/>
            <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
            <property name="hibernate.connection.username" value="root"/>
            <property name="hibernate.connection.password" value=""/>
            <property name="hibernate.archive.autodetection" value="class"/>
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
            <property name="hbm2ddl.auto" value="create"/>
        </properties>
        <class>net.interaxia.icarus.data.models.ServerNode</class>
    </persistence-unit>
</persistence>

and the script:

import javax.persistence.EntityManager
import javax.persistence.EntityManagerFactory
import javax.persistence.Persistence
import net.interaxia.icarus.data.models.ServerNode

def factory = Persistence.createEntityManagerFactory("NewPersistenceUnit")
def manager = factory.createEntityManager()

manager.getTransaction().begin()

manager.persist new ServerNode(firstName: "Test", lastName: "Server")

manager.getTransaction().commit()

the database Icarus exists, but currently has no tables. I would like Hibernate to automatically create and/or update the tables based on the entity classes. How would I accomplish this?

Answer

I don’t know if leaving hibernate off the front makes a difference.

The reference suggests it should be hibernate.hbm2ddl.auto

A value of create will create your tables at sessionFactory creation, and leave them intact.

A value of create-drop will create your tables, and then drop them when you close the sessionFactory.

Perhaps you should set the javax.persistence.Table annotation explicitly?

Hope this helps.

Categories
discuss

How to check if a variable exists in a FreeMarker template?

I have a Freemarker template which contains a bunch of placeholders for which values are supplied when the template is processed. I want to conditionally include part of the template if the userName variable is supplied, something like:

[#if_exists userName]
  Hi ${userName}, How are you?
[/#if_exists]

However, the FreeMarker manual seems to indicate that if_exists is deprecated, but I can’t find another way to achieve this. Of course, I could simple providing an additional boolean variable isUserName and use that like this:

[#if isUserName]
  Hi ${userName}, How are you?
[/#if]

But if there’s a way of checking whether userName exists then I can avoid adding this extra variable.

Answer

To check if the value exists:

[#if userName??]
   Hi ${userName}, How are you?
[/#if]

Or with the standard freemarker syntax:

<#if userName??>
   Hi ${userName}, How are you?
</#if>

To check if the value exists and is not empty:

<#if userName?has_content>
    Hi ${userName}, How are you?
</#if>
Source: stackoverflow
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