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Get a value from an input in JavaScript and use it in Java

I work on an application and I want to connect the front end (JavaScript) with the back end (Java). I have an input in HTML. When a user inputs something, by clicking on a button, I want to get that value in Java, so I can perform some actions on it. How can I do this? Thank you!
<input type="text" id="input" placeholder="Input...">
<button type="button" id="button" onclick="loadResults()">Search</button>

the easiest way I can see is getting the value from the input using the id property, like so:
const inputValue = document.getElementById("input").value
and then it is expected that you have some kind of API setup on your java backend, if so you can use the fetch built in function to make a HTTP request to you java backend
https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API

Sending data to php script without ajax

I am required to send data from an html page, via input elements to a php script and I cannot use ajax for some reason. How to accomplish this?
My code is something like this:
HTML:
First Name : <input type"text" name="first_name">
<br><br>
Last Name : <input type"text" name="last_name">
<br><br>
<button id="submit_btn">Submit</button>
Now, I want the javascript to be something like :
function redirect_from_here() {
close(); //close the current window
window.location='./phpfile.php'; //load the new page which will process the data sent to it.
}
My question is how do I send the value in the input elements in the HTML portion, as data to be processed, to the php script (phpfile.php in this case).
Please note that I prefer not to use html form for doing the job.

You are using HTML form as there are input fields in you code.
Inputs are part of a form and should be wrapped by a form element
Why using JS for submitting the form when you can use <form action='script.php'.. for that?
I suggest that you revise your requirements instead of trying to come up with a hackish way of how to send the data..

Just submit a form with action="phpfile.php"
if not interested with html,
then in the window location bind the values as a GET form method do

getting value of input box on another page

I have two pages; page1 and page2. page1 holds a form while page2 has some images. I want to enter a sentence into the form on page1 and get the value of that text box on page2.
the form on page1
<form name="input" action="page2.html" method="get">
<input type="text" name="textbox" id="textbox">
<input type="submit" value="Submit">
</form>
what would be the proper way to go about this?
Would I use the get method along with the action method to send the value through the url and then extract it using window.location.href then splitting the text after the ? or is there a simpler way of achieving this?
Thanks

Just send it in the query string and read it in page2, as you said.
See this to have an idea of how to read the query string parameter:
How can I get query string values in JavaScript?

Your way of doing this would work, although it's certainly not a good long term solution. If your site is going to get more complicated, it would be better to use a server-side language like .NET, PHP, Ruby, etc.

If you have to get thed data using only JS, then you need to use URL. Otherwise you can use server side script like jsp.
When you use URL, make sure the text is URL-encoded before appending to the URL.

Accessing data before sending it to server with Form.Request?

Is there a way to access the whole data array/object (which contains all the data form input fields etc) before sending it to the server with Form.Request? I know there is the extraData option which allows you to pass in other data, but I would like to access the actual form data and to some manipulation with it.

Before being sent to the server? I'm not entirely sure, the send method attached to Form.Request sends in the form itself, and the data, but this is probably after the request has already been fired off.
I tend not to use Form.Request, precisely because I have to do these form manipulations and such.
Instead of using a Form.Request Class, I simply iterate through the form's inputs as necessary.
e.g.
<form id="foobar">
<input type="text" id="lorem" />
<select id="ipsum">
<option>...
</select>
</form>
<script>
var formValues = document.id('foobar').getElements('input, select');
</script>

Making sticky javascript

I have a javascript form where I am using innerHTML to set some text.
When the form submits that information is lost.Is there anyway I can make it "sticky".I was thinking a cookie but that's about all I know.
Thanks
<form "action="" name="myform">
<input type="text" name='name">
<div id="theName"></div>
</form>
Quick example I am capturing the name and need the div to show the name after the form submits.

You will need to persist the data somehow. There are several options:
Store it on the server. When the form is submitted, your server-side script will receive the data; it can persist it in a database, session variable, or some other form of storage that's appropriate for your application. Whenever the client re-visits the page with the form, have the server generate the form's HTML with the persisted data.
Use HTML5's local storage. While not supported in legacy browsers, all modern ones provide the local storage API. When the user submits the form (attach an event listener to the form's "submit" event), you can store the form data by making calls to localStorage[key] = value and retrieving it with localStorage[key].
Store it in a cookie. Although I don't recommend this approach, you can create a cookie with the form data. The only restriction is that the data needs to be represented as a string, but I recommend JSON. However, you probably should not use this approach since cookies are sent to the server for each request; if the form fields contain a lot of data, then you're also unnecessarily sending a lot of data to the server.
Using HTML5's local storage gives you a self-encapsulated approach that requires no server-side configuration:
<form action="" name="myform">
<input type="text" name="name">
<div id="theName"></div>
</form>
<script type="text/javascript">
(function() {
var form = document.getElementsByName('myform')[0];
if (localStorage['name'] !== undefined) {
var displayArea = document.getElementById('theName');
displayArea.textContent = localStorage['name'];
}
form.addEventListener('submit', function() {
var nameField = document.getElementsByName('name')[0];
localStorage['name'] = nameField.value;
}, false);
})();
</script>

Are you setting the "value" attribute of the input tags to something or blank? you can just remove (remove the attribute itself) that so that the last value set will be used (true only for non-password type inputs. also, haven't tried it in all browsers.).
Or better yet, you can use serverside script like (PHP, ASP, RUBY, etc) to set the attribute value to the previously submitted.
<form method="post">
<input type="text" name="txtinput" id="txtinput" value="<?php echo $_POST['txtinput']?>"/>
<input type="submit" value="submit">
</form>
doing it in js only is much more complicated and unreliable since your going to use cookies.
PS: I'm assuming your not using XHR(AJAX) to submit your forms since XHR's don't refresh pages or re-initializes inputs unless you told them to.

This should be happening server-side. Javascript is for enhancing a page, it's not to be depended on for data manipulation.
Your script, converted to PHP, would look like
<form action="" method="post" name="myform">
<input type="text" name='name">
<div id="theName"><?php
if(isset($_POST['name'])) { echo $_POST['name']; }
?></div>
</form>
...and it would work every time, without having to call any JS.
You'll have to handle the form data somehow anyway - how were you intending to retrieve the data without a server-side script?