Iterate over a List of objects in java-CodePudding

I would like to iterate over a csv file and save rows to the database with Spring CrudRepository. From my data I only need the data object, error and meta is not important. When I run a for-loop on my data it looks like this:

for (int i = 0; i < file.size(); i  ) {
            System.out.println(file.get(i));
        }

[{data=[column1,column2,column3,column4], errors=[], meta={delimiter=,, linebreak=
, aborted=false, truncated=false, cursor=41}}
{data=[data1,data2,data3,data4], errors=[], meta={delimiter=,, linebreak=
, aborted=false, truncated=false, cursor=78}}
{data=[data1,data2,data3,data4], errors=[], meta={delimiter=,, linebreak=
, aborted=false, truncated=false, cursor=134}}]

I tried to write an other loop in the first one, unfortunately it didn't work. How could I read and reference to this section of my list?

[column1,column2,column3,column4],[data1,data2,data3,data4],[data1,data2,data3,data4]

Edit to make it clear: I get a csv file from user on frontend, it should has 4 columns. Something like this one:

When I send it to the backend I got this List collection:

On my frontend:

On my backend:

CodePudding user response：

You should find out what the objects that you are iterating look like before trying to iterate them. In Java, you can get the type of an object o using o.getClass(), which when printed will tell you the name of the class so you can look up its API. Using a debugger can also let you explore inspect any object (and its class), without having to print anything.

Whatever the class of those objects, they appear to have data, errors, and a meta field - so maybe you can iterate the data by using:

for (int i = 0; i < file.size(); i  ) {
  // file.get(i) probably has a getData() method
  for (Object o : file.get(i).getData()) {
     System.out.println(o);
  }
}

CodePudding user response：

Use this to parse array or every element of array.

 for (int i = 0; i < file.size(); i  ) {
    for (Object key : file.get(i).keySet()) {
        String keyString = (String) key;
        Object keyvalue = file.get(i).get(keyString);
        if(keyvalue instanceof JSONArray){
        JSONArray array = (JSONArray) keyvalue;
        for (int j = 0; j < array.length(); j  ) {
           System.out.println(array.get(j));
        }
     }
}