David Landup

Branko Ilic

In this tutorial, we'll take a look at how to convert and collect a stream into a map in Java 8 with a pair of mapper functions, a merge function and a supplier function, with practical examples and tips.

Guide to Java 8 Collectors: toMap()

<h2 id="introduction">Introduction</h2>
A stream represents a sequence of elements and supports different kinds of operations that lead to the desired result. The source of a stream is usually a Collection or an Array, from which data is streamed from.
Streams differ from collections in several ways; most notably in that the streams are not a data structure that stores elements. They're functional in nature, and it's worth noting that operations on a stream produce a result and typically return another stream, but do not modify its source.
To &quot;solidify&quot; the changes, you collect the elements of a stream back into a <code>Collection</code>.
<blockquote>
In this guide, we'll take a look at how to collect a stream into an unmodifiable collections.
</blockquote>
<h2 id="collectstreamintounmodifiablecollections">Collect Stream into Unmodifiable Collections</h2>
It's worth noting that there's a difference between an immutable and unmodifiable collection.
<blockquote>
You can't change the contents of an unmodifiable collection. But, if the source collection changes, the unmodifiable collection changes too. An immutable collection is one that is a result of copying a source collection to create a new one. This new one should be unmodifiable too.
</blockquote>
In the proceeding sections, we'll take a look at how you can collect a stream into an unmodifiable list, set or map. For this purpose, the regular <code>collect()</code> and <code>collectingAndThen()</code> methods do the trick. The former allows you to directly convert a stream into a collection, while the latter allows us to collect a stream into a regular collection, and then convert it into its unmodifiable counterpart through a separate function.
Instead, you could introduce other functions or chain the <code>collectingAndThen()</code> method to introduce new changes in the pipeline before collecting into an unmodifiable collection.
<blockquote>
If you'd like to read more about the regular <code>collect()</code> and advanced <code>collectingAndThen()</code>, read our <a target="_blank" href="https://stackabuse.com/java-8-streams-convert-a-stream-to-list/">Java 8 Streams: Convert a Stream to List</a> and <a target="_blank" href="https://stackabuse.com/guide-to-java-8-collectors-collectingandthen/">Guide to Java 8 Collectors: collectingAndThen()</a>!
</blockquote>
<h3 id="collectstreamintounmodifiablelist">Collect Stream into Unmodifiable List</h3>
Let's start out with a list. We'll use the standard <code>Collectors.toList()</code> collector, followed by a call to <code>unmodifiableList()</code> of the <code>Collections</code> class. Alternatively, you can supply a <code>toUnmodifiableList()</code> collector to the <code>collect()</code> method:
<pre><code class="hljs">Stream&lt;Integer&gt; intStream = Stream.of(1, 2, 3);

List&lt;Integer&gt; unmodifiableIntegerList1 = intStream.collect(Collectors.toUnmodifiableList());

List&lt;Integer&gt; unmodifiableIntegerList2 = intStream.collect(
 Collectors.collectingAndThen(
 Collectors.toList(),
 Collections::unmodifiableList
 )
);
</code></pre>
If we try to modify these lists, an <code>UnsupportedOperationException</code> should be thrown. Their <code>simpleName</code> should be <code>UnmodifiableRandomAccessList</code> and they should contain the exact same elements as seen in the stream:
<pre><code class="hljs">@Test
public void listShouldBeImmutable() {
 // Should contain elements 1, 2, and 3
 assertEquals(
 &quot;[1, 2, 3]&quot;,
 unmodifiableIntegerList1 .toString()
 );
 // Should be of type UnmodifiableList
 assertEquals(
 &quot;UnmodifiableRandomAccessList&quot;,
 unmodifiableIntegerList1 .getClass().getSimpleName()
 );
 // Should throw an exception when you attempt to modify it
 assertThrows(
 UnsupportedOperationException.class,
 () -&gt; unmodifiableIntegerList1 .add(4)
 );
}
</code></pre>
<h3 id="collectstreamintounmodifiableset">Collect Stream into Unmodifiable Set</h3>
If a stream you're dealing with has duplicates, and you'd like to get rid of them - the easiest way isn't to filter the list or keep track of the encountered elements in another list. The easiest solution to remove duplicates from a list is to box the list into a set, which doesn't allow for duplicates!
Again, the <code>collectingAndThen()</code> collector works wonders here, as you can collect the stream into a <code>Set</code> and convert it into an unmodifiable set in the downstream function:
<pre><code class="hljs">Stream&lt;Integer&gt; intStream = Stream.of(1, 1, 3, 2, 3);

Set&lt;Integer&gt; integerSet1 = intStream.collect(Collectors.toUnmodifiableSet());

Set&lt;Integer&gt; integerSet2 = intStream.collect(
 Collectors.collectingAndThen(
 Collectors.toSet(),
 Collections::unmodifiableSet
 )
);
</code></pre>
Then, the <code>Set</code> should be unmodifiable. Any change attempt should throw an <code>UnsupportedOperationException</code>:
<pre><code class="hljs">@Test
public void setShouldBeImmutable() {
 // Set shouldn&#x27;t contain duplicates
 assertEquals(
 &quot;[1, 2, 3]&quot;,
 integerSet1.toString()
 );
 // Set should be of type UnmodifiableSet
 assertEquals(
 &quot;UnmodifiableSet&quot;,
 integerSet1.getClass().getSimpleName()
 );
 // Set should not be modifiable
 assertThrows(
 UnsupportedOperationException.class,
 () -&gt; integerSet1.add(3)
 );
}
</code></pre>
<h3 id="collectstreamintounmodifiablemap">Collect Stream into Unmodifiable Map</h3>
Collecting to an unmodifiable map works in much the same way as the previous two, so let's try to spice it up a bit. Say you have a case where you want to store numbers and their square-value equivalents:
<table class="table table-striped">
 <tr>
 <td>Key</td>
 <td>Value</td>
 </tr>
 <tr>
 <td>2</td>
 <td>4</td>
 </tr>
 <tr>
 <td>3</td>
 <td>9</td>
 </tr>
 <tr>
 <td>4</td>
 <td>16</td>
 </tr>
</table>
But, when you receive duplicate keys, you don't want to repeat entries:
<table class="table table-striped">
 <tr>
 <td>Key</td>
 <td>Value</td>
 <td>Passes?</td>
 </tr>
 <tr>
 <td>2</td>
 <td>4</td>
 <td>YES</td>
 </tr>
 <tr>
 <td>3</td>
 <td>9</td>
 <td>YES</td>
 </tr>
 <tr>
 <td>4</td>
 <td>16</td>
 <td>YES</td>
 </tr>
 <tr>
 <td>4</td>
 <td>16</td>
 <td>NO</td>
 </tr>
</table>
Yet, the method, when converting into a map using the same approach we used before, there's no place to check for duplicate entries:
<pre><code class="hljs">Stream&lt;Integer&gt; stream = Stream.of(1, 2, 3, 4, 4);

Map&lt;Integer, Integer&gt; map1 = stream.collect(
 Collectors.toUnmodifiableMap(
 Function.identity(), 
 i -&gt; (int)Math.pow(i, 2)
 )
);

Map&lt;Integer, Integer&gt; map2 = stream.collect(
 Collectors.collectingAndThen(
 Collectors.toMap(
 // Key
 Function.identity(),
 // Value
 i -&gt; (int) Math.pow(i, 2)
 ),
 Collections::unmodifiableMap
 )
);
</code></pre>
Note the use of <code>Function.identity()</code> in the key mapper of the <code>Collectors.toMap()</code> method. The method <code>identity()</code> makes the mapper use the <code>Integer</code> element itself as the key of the map entry.
Thus, when you call it with duplicate entries, it always throws an <code>IllegalStateException</code>:
<pre><code class="hljs">Exception in thread &quot;main&quot; java.lang.IllegalStateException: 
Duplicate key 4 (attempted merging values 16 and 16)
</code></pre>
It's easy to remedy this problem with stream operations themselves, so the client doesn't have to worry about providing a clean list! Just by adding an intermediate <code>distinct()</code> operation to the stream, we can filter out duplicate values before collecting:
<pre><code class="hljs">Map&lt;Integer, Integer&gt; map1 = stream.distinct().collect(
 Collectors.toUnmodifiableMap(
 Function.identity(), 
 i -&gt; (int)Math.pow(i, 2)
 )
);

Map&lt;Integer, Integer&gt; map2 = stream.distinct().collect(
 Collectors.collectingAndThen(
 Collectors.toMap(
 // Key
 Function.identity(),
 // Value
 i -&gt; (int) Math.pow(i, 2)
 ),
 Collections::unmodifiableMap
 )
);
</code></pre>
Let's test the result:
<pre><code class="hljs">@Test
public void mapShouldBeImmutable() { 
 assertEquals(
 &quot;{1=1, 2=4, 3=9, 4=16}&quot;,
 map1.toString()
 );
 assertEquals(
 &quot;UnmodifiableMap&quot;,
 map1.getClass().getSimpleName()
 );
 assertThrows(
 UnsupportedOperationException.class,
 () -&gt; map1.put(5, 25)
 );
}
</code></pre>
<h2 id="conclusion">Conclusion</h2>
In this short guide, we've taken a look at how to collect streams into unmodifiable collections - a list, set and map!
We've also taken a quick look at how to handle duplicate values, which can in some data structures, raise exceptions, and in others, cause silent failure.

Hiram Kamau

In this guide, learn how to convert/collect a Java 8 Stream into a Collection such as a List, Map or Set, with practical code examples!

Java 8 - Collect Stream into Unmodifiable List, Set or Map

Checkboxes are one of the several types of input fields we use very commonly to allow users to interact with web pages and typically POST data to a backend, by checking any box that applies to a given situation.
As opposed to Radio Buttons (which belong to Radio Groups) - checkboxes belonging to a single group aren't mutually exclusive so a user can select multiple boxes that apply. You should keep this in mind when checking whether any options are selected.
<blockquote>
In this guide, we'll take a look at how to check if a checkbox is checked in jQuery - a popular library for JavaScript, and Vanilla JavaScript.
</blockquote>
<h2 id="elementchecked">element.checked</h2>
Let's start out with a simple Checkbox setup:
<pre><code class="hljs">&lt;h1&gt;Check if Checkbox is Checked&lt;/h1&gt;
&lt;p&gt;Have you taken this test before?&lt;/p&gt;
&lt;input type=&quot;checkbox&quot; id=&quot;takenBefore&quot; placeholder=&quot;Yes&quot;&gt;
&lt;label for=&quot;takenBefore&quot;&gt;Yes&lt;/label&gt;

&lt;button id=&quot;button&quot;&gt; Check Checkbox &lt;/button&gt;
</code></pre>
In pure JavaScript - checking if a Checkbox is checked is as easy as accessing the <code>checked</code> field, represented by a boolean:
<pre><code class="hljs">// Get relevant element
checkBox = document.getElementById(&#x27;takenBefore&#x27;);
// Check if the element is selected/checked
if(checkBox.checked) {
 // Respond to the result
 alert(&quot;Checkbox checked!&quot;);
}
</code></pre>
However, this code will only ever run if you specifically run it. For instance, you could run it when the user submits a form or wishes to proceed to the next page as a validation step. Validation isn't commonly done this way, and typically delegated to libraries that are more robust. More commonly, you want to react to the checkbox to change something on the page - such as providing other input options.
In these cases, you'll want to add an event listener to the button, and let it listen to events, such as being clicked. When such an event occurs - you can decide to respond to the event:
<pre><code class="hljs">checkBox = document.getElementById(&#x27;takenBefore&#x27;).addEventListener(&#x27;click&#x27;, event =&gt; {
 if(event.target.checked) {
 alert(&quot;Checkbox checked!&quot;);
 }
});
</code></pre>
In this setup - the code is constantly waiting and listening to events on the element! Now as soon as someone clicks the Checkbox, your <code>alert()</code> method will execute. This time, however, to obtain the <code>checkBox</code> element, we procure it from the <code>event</code> instance, as the <code>target</code> of that event.
When you open the page up in a browser, and click the button, you'll be greeted with:
<img src="https://s3.stackabuse.com/media/articles/how-to-check-if-checkbox-is-checked-in-jquery-and-javascript-1.gif" alt="check if checkbox is checked with javascript">
<h2 id="element0checked">$('#element')[0].checked</h2>

 <div class="alert alert-note">
 <div class="flex">
 
 <div class="flex-shrink-0 mr-3">
 <img src="/assets/images/icon-information-circle-solid.svg" class="icon" aria-hidden="true" />
 </div>
 
 <div class="w-full">
 Note: jQuery is a popular JavaScript library, present in many projects around the world. Due to its light weight and features that expand the scope of JavaScript's built-in capabilities - it's become a staple. Not surprisingly - it can be used to check whether a Checkbox is selected or not.

 </div>
 </div>
 </div>
 We can use jQuery's selector syntax instead of pure JavaScript's to simplify both the selection of the element and the event listener attached to it.
<blockquote>
You can import jQuery in your page via a Content Delivery Network (CDN). You can use jQuery's own CDN for importing the library as well.
</blockquote>
<pre><code class="hljs">&lt;script src=&quot;https://code.jquery.com/jquery-3.6.0.min.js&quot;
 integrity=&quot;sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=&quot; 
 crossorigin=&quot;anonymous&quot;&gt;
&lt;/script&gt;
</code></pre>
To check whether a Checkbox has been checked, in jQuery, you can simply select the element, get its underlying object, instead of the jQuery object (<code>[0]</code>) and use the built-in <code>checked</code> property:
<pre><code class="hljs">let isChecked = $(&#x27;#takenBefore&#x27;)[0].checked
console.log(isChecked);
</code></pre>
Here, we'd be testing if the <code>takenBefore</code> checkbox is checked, by accessing it via jQuery's selector, and using the innate <code>checked</code> field as before.
Alternatively, you can define a listener that detects a change:
<pre><code class="hljs">$(&#x27;#takenBefore&#x27;).change(function() {
 alert(&#x27;Checkbox checked!&#x27;);
});
</code></pre>
This code is much simpler, but performs in much the same way as the pure JS solution! Clicking the checkbox will trigger an alert as before:
<img src="https://s3.stackabuse.com/media/articles/how-to-check-if-checkbox-is-checked-in-jquery-and-javascript-2.gif" alt="check if checkbox is checked with jquery">
<h2 id="elementischecked">$('#element').is(':checked'))</h2>
Instead of checking via the built-in <code>checked</code> property - we can offload the logic to the <code>is()</code> method. The <code>is()</code> method is a general method that can be used for many purposes - and returns a <code>true</code>/<code>false</code> based on the comparison criteria. The <code>:checked</code> selector was specifically made for checking whether a radio button or checkbox element had been selected or not.
So, if you combine these together, it's easy to check whether a checkbox <code>is(':checked')</code>:
<pre><code class="hljs">let isChecked = $(&#x27;#takenBefore&#x27;).is(&#x27;:checked&#x27;);
console.log(isChecked); // true (if checked at the time)
</code></pre>
<h2 id="elementpropchecked">$('#element').prop(&quot;checked&quot;)</h2>
In earlier versions of jQuery - <code>attr()</code> was used to access and manipulate fields of elements. In newer versions, the method was replaced with <code>prop()</code>. It can work as a getter/setter for properties of elements, and in our case - as a wrapper for getting the <code>checked</code> property of an element.
<pre><code class="hljs">let isChecked = $(&#x27;#takenBefore&#x27;).prop(&#x27;checked&#x27;);
console.log(isChecked); // true (if checked at the time)
</code></pre>
<h2 id="isvsprop">is() vs prop()?</h2>
<blockquote>
So, what's the difference between <code>is()</code> and <code>prop()</code> in this context?
</blockquote>
Both methods appear to work in much the same way from our perspective. So, what's the difference?
<code>is()</code> has a bit more overhead processing and parsing than <code>prop()</code>. Even considering the fact that <code>prop()</code> doesn't just access the property and does some validation, it's more performant when it comes to processing a large number of inputs, say, in a loop.
On the other hand - <code>is()</code> always returns a <code>boolean</code> value, whereas <code>prop()</code> just returns the underlying property's type. You can't guarantee that something else sneaked in, and your code may fail at runtime if you don't use <code>prop()</code> properly. <code>is()</code> also semantically indicates the return value - a <code>boolean</code>, which makes it a more readable option.
<h2 id="conclusion">Conclusion</h2>
In this short guide, we've taken a look at how to check whether a checkbox is checked with JavaScript and jQuery.
We've used the <code>checked</code> property - directly and indirectly, followed by the <code>is()</code> and <code>prop()</code> methods, as well as taken a quick look as to which you could prefer.

In this short guide, learn how to check if a checkbox has been checked in JavaScript with jQuery - using the checked property, is() and prop() methods!

How to Check if Checkbox is Checked in jQuery and JavaScript

<h2 id="introduction">Introduction</h2>
The <code>or</code> operator is one of the three existing logical operators in Python (<code>and</code>, <code>or</code>, <code>not</code>), which perform a logical evaluation of the passed operands.
In simple terms, when two operands are passed, it will decide whether the final value of the created logical expression is <code>True</code> or <code>False</code>. The mechanism used to evaluate the value of the final expression is based on the set of rules known as Boolean algebra.
<blockquote>
In this guide, we'll cover the <code>or</code> operator in Python as well as its most common use cases.
</blockquote>
<h2 id="oroperatorbasics">or Operator Basics</h2>
Python's <code>or</code> operator just performs logical disjunction on the two provided operands. Assuming that the operands are simply two Boolean values, the rule on how to use the <code>or</code> operator is pretty straight-forward:
<blockquote>
If either one of two operands has the value <code>True</code>, the whole expression has the value <code>True</code>. In all other cases, the whole expression has the value <code>False</code>.
</blockquote>
Now let's take a look at the truth table of the <code>or</code> operator:
<table class="table table-striped">
 <tr>
 <td>Operand 1</td>
 <td>Operand 2</td>
 <td>OR Expression Value</td>
 </tr>
 <tr>
 <td>True</td>
 <td>True</td>
 <td>True</td>
 </tr>
 <tr>
 <td>True</td>
 <td>False</td>
 <td>True</td>
 </tr>
 <tr>
 <td>False</td>
 <td>True</td>
 <td>True</td>
 </tr>
 <tr>
 <td>False</td>
 <td>False</td>
 <td>False</td>
 </tr>
</table>
This table describes the law of logical disjunction. By looking at this table, we can see that the <code>or</code> operator produces <code>False</code> as the result only if both of the operands are <code>False</code> as well.
All of this leads us to the concept of lazy evaluation. A mechanism used to optimize calculations of mathematical operations. In this particular case, it is used to speed up the process of evaluating Boolean expressions with the <code>or</code> operator.
We already know that an <code>or</code> expression results in a <code>True</code> value if either of its two operands is <code>True</code>. Therefore, in a Boolean expression consisting of multiple operands, it is completely unnecessary to evaluate each one of them individually.
It's enough just to read the values of operands one after the other. When we come across a <code>True</code> for the first time, we can safely just ignore all of the other operands and just evaluate the whole expression as <code>True</code>.
On the other hand, if there is no operand with the value <code>True</code>, we must evaluate the whole expression with the value <code>False</code>. That is the basic principle of lazy evaluation - don't evaluate if you don't have to.
<h2 id="usingoronbooleanvariables">Using or on Boolean Variables</h2>
The <code>or</code> operator in Python is used to evaluate two of its operands. In this section, we'll focus on the case where both of the operands have Boolean values. In some cases, the <code>or</code> operator can be used with non-Boolean values, which we'll discuss in the following sections.
Let's take a look at how to use the <code>or</code> operator with two Boolean values:
<pre><code class="hljs"># Two Boolean values
result1 = True or False # Expected to be `True`
result2 = False or False # Expected to be `False`

print(&#x27;R1:&#x27;, result1)
print(&#x27;R2:&#x27;, result2)
</code></pre>
In this example, we can see how the <code>or</code> operator evaluates expressions consisting only of simple Boolean values. As described in the previous sections, this piece of code will have the following output:
<pre><code class="hljs">R1: True
R2: False
</code></pre>
In the previous example, we've named Boolean expressions <code>result1</code> and <code>result2</code>. That way, we've created two Boolean variables with values <code>True</code> and <code>False</code>, respectively.
Those two variables can be used as the operands of another Boolean expression and, therefore, could be considered as subexpressions of the more complex Boolean expression. That's the general principle used to build more complex Boolean expressions layer by layer:
<pre><code class="hljs">result3 = result1 or result2 # `True` or `False` &lt;=&gt; `True`
result4 = result3 or True # `True` or `True` &lt;=&gt; `True`

print(&#x27;R3:&#x27;, result3)
print(&#x27;R4:&#x27;, result4)
</code></pre>
As expected, this will output:
<pre><code class="hljs">R3: True
R4: True
</code></pre>
<code>result4</code> is a complex Boolean expression consisting of multiple subexpressions and Boolean values. Let's take a look at the process of unfolding it:
<pre><code class="hljs">1. result4 = result3 or True
2. result4 = (result1 or result2) or True
3. result4 = ((True or False) or (False or False)) or True
</code></pre>
Based on the associative law for the <code>or</code> operator, we know that the order in which we apply the operator doesn't have an impact on the value of the Boolean expression, so there is no need for brackets. Therefore, we can transform <code>result4</code> one step further by deleting all of the brackets:
<pre><code class="hljs"> result4 = True or False or False or False or True
</code></pre>
<h2 id="usingoronnonbooleanvariables">Using or on Non-Boolean Variables</h2>
The <code>or</code> operator in Python can also be used with variables other than Boolean. You can even mix and match Boolean variables with non-Boolean variables. In this section, we'll go over some examples that illustrate the usage of the <code>or</code> operator with variables of different data types.
Generally speaking, any object or a variable in Python is considered to be <code>True</code> unless its class has a predefined <code>__bool__()</code> method that returns <code>False</code> or a <code>__len__()</code> method that returns <code>0</code>.
In simple terms that means that only objects considered to be <code>False</code> are those which are predefined to be <code>False</code> or those which are empty - empty lists, tuples, strings, dictionaries... <a target="_blank" rel="nofollow noopener noreferrer" href="https://docs.python.org/3/library/stdtypes.html">The Official Python documentation</a> gives us the list of some of the most common built-in objects considered to be <code>False</code>:
<ul>
<li>Constants defined to be false: <code>None</code> and <code>False</code>.</li>
<li>Zero of any numeric type: <code>0</code>, <code>0.0</code>, <code>0j</code>, <code>Decimal(0)</code>, <code>Fraction(0, 1)</code></li>
<li>Empty sequences and collections: <code>''</code>, <code>()</code>, <code>[]</code>, <code>{}</code>, <code>set()</code>, <code>range(0)</code></li>
</ul>

 <div class="alert alert-note">
 <div class="flex">
 
 <div class="flex-shrink-0 mr-3">
 <img src="/assets/images/icon-information-circle-solid.svg" class="icon" aria-hidden="true" />
 </div>
 
 <div class="w-full">
 Note: These are also known as Falsy Values - the ones you'd intuitively be able to reduce to a <code>False</code> boolean value. The opposite values are Truthy Values.

 </div>
 </div>
 </div>
 <blockquote>
Another very important fact is that the <code>or</code> operator in this case returns the actual object, not the <code>True/False</code> value of the object.
</blockquote>
Let's take a look at the example that illustrates mentioned behavior:
<pre><code class="hljs">exp = {} or &#x27;This is a string&#x27;
</code></pre>
As stated before, the first operand - <code>{}</code> (empty dictionary) is considered to be <code>False</code> and the second operand - <code>'This is a string'</code> (not an empty string) is considered to be <code>True</code>. This means that the previous expression is implicitly transformed to:
<pre><code class="hljs"># exp = False or True
</code></pre>
Here, <code>exp</code> is evaluated to <code>True</code>. But, when we try to print the original <code>exp</code> value, instead of <code>True</code>, the output will be:
<pre><code class="hljs">&#x27;This is a string&#x27;
</code></pre>
This example illustrates the case when the <code>or</code> operator returns the object itself instead of its <code>True/False</code> value. To sum this behavior up, we can illustrate it with the altered (truth) table of the <code>or</code> operator:
<table class='table table-striped'>
 <tr>
 <td>object1 (value)</td>
 <td>object2 (value)</td>
 <td>object1 `or` object2 (return value)</td>
 </tr>
 <tr>
 <td>True</td>
 <td>True</td>
 <td>object1</td>
 </tr>
 <tr>
 <td>True</td>
 <td>False</td>
 <td>object1</td>
 </tr>
 <tr>
 <td>False</td>
 <td>True</td>
 <td>object2</td>
 </tr>
 <tr>
 <td>False</td>
 <td>False</td>
 <td>object2</td>
 </tr>
</table>
This also applies when we combine usual Boolean values and objects in Boolean expressions.
If the expression contains at least one value that is considered to be <code>True</code>, the value of the expression is <code>True</code>, but the return value can vary based on the first <code>True</code> element in it.
If the first <code>True</code> operand to be found in the expression is a simple Boolean value, the return value will be <code>True</code>, but if the first <code>True</code> element is some sort of an object, the return value will be that object itself. For example, the following expression will return <code>True</code>:
<pre><code class="hljs">0 or True 
</code></pre>
And the following expression will return <code>[1, 2, 3]</code>, which is the first <code>True</code> operand found:
<pre><code class="hljs">False or [1, 2, 3]
</code></pre>
On the other hand, if a Boolean expression is <code>False</code>, meaning that no <code>True</code> operand was found, its return value will be its last operand, either object or <code>False</code>:
<pre><code class="hljs">{} or 0.0 or [] or False or ()
# Output &gt;&gt; ()
</code></pre>
<h2 id="conclusion">Conclusion</h2>
In this guide, we've explained the usage of the <code>or</code> operator in Python. We've introduced the syntax in Python and described how the <code>or</code> operator evaluates Boolean expressions and how it determine the proper return value based on the operands.
Besides its primary usage for evaluating Boolean expressions, the <code>or</code> operator can also be pretty useful in some other use cases.
Its features make it a good choice when you need to set default values for some variables or a default return value of a function and much more, but those special use cases are far beyond the scope of this article, so we'll let you explore all the use cases that the <code>or</code> operator can be utilized in.

Vladimir Batoćanin

Dimitrije Stamenic

In this guide, we'll take a look at how to use the Python or operator, how Boolean algebra works, what truthy and falsy values are in Python, as well as how to use the or operator both with boolean and non-boolean values.

Guide to the Python or Operator

Marcus Sanatan

Reham Muzzamil

In this guide, learn what the difference is between Java 8's map() and flatMap() methods are, in the context of Optionals and the Stream API, with practical code examples and use cases.

Java 8 - Difference Between map() and flatMap()

<h2 id="introduction">Introduction</h2>
A stream represents a sequence of elements and supports different kinds of operations that lead to the desired result. The source of a stream is usually a Collection or an Array, from which data is streamed from.
Streams differ from collections in several ways; most notably in that the streams are not a data structure that stores elements. They're functional in nature, and it's worth noting that operations on a stream produce a result and typically return another stream, but do not modify its source.
To &quot;solidify&quot; the changes, you collect the elements of a stream back into a <code>Collection</code>.
<blockquote>
In this guide, we'll take a look at how to collect and reverse a list in Java.
</blockquote>
<h2 id="collectandreversealistinjava">Collect and Reverse a List in Java</h2>
The first approach would be to <code>collect()</code> the stream into a list - and then use the <code>Collections.reverse()</code> method on the list, which reverses it in-place.

 <div class="alert alert-note">
 <div class="flex">
 
 <div class="flex-shrink-0 mr-3">
 <img src="/assets/images/icon-information-circle-solid.svg" class="icon" aria-hidden="true" />
 </div>
 
 <div class="w-full">
 Note: If you also want to sort the collection while reversing it, you can use the <code>sorted(Comparator.reverseOrder())</code> method in the chain. However, this will impose a completely new relative order of elements.

 </div>
 </div>
 </div>
 Let's define a stream with some elements, collect it into a list, reverse the list and print it:
<pre><code class="hljs">Stream&lt;?&gt; s = Stream.of(1, 2, 3, 4, 5);

List&lt;?&gt; list = s.collect(Collectors.toList());
Collections.reverse(list);
System.out.println(list);
</code></pre>
This results in:
<pre><code class="hljs">[5, 4, 3, 2, 1]
</code></pre>
This works well, but there's a simpler way to do this - all in one method. The <code>collectingAndThen()</code> method allows us to chuck in an additional finishing transformation <code>Function</code> besides the <code>collect()</code> method. This way, we can collect the stream and perform the <code>Collections.reverse()</code> call in the same <code>collect()</code> call!
<h2 id="collectandreversealistwithcollectingandthen">Collect and Reverse a List with collectingAndThen()</h2>
<blockquote>
If you'd like to read more about <code>collectingAndThen()</code> - read our detailed, definitive <a target="_blank" href="https://stackabuse.com/guide-to-java-8-collectors-collectingandthen/">Guide to Java 8 Collectors: collectingAndThen()</a>!
</blockquote>
Let's take a look at how we can use <code>collectingAndThen()</code> to bundle the transformations into a single call:
<pre><code class="hljs">Stream&lt;?&gt; s = Stream.of(1, 2, 3, 4, 5);

List&lt;?&gt; list = s.collect(
 Collectors.collectingAndThen(
 Collectors.toList(),
 l -&gt; {Collections.reverse(l); return l; }
 )
);

System.out.println(list);
</code></pre>
Here, we've supplied an adapter <code>Collector</code> to the <code>collect()</code> method. The adapted collector runs <code>Collectors.toList()</code>, and a Lambda function to reverse and return the reversed list. Finally - the returned value is assigned to the <code>list</code> reference variable!
<blockquote>
If you'd like to read more about Functional Interfaces and Lambda Expressions in Java - read our <a target="_blank" href="https://stackabuse.com/guide-to-functional-interfaces-and-lambda-expressions-in-java/">Guide to Functional Interfaces and Lambda Expressions in Java</a>!
</blockquote>
This results in:
<pre><code class="hljs">[5, 4, 3, 2, 1]
</code></pre>
<h2 id="conclusion">Conclusion</h2>
In this short guide, we've taken a look at how you can collect and reverse a stream/list in Java 8, using <code>collect()</code> and <code>Collections.reverse()</code> - individually, and together through the <code>collectingAndThen()</code> method.

In this tutorial, learn how to collect and reverse a stream into a list in Java 8 with Collectors.toList() and Collections.reverse(), as well as Collectors.collectingAndThen()!

Java 8 Streams - Collect and Reverse Stream into List

Olivera Popović

In this parent-guide - learn what you need to know about Graphs in Computer Science and focus on graph-related algorithm's theory and implementation - DFS, BFS, Dijkstra, A*, Minimum Spanning Trees, etc...

Graph Theory and Graph-Related Algorithm's Theory and Implementation

<h2 id="introduction">Introduction</h2>
Flask is one of the most popular Python web development frameworks. It's a lightweight web framework, yet it offers a variety of built-in methods that can be used for hassle-free deployment of efficient web applications.
<blockquote>
In this guide, we will get the IP address of the user who visits the web application created using Flask.
</blockquote>
We'll create a simple REST API that handles the incoming requests to the <code>/</code> endpoint, returning the IP address of the user as the response.
<h2 id="creatingabasicwebappusingflask">Creating a Basic Web App Using Flask</h2>
Create a new Flask project for this tutorial. We'll create a basic app that returns &quot;Hello World&quot; with requests to the <code>/</code> endpoint:
<pre><code class="hljs">from flask import Flask

# Creating the instance of the class Flask
app = Flask(__name__)

# Using the decorator to create the URL for the web application
@app.route(&#x27;/&#x27;)
# The function will return the information displayed on the webpage
def hello_world():
 return &#x27;&lt;h1&gt; Hello World&lt;/h1&gt;&#x27;

# Run the application
if __name__ == &#x27;__main__&#x27;:
 app.run(debug=True)
</code></pre>
Now that we have our app set up, let's collect the IP addresses associated with a user's request.
<h2 id="howtofindtheipaddressofauserinflask">How to Find the IP Address Of a User in Flask?</h2>
Whenever a client requests a page - they send an HTTP GET request, with a certain payload. Amongst other information - their distinctive IP address is included. In Flask - every route has access to the <code>request</code> variable, which represents the incoming request from the user.
From this <code>request</code>, we can extract various elements - and the IP address is denoted as the <code>remote_addr</code> property:
<pre><code class="hljs">@app.route(&#x27;/&#x27;)
def hello_world():
 ip_addr = request.remote_addr
 return &#x27;&lt;h1&gt; Your IP address is:&#x27; + ip_addr
</code></pre>
We can also get the remote address through the request context's environment. The <code>environ</code> dictionary of the <code>request</code> object holds various keys and their respective values, pertaining to the request. The <code>REMOTE_ADDR</code> is one of the server variables (keys) that is mapped to the IP address of the client or the server:
<pre><code class="hljs">@app.route(&#x27;/client&#x27;)
def client():
 ip_addr = request.environ[&#x27;REMOTE_ADDR&#x27;]
 return &#x27;&lt;h1&gt; Your IP address is:&#x27; + ip_addr
</code></pre>
But the above two options won't work if your user is making a request behind a proxy. In that case we have to check the headers of the request. In particular, we're looking for the <code>HTTP_X_FORWARDED_FOR</code> header:
<pre><code class="hljs">@app.route(&#x27;/proxy-client&#x27;)
def proxy_client():
 ip_addr = request.environ[&#x27;HTTP_X_FORWARDED_FOR&#x27;]
 return &#x27;&lt;h1&gt; Your IP address is:&#x27; + ip_addr
</code></pre>
Now, even this approach would not work if the header is not set. So a common strategy is to use the <code>get()</code> method so that if the header is not set, we can default to the remote address:
<pre><code class="hljs">@app.route(&#x27;/proxy-client&#x27;)
def proxy_client():
 ip_addr = request.environ.get(&#x27;HTTP_X_FORWARDED_FOR&#x27;, request.remote_addr)
 return &#x27;&lt;h1&gt; Your IP address is:&#x27; + ip_addr
</code></pre>

 <div class="alert alert-note">
 <div class="flex">
 
 <div class="flex-shrink-0 mr-3">
 <img src="/assets/images/icon-information-circle-solid.svg" class="icon" aria-hidden="true" />
 </div>
 
 <div class="w-full">
 Note: Trying to get a value from a non-existent key returns <code>None</code>. The <code>get()</code> method of the <code>dict</code> class allows you to set a default value to be used if <code>get()</code> fails to find a key. If <code>HTTP_X_FORWARDED_FOR</code> is <code>None</code>, the <code>request.remote_addr</code> value is used instead.

 </div>
 </div>
 </div>
 With these in mind, we can choose from any of the following three approaches to get the IP address of a user, via their incoming request:
<pre><code class="hljs">from flask import Flask, request

app = Flask(__name__)

@app.route(&#x27;/&#x27;)
def hello_world():
 ip_addr = request.remote_addr
 return &#x27;&lt;h1&gt; Your IP address is:&#x27; + ip_addr


@app.route(&#x27;/client&#x27;)
def client():
 ip_addr = request.environ[&#x27;REMOTE_ADDR&#x27;]
 return &#x27;&lt;h1&gt; Your IP address is:&#x27; + ip_addr


@app.route(&#x27;/client&#x27;)
def proxy_client():
 ip_addr = request.environ.get(&#x27;HTTP_X_FORWARDED_FOR&#x27;, request.remote_addr)
 return &#x27;&lt;h1&gt; Your IP address is:&#x27; + ip_addr


if __name__ == &#x27;__main__&#x27;:
 app.run(debug=True)
</code></pre>
<h2 id="conclusion">Conclusion</h2>
In this article, we learned three different methods to get the IP address of a user. We access the remote address directly with <code>request.remote_addr</code>, through the <code>REMOTE_ADDR</code> key from <code>request.environ</code>, and in the cases where the user is using a proxy we should check the <code>HTTP_X_FORWARDED_FOR</code> key of <code>request.environ</code>.

Mohammad Waseem

In this tutorial, we will take a look at how to get the IP address of the user who visits a web application created using Flask.

How To Get User's IP Address Using Flask?

Jovana Ninkovic

In this definitive, detailed guide - learn all you need to know about Java Collectors' collectingAndThen(), whether it's efficient, and try out a hands-on data analysis project.