How can I add the same key to an ArrayList of Maps<String, String>

Question

Is there anyway to have a map defined as Map<String, String> that can be added to an array list defined as List<Map<String, String> that has the same key? Ideally we would have an arraylist of maps

[{firstName=john}, {firstName=Kelly}, {firstName=Jack}]

The reason I need the same key is for react.js ui I will need to map over these items and I need to have the same key in each object so I can call

List.map(key, index) = > key.firstName

To build the ui component. Is this possible?

Map<String,String> names = new HashMap<>();

List<Map<String,String> firstNamesLastNamesList = new ArrayList<>();

while(resultSet.next()){
names.put("firstName", resultSet.getString("firstName");
names.put("lastName", resultSet.getString("lastName");
firstNamesLastNamesList.add(names); 
}

The values to populate each map will come from a database. Currently when I add each object to the List of maps as you might imagine, the previous map is overwritten due to the same key of firstName. I am left with a List like [{firstName=Jack, lastName=Hammer}, {firstName=Jack, lastName=Hammer}, {firstName=Jack, lastName=Hammer}] not [{firstName=John, lastName=Carpenter}, {firstName=Kelly, lastName=Johnson}, {firstName=Jack, lastName=Black}] which is the desired out put. Any help would be greatly appreciated.

The key/value mappings will overwrite, if you give a new key/value with the same key, within a Map. That's just how they work. But adding those maps to a list should be independent of that. — djb
– djb, Commented Oct 21, 2022 at 3:25

devin · Accepted Answer · 2022-10-21 03:43:42Z

1

Your code is doing what is expected. the object that you are creating is before the loop

Map<String,String> names = new HashMap<>();

when you create this object you are saying make a new object called names at memory address 0x84927 - some random address.

then you are adding a reference to that object at index i for each item in the list. when you change the object names the list will look for memory address 0x84927 for each index in the firstNamesLastNamesList.

you need to put that piece of code in the while loop

answered Oct 21, 2022 at 3:43

devin

4322 gold badges6 silver badges20 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

questionsneedanswers Over a year ago

right but wouldn't that be a lot of overhead? Each time creating a new object?

devin Over a year ago

@questionsneedanswers there is possibly a more elegant solution, possibly by using the stream() functionality that arrays have. I'd look into that but really you need to have some point in memory for each map

iamgirdhar · Accepted Answer · 2022-10-21 03:43:23Z

1

You can update your code like this:

Here,

You need to create a new Map for each resultSet inside the loop and then add it to the list. (And as you are creating map outside the loop which is creating a problem.)

List<Map<String,String>> firstNamesLastNamesList = new ArrayList<>();

        while(resultSet.next()){
            Map<String,String> names = new HashMap<>();
            names.put("firstName", resultSet.getString("firstName"));
            names.put("lastName", resultSet.getString("lastName"));
            firstNamesLastNamesList.add(names);
        }

edited Oct 21, 2022 at 3:43

answered Oct 21, 2022 at 3:37

iamgirdhar

1,1631 gold badge7 silver badges10 bronze badges

Collectives™ on Stack Overflow

How can I add the same key to an ArrayList of Maps<String, String>

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related