I'm having big problem with making Java stream work the way I want it to. I have a class with fields: title(string), value(BigDecimal). I get objects of this class in a form of list. Titles on the list are not unique, and I want to get list where titles ARE unique, and values for the same titles(of input list) are summed.
So I make something like:
Map<String, BigDecimal> groupedAndSummed = inputList.stream()
.collect(Collectors.groupingBy(MyClass::getTitle,
Collectors.reducing(BigDecimal.ZERO, MyClass::getValue, BigDecimal::add)));
From what I found out - such code should return Map where titles are keys(so they are unique), values are values, but for the same keys they are summed. I tried also:
Map<String, BigDecimal> groupedAndSummed = inputList.stream()
.collect(Collectors.toMap(MyClass::getTitle, MyClass::getValue, BigDecimal::add));
Which from my point of view should do the same thing. So, once again, I get list with objects, that can have the same titles. I want to get list with objects where titles are unique, and for initial objects with the same titles I want to make sum of values of duplicates. I've searched for solutions, and the way I'm doing it should work, but it doesn't. I get list of objects with distinct titles, but theirs values are not summed. It's always simply first value to be summed, from the all that should be summed, for that title. I dont know where I'm doing something wrong. When I do it manually it works, I use something like
sum = sum.add(object.getValue());
And the sums are ok after whole loop. The problem is, that I'm looking for automatic way to do it, no some nested loops in loops checking suspicious conditions. I would like it to work with stream. Can anyone help?
Related
I am doing a project for my AP Computer Science class. The current project asks us to create a rip off version of excel that prints to the console. I am wondering if theres a way for me to create an arraylist of empty values that I can assign cell names to (A1, A2, F6, etc.) and then use those names to call values from their place in the arraylist. For example, I'd give the cell D4 a value of 6, store that 6 in an arraylist, then call it back using "D6".
A link to the directions of the project: https://issaquahwednet-my.sharepoint.com/:w:/g/personal/stutlerk_issaquah_wednet_edu/EQOW8BFzHIhIsdvXGP-qKDsBbN7BFa-kUCiMeeq9BZbbwg?e=Nc1Jcs
Maybe I'm not making sense, for which I am very sorry.
//what i would like to be able to do:
arraylist "cells" = (A1 through L20)
user input = "A1 = hi"
set index "A1" of "cells" to "hi"
repeat that stuff until user input = quit
is this possible? or would I just have to create two arraylists, one for cell numbers and one for values? I guess I could do that and then if a user wants to see what a cell says then they could Say "A1" and I could search my cell arraylist for a value of A1 then compare that index with the other arraylist.
I'm completely lost one this and, again, sorry if I'm not making any sense.
To do what you want, you'd need a map rather than a list. Lists always have integer numbers as the indices and there's no way to change that. (You could technically do this with two arrayLists, but it would be a pain to maintain it so that "A1" and "hi" stayed in the same index on two different lists if you ever had to delete/move anything.)
The HashMap documentation is here: https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html
And you'd do something like this:
Map<String, String> cells = new HashMap<String,String>();
cells.put("A1", "hi");
That would create a key-value pair with key "A1" and value "hi".
You can use structure map where for every key you have associated a value.
https://docs.oracle.com/javase/8/docs/api/java/util/Map.html
The problem I'm facing is more like of algorithmic nature.
Let's say that I have a list of pair objects containing integers. Is there a way to sort the list so that the second part of the pair is equal to first part of the next pair?
For instance given this list of pairs:
A = Pair(2,1),Pair(2,3),Pair(1,3).
After sorting the list becomes:
A = Pair(1,3), Pair(3,2),Pair(2,1).
As you can see it is allowed to change the order of values inside the pair like the Pair(2,3) which became Pair(3,2).
I though about using comparator or comparable interfaces but they dont cover complex cases like the above.
I'm completely new to programming and to java in particular and I am trying to determine which data structure to use for a specific situation. Since I'm not familiar with Data Structures in general, I have no idea what structure does what and what the limitations are with each.
So I have a CSV file with a bunch of items on it, lets say Characters and matching Numbers. So my list looks like this:
A,1,B,2,B,3,C,4,D,5,E,6,E,7,E,8,E,9,F,10......etc.
I need to be able to read this in, and then:
1)display just the letters or just the numbers sorted alphabetically or numerically
2)search to see if an element is contained in either list.
3)search to see if an element pair (for example A - 1 or B-10) is contained in the matching list.
Think of it as an excel spreadsheet with two columns. I need to be able to sort by either column while maintaining the relationship and I need to be able to do an IF column A = some variable AND the corresponding column B contains some other variable, then do such and such.
I need to also be able to insert a pair into the original list at any location. So insert A into list 1 and insert 10 into list 2 but make sure they retain the relationship A-10.
I hope this makes sense and thank you for any help! I am working on purchasing a Data Structures in Java book to work through and trying to sign up for the class at our local college but its only offered every spring...
You could use two sorted Maps such as TreeMap.
One would map Characters to numbers (Map<Character,Number> or something similar). The other would perform the reverse mapping (Map<Number, Character>)
Let's look at your requirements:
1)display just the letters or just the numbers sorted alphabetically
or numerically
Just iterate over one of the maps. The iteration will be ordered.
2)search to see if an element is contained in either list.
Just check the corresponding map. Looking for a number? Check the Map whose keys are numbers.
3)search to see if an element pair (for example A - 1 or B-10) is
contained in the matching list.
Just get() the value for A from the Character map, and check whether that value is 10. If so, then A-10 exists. If there's no value, or the value is not 10, then A-10 doesn't exist.
When adding or removing elements you'd need to take care to modify both maps to keep them in sync.
I have a HashMap of this type
Map<String, Set<String>> list_names = new HashMap<String,Set<String>>();
that I have constructed and added its elements from a txt file that has a list's name and a set of names in it.
98298XSD98 N9823923 N123781 N723872 ....
13214FS923 N9818324 N982389
... ...
I made another HashMap, called names_list that pretty much replaces the order of the list_names HashMap such that I can get all the lists that a given name is in.
now the HashMap I have is pretty big, and there are over 400k items and 60k lists.
somewhere in my code im trying to get the Set of different lists many many times and then getting the intersection of these two lists for computational purposes,
a_list = this.names_lists.get(a);
b_list = this.names_lists.get(b);
// printing lists
//intersection stuff
but whats weird is the HashMap didn't recognizance one of its keys(or maybe many of its keys) and treated it as null after one retrieval or sometimes 0 retrievals.
a:0122211029:[R3DDZP35ERRSA] b:1159829805:[R3ALX1GRMY5IMX, R1204YEYK4MBCA]
a:0122211029:[] b:1593072570:[R222JSDL42MS64]
here, im just printing the name and names_list.get(key).toString();
and yes i'm printing these before doing any intersection stuff.
any idea why is it doing that?
When you calculate the intersection of two sets, you actually modify one of the sets. You have to create a temporary set to hold the intersection, e.g.:
a_list = this.names_lists.get(a);
b_list = this.names_lists.get(b);
Set<String> intersection = new HashSet<>(a_list).retainAll(b_list);
Currently I'm using an ArrayList to store a list of elements, whereby I will need to insert new elements at specific positions. There is a need for me to enter elements at a position larger than the current size. For e.g:
ArrayList<String> arr = new ArrayList<String>();
arr.add(3,"hi");
Now I already know there will be an OutOfBoundsException. Is there another way or another object where I can do this while still keeping the order? This is because I have methods that finds elements based on their index. For e.g.:
ArrayList<String> arr = new ArrayList<String>();
arr.add("hi");
arr.add(0,"hello");
I would expect to find "hi" at index 1 instead of index 0 now.
So in summary, short of manually inserting null into the elements in-between, is there any way to satisfy these two requirements:
Insert elements into position larger than current size
Push existing elements to the right when I insert elements in the middle of the list
I've looked at Java ArrayList add item outside current size, as well as HashMap, but HashMap doesn't satisfy my second criteria. Any help would be greatly appreciated.
P.S. Performance is not really an issue right now.
UPDATE: There have been some questions on why I have these particular requirements, it is because I'm working on operational transformation, where I'm inserting a set of operations into, say, my list (a math formula). Each operation contains a string. As I insert/delete strings into my list, I will dynamically update the unapplied operations (if necessary) through the tracking of each operation that has already been applied. My current solution now is to use a subclass of ArrayList and override some of the methods. I would certainly like to know if there is a more elegant way of doing so though.
Your requirements are contradictory:
... I will need to insert new elements at specific positions.
There is a need for me to enter elements at a position larger than the current size.
These imply that positions are stable; i.e. that an element at a given position remains at that position.
I would expect to find "hi" at index 1 instead of index 0 now.
This states that positions are not stable under some circumstances.
You really need to make up your mind which alternative you need.
If you must have stable positions, use a TreeMap or HashMap. (A TreeMap allows you to iterate the keys in order, but at the cost of more expensive insertion and lookup ... for a large collection.) If necessary, use a "position" key type that allows you to "always" generate a new key that goes between any existing pair of keys.
If you don't have to have stable positions, use an ArrayList, and deal with the case where you have to insert beyond the end position using append.
I fail to see how it is sensible for positions to be stable if you insert beyond the end, and allow instability if you insert in the middle. (Besides, the latter is going to make the former unstable eventually ...)
even you can use TreeMap for maintaining order of keys.
First and foremost, I would say use Map instead of List. I guess your problem can be solved in better way if you use Map. But in any case if you really want to do this with Arraylist
ArrayList<String> a = new ArrayList<String>(); //Create empty list
a.addAll(Arrays.asList( new String[100])); // add n number of strings, actually null . here n is 100, but you will have to decide the ideal value of this, depending upon your requirement.
a.add(7,"hello");
a.add(2,"hi");
a.add(1,"hi2");
Use Vector class to solve this issue.
Vector vector = new Vector();
vector.setSize(100);
vector.set(98, "a");
When "setSize" is set to 100 then all 100 elements gets initialized with null values.
For those who are still dealing with this, you may do it like this.
Object[] array= new Object[10];
array[0]="1";
array[3]= "3";
array[2]="2";
array[7]="7";
List<Object> list= Arrays.asList(array);
But the thing is you need to identify the total size first, this should be just a comment but I do not have much reputation to do that.