Access The Java Map In Javascript

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In programming, data types are used to classify particular types of data. Each type of data is stored differently, and the type of data in which a value is stored will determine what operations can be performed on the value.

When working in Java, one class that you may come across is the Java HashMap Class. This class is part of the collections framework and allows developers to store data using the Map data type.

This tutorial will cover the basics of Java HashMaps, how to create a HashMap and explore the main ones. that can be used when working with the HashMap class. This article will refer to examples so that we can explain the HashMap class in more depth.

Java Maps and HashMap

The Java Map interface is used to store map values ‚Äã‚Äãin a key / value pair. Keys are unique values ‚Äã‚Äãassociated with a specific value. In Java, a map cannot contain duplicate keys and each key must be associated with a particular value.

The key / value structure proposed by Map allows access to values ‚Äã‚Äãaccording to their keys. So, if you had a card with the key gbp and the value United Kingdom, when you refer to the key gbp the value " United Kingdom & rdquo; will be returned.

The HashMap class is part of the collections framework and allows you to store data using the Map interface and hash tables. Hash tables are special collections used to store key / value elements.

Before we can create a HashMap, we must first import the HashMap package. Here’s how to do it in a Java program:

import java.util.hashmap ;

Now that we have imported the HashMap package, we can start creating HashMaps in Java.

Create a HashMap

To create a HashMap in Java, you can use the following syntax:

HashMap map_name = new HashMap (capacity, loadFactor);

Let’s break it down into its basic components:

  • HashMap is used to tell our code that we declare a hashmap.
  • stores the data types for the key and values, respectively.
  • map_name is the name of the hashmap we declared.
  • new HashMap tells our code to initialize a HashMap with the data types we specified.
  • capacity tells our code how many entries it can store. By default it is set to 16. (optional)
  • loadFactor tells our code that when our hash table reaches a certain capacity, a new hash table of double size against the original hash table must be created. By default, it is set to 0.75 (or 75% of capacity). (optional)

Suppose you are creating a program for a local currency exchange company. They want to create a program that stores the names of the countries and currency codes in which they offer exchange services. Using a HashMap is a good idea to remember this data as we have two things that we want to store together: the country name and the currency code.

Here is the code we would use to create a HashMap for this purpose:

In this example, we have declared a HashMap called currencyCodes which stores two String values. Now that we have our HashMap, we can start adding elements and manipulating its content.

The HashMap class offers aw range Ide methods that can be used to store and manipulate data. The put () method is used to add values ‚Äã‚Äãto a HashMap using the key / value structure.

Back to currency exchange. Let’s say we want to add the GBP / United Kingdom entry in our program, which will store the currency value for the United Kingdom. The key GBP maps to the value United Kingdom in this example. We could do it using this code:

In our code, we initialize a hash map called currencyCodes, then use the put () method to add an entry to the hash map. This entry has the key GBP and the value United Kingdom. Next, we print the HashMap value, which returns the following:

{GBP = UK, USD = US}

As you can see our HashMap now contains two values: GBP = UK and USD = United States.

Accessing an element

To access an element in a HashMap, you can use the get () method . The> get method takes one parameter: the name of the key for the value you want to retrieve.

Suppose we wanted to retrieve the name of the country associated with GBP. We could do this using this code:

Our code returns: United Kingdom.

Delete item

The remove () method is used e to remove an item from a HashMap. remove () takes one parameter: the name of the key that you want to delete the entry .

Suppose we want to remove GBP from our HashMap. We could do it using this code:

When we run our code, GBP is removed from our HashMap and the following response is returned: {USD = United States}

Additionally, the clear () method is used to remove all items from a HashMap. clear () takes no parameters. Here is an example of the clear () method in action:

Our code returns an empty HashMap: {}.

Replace HashMap Elements

Replacement () is used to replace a value associated with a specific key with a new value. replace () takes two parameters: the key of the value you want to replace and the new value with which you want to replace the old value.

For example, suppose you want to replace the value United Kingdom with Great Britain in our HashMap. We could do it using this code:

When we run our code, the value of the GBP key (which in this case is United Kingdom) is replaced by Great Britain and our program returns the following:

{GBP = Great Britain, USD = United States}

Iterate through a HashMap

You can also browse a HashMap in Java. three methods that can be used to iterate a HashMap:

The easiest way to iterate through a HashMap is to use a for-each loop. If you want to learn more about Java for-each loops, you can read our tutorial on the topic here.

Suppose you want to print each value in our currencyCodes ‚"HashMap on the console so that you can show the currency conversion company a list of currencies that ’it offers is stored in HashMap. We could use the following code to do this:

When we run our code, the following response is returned:

Great Britain

United States

In our code, we use a for-each loop to iterate through each item in the list of currencyCodes .values ‚Äã‚Äã(). Then we print each element on a new line.

If we wanted to iterate through each key and print the name of each key in our HashMap, we could replace the values ‚Äã‚Äã() with keySet () in our code above. This is what our program would return:

GBP

USD

Conclusion

The Java HashMap class is used to store data using the key / value collection structure. This structure is useful if you want to store two values ‚Äã‚Äãthat need to be associated with each other.

This tutorial covered the basics of HashMaps. We’ve shown you how to create a HashMap and explored some examples of common HashMap methods in action. You are now equipped with the information you need to work with Java HashMaps like an expert!

Access The Java Map In Javascript __del__: Questions

__del__

How can I make a time delay in Python?

5 answers

I would like to know how to put a time delay in a Python script.

2973

Answer #1

import time
time.sleep(5)   # Delays for 5 seconds. You can also use a float value.

Here is another example where something is run approximately once a minute:

import time
while True:
    print("This prints once a minute.")
    time.sleep(60) # Delay for 1 minute (60 seconds).

2973

Answer #2

You can use the sleep() function in the time module. It can take a float argument for sub-second resolution.

from time import sleep
sleep(0.1) # Time in seconds

__del__

How to delete a file or folder in Python?

5 answers

How do I delete a file or folder in Python?

2639

Answer #1


Path objects from the Python 3.4+ pathlib module also expose these instance methods:

Access The Java Map In Javascript __delete__: Questions

2639

Answer #2


Path objects from the Python 3.4+ pathlib module also expose these instance methods:

2639

Answer #3

Python syntax to delete a file

import os
os.remove("/tmp/<file_name>.txt")

Or

import os
os.unlink("/tmp/<file_name>.txt")

Or

pathlib Library for Python version >= 3.4

file_to_rem = pathlib.Path("/tmp/<file_name>.txt")
file_to_rem.unlink()

Path.unlink(missing_ok=False)

Unlink method used to remove the file or the symbolik link.

If missing_ok is false (the default), FileNotFoundError is raised if the path does not exist.
If missing_ok is true, FileNotFoundError exceptions will be ignored (same behavior as the POSIX rm -f command).
Changed in version 3.8: The missing_ok parameter was added.

Best practice

  1. First, check whether the file or folder exists or not then only delete that file. This can be achieved in two ways :
    a. os.path.isfile("/path/to/file")
    b. Use exception handling.

EXAMPLE for os.path.isfile

#!/usr/bin/python
import os
myfile="/tmp/foo.txt"

## If file exists, delete it ##
if os.path.isfile(myfile):
    os.remove(myfile)
else:    ## Show an error ##
    print("Error: %s file not found" % myfile)

Exception Handling

#!/usr/bin/python
import os

## Get input ##
myfile= raw_input("Enter file name to delete: ")

## Try to delete the file ##
try:
    os.remove(myfile)
except OSError as e:  ## if failed, report it back to the user ##
    print ("Error: %s - %s." % (e.filename, e.strerror))

RESPECTIVE OUTPUT

Enter file name to delete : demo.txt
Error: demo.txt - No such file or directory.

Enter file name to delete : rrr.txt
Error: rrr.txt - Operation not permitted.

Enter file name to delete : foo.txt

Python syntax to delete a folder

shutil.rmtree()

Example for shutil.rmtree()

#!/usr/bin/python
import os
import sys
import shutil

# Get directory name
mydir= raw_input("Enter directory name: ")

## Try to remove tree; if failed show an error using try...except on screen
try:
    shutil.rmtree(mydir)
except OSError as e:
    print ("Error: %s - %s." % (e.filename, e.strerror))

__delete__

Is there a simple way to delete a list element by value?

5 answers

I want to remove a value from a list if it exists in the list (which it may not).

a = [1, 2, 3, 4]
b = a.index(6)

del a[b]
print(a)

The above case (in which it does not exist) shows the following error:

Traceback (most recent call last):
  File "D:zjm_codea.py", line 6, in <module>
    b = a.index(6)
ValueError: list.index(x): x not in list

So I have to do this:

a = [1, 2, 3, 4]

try:
    b = a.index(6)
    del a[b]
except:
    pass

print(a)

But is there not a simpler way to do this?

1055

Answer #1

To remove an element"s first occurrence in a list, simply use list.remove:

>>> a = ["a", "b", "c", "d"]
>>> a.remove("b")
>>> print(a)
["a", "c", "d"]

Mind that it does not remove all occurrences of your element. Use a list comprehension for that.

>>> a = [10, 20, 30, 40, 20, 30, 40, 20, 70, 20]
>>> a = [x for x in a if x != 20]
>>> print(a)
[10, 30, 40, 30, 40, 70]

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