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How to find last non-empty index in an array

I am using an input file to populate my array with different strings. how do I check up to where the array is filled? I initialize the size of my array to 30000, but how do I check up to which index it contains a string?
Thank you for your help!

Low-level approach: while populating the array, maintain a count variable, adding 1 to it on every element you insert into the array.
Professional approach: instead of an array, use a java.util.List, e.g. a java.util.ArrayList. List instances do the counting for you (and by the way, don't waste 25000 empty elements if your file only contains 5000).

Remove all the strings in an array list containing certain characters

so i have an array list that contains strings such as:
ArrayList<String> list = new ArrayList<>();
list.add("bookshelf");
list.add("bookstore");
list.add("library");
list.add("pencil");
Now i wanna search and remove all the strings in the arraylist that contain the word "book" in them. As far as i understand list.remove("book"); will only search for the particular string "book" and not the strings that contain the word "book". How can i solve this?

You can use removeIf like this:
list.removeIf(s -> s.contains("book"));

Note: this answers applies to Java version 7 and below (of course that it will work for higher versions as well but YCF_L's answer is simpler to implement in versions 8 and above).
The requirement is to iterate the list, check every element, and if it answers a certain condition: remove it.
Since this is the case we fall into a risky scenario where we modify the list while iterating it which is problematic because when we remove an element in the list its size changes.
In order to work around this problem we can iterate the list by index from the last element and back until the first one, this way, removing an element at index n will not effect accessing any element at index < n.
I'll leave the implementation details to you in order not to "spoon feed" and destroy your exercise :)

String array Categorizing

I have a string array where I want to check each element in the array with 8 other array elements to see if any of those element in the first array element categorize under any of them.
Simply I want to categorize a string array. So in order to do that I have to check with 8 other arrays (Because I have 8 categories) I want to know an efficient way to do this without looping one by one

You can use HashMap instead of array or arraylist.
Implement the 8 categories as HashMap.
From the element you want to check, match it against the the 8 HashMaps. This will give you a worst case of 8 checks.
If you can combine the values of all 8 categories into one HashMap, the worst case will be 1.
I believe you could combine the values from various categories and have only 1 HashMap by appending a checksum to each value. Something like:
//values in the hash map
xxxx_cat1
yyyy_cat1
zzzz_cat1
xxxx_cat2
yyyy_cat2
zzzz_cat2
After you get the value from the HashMap, just based on the checksum (the appended text) to get its category.

You can sort the array and then use Arrays.binarySearch() method instead of looping one by one. It is more efficient way to search a particular element.

Remove element from Array Java

I am currently making a tiny little program that should be able to select a random element i put into the array using a Random of course (For practice purposes) and when a element in the array has been chosen at random. i want to remove this element in the array, so how do you remove a element in a array the easiest way?
Its the only thing i want to know. I got everything else sorted. It's just removing the element it has chosen (the random takes a random number between 0 and the amount of elements in the array, so if it chooses 0, it will take the first element in the array, and so on)

You can use ArrayList which support remove or add function, which actually is an resizable array.

You can't remove an element from an array. You can replace it with some other value that indicates "nothing", null for example.

An easy solution is to convert the array into a list.
list = Arrays.asList(array);
Remove any element from the list and then revert it back to an array using
array = list.toArray();
Hope it helps.

Use List instead of Array, and if you want to stay on Array than there is 2 solution,
Create another Array ignoring your element which you want to delete.
create a List using Array.asList(...) than remove element from list and convert back to Array.
but according to me its better for you as well as java to use List. because List provide a many build-in functions.

Arraylist vs Array with custom objects in java

I am trying to convert a section of code from using an ArrayList of custom objects to a regular array.
Previous my definition was
ArrayList<Room> rooms = new ArrayList<Room>();
Which I have now changed to
Room[] rooms;
Previously I used the below line to add items to the array list
rooms.add(new Room(1,1,30,false,true,true,false));
But I am now struggling to find the way I should simply add individual items to the array throughout code.

I think you are best sticking with an arrayList here, but just to give you a bit more light on it.
To do what you are trying to do, you will have to keep a index integer which will just point to the current position in the array, then when you add you can increment this and add the new object into the next poisition.
When you get to the maximum size of your array, you will need to expand it.
You will find that there has been questions on expanding an array which have been asked already and you can find the answers here:
Expanding an Array?

If you can live with a fixed-size array, that gives you at least a slight chance of success. If not, you can't beat ArrayList and if your mission is to succeed without reimplementing it, then it is an impossible mission.
You should really give more insight into the exact rationale for rewriting your code like that, it would give us some chance to properly help you.

I can recommend:
Use Arraylist as Long you Need to insert Elements. Once th Array is final Convert to Array.

If you need to increase the size of the array when adding another element, you have to construct another array with the size of the old array +1.
Afterwards, you would copy the contents of the old array over to the new array (the bigger array).