Adding elements to array list - Java

Question

I have a class file

public class Name {
    private String id;

    public String getId() {
        return id;
    }
}

This is the test file

CollectionType javaType = mapper.getTypeFactory().constructCollectionType(ArrayList.class, Name.class);
ArrayList<Names> nameList = mapper.readValue(json, javaType);
for (Names nameValue : nameList) {
    List<String> id = new ArrayList<>();
    id.add(nameValue.getId());
System.out.println(id);

json

[{"id":"1"},{"id":"2"}]

In the JSON output there is more than one id, and I am trying to store them in one list but it only adds the first one ("1"), how do I store both in one list. Please help

Where are you closing the for loop? Your identation doesn't help too much to figure it out. In every iteration you're creating a new List, take the id outside the for loop — Frakcool
– Frakcool, Commented Apr 29, 2021 at 21:20

Joris Kinable · Accepted Answer · 2021-04-29 21:20:51Z

1

The problem is that you iterate over the list of names, and inside the for loop you create a new ArrayList during every iteration! You want to create your ArrayList outside of the loop:

CollectionType javaType = mapper.getTypeFactory().constructCollectionType(ArrayList.class, Name.class);
ArrayList<Names> nameList = mapper.readValue(json, javaType);
List<String> id = new ArrayList<>();
for (Names nameValue : nameList) {
    id.add(nameValue.getId());
}
System.out.println(id);

answered Apr 29, 2021 at 21:20

Joris Kinable

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2 Comments

kaya Over a year ago

Hi, thank you for the answer. Is there a way to just print the last list without printing it twice as [1], [1,2]

kaya Over a year ago

Actually got it, just moved the println from for loop, thank you!

DLynch · Accepted Answer · 2021-04-29 21:21:11Z

0

Each iteration of the loop is creating a new id list. Declare that list outside of the loop and it will add the elements to it.

answered Apr 29, 2021 at 21:21

DLynch

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