64bit HashMap in Java

By : AshleynRyan Hicks

Date : December 01 2020, 05:00 PM

I wish this helpful for you Collisions are indeed an issue, but simply increasing table size is not a viable solution. A hash table (as defined in Intro to Algorithms) uses a direct-address table to store the hash buckets. Because it's a direct-address table, the size before you actually start storing objects is relative to the total possible number of hashes in the 'universe' (by universe, I mean the universe as far the hash table is concerned). If you were to actually use all of that available address space, your hash table, before you put anything in it, would be 2^30 * memory_address_size, which is a lot (note that OpenJDK sets the limit on the number of objects in a hash map to 2^30, not 2^32).
The Java HashMap implementation actually starts, by default, with a hash universe size of 16 (I remember reading 8, but OpenJDK 8 JVM implementation is 16). So when you put in an object, Java gets the integer result from hashCode() and finds the remainder from dividing by 16. And that's the hash it uses. By default, the Java hash map also uses a load factor of 0.75. So it doesn't attempt to increase the size of the 'hash universe' until it's 75% full. When it gets to that point, a new hash table is created, recalculating all of the hashes in the process with a new universe size double that of the previous table. Which is an expensive operation. So what I'm saying is, the Java HashMap aims to keep the map 75% full, and expects collisions. And that correctly setting up your hash map can improve performance. You can instantiate a hash map with an initial value, and also a load factor of your choosing.

code :

Map<String, String> myMap = new HashMap(128, 0.5f);

float lf = Math.min(Math.max(0.25f, loadFactor), 4.0f);

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Java 64bit install throwing non compatible 64bit error in 64bit Windows 7

By : user3790106

Date : March 29 2020, 07:55 AM

To fix the issue you can do Most probably the file that you downloaded is corrupted. Check that it downloaded completely, or try to use another browser to download the installer. Unfortunately there are no checksums on the JRE download pages.
Also check the digital signature is valid and shows no warning: open the properties dialog of the installer and then click Digital Signatures tab.

Python 2.7.8 (64bit) on Windows (64bit) is half as fast as 2.7.8 (64bit) on Ubuntu (64bit)

By : user3254488

Date : March 29 2020, 07:55 AM

Does that help It is because of the size of the long in windows, in windows a long is 32 bits in unix it is 64 bits so you are hitting Arbitrary-precision_arithmetic issues sooner which are costlier to allocate.
Related question Why are python's for loops so non-linear for large inputs

convert JSON file to a hashmap in java with error Can not deserialize instance of java.util.HashMap out of START_ARRAY t

By : user3804713

Date : March 29 2020, 07:55 AM

may help you . Try reading it in as an array of maps.
A json map would look like this:

code :

{ "AC": 1.1324, "AD": 0.64956, "AE": 1.4508, "AF": -0.94481 }

HashMap: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.HashMap.put'

By : Luke M

Date : March 29 2020, 07:55 AM

may help you . In this case the addLocation is being called before the onCreate() method is executed.
I guess you are accessing an instance of InitLocationsOnStartup which is not managed by the Android framework.

code :

InitLocationsOnStartup startup = new InitLocationsOnStartup();
startup.addLocation(..., ...);

getApplication()

public static InitLocationsOnStartup getInitLocationsOnStartup(Activity activity) {
    return (InitLocationsOnStartup) activity.getApplication();
}

InitLocationsOnStartup startup = getInitLocationsOnStartup(this);

JAVA HashMap 2D, cant get the right approach to make a 2D HashMap, i mean a HashMap into another HashMap

By : perfectscript

Date : March 29 2020, 07:55 AM

I wish this help you I want to make a board of Students' names and Subjects and each student has a grade in each subject (or not.. he can leave the exam and doesnt write it, and then his case will be empty). I want to use just HashMaps. I mean, it will be something like that: , You can use this class: