How to join when using map with filter with stream

Question

I want to create a query if the getData() return "yes". So added the filter for the same.

Now if the value of getData() is "yes" then I want to concat the three parameter which I am getting from the object.

Now for the list of object I want to join all the value which is coming from

(f -> f.getF()+" "+f.getO()+" "+f.getV())

with " and " as delimiter but not able to figure it out.

String query = data.stream()
    .filter(f -> f.getData().equalsIgnoreCase("yes"))
    .map(f -> f.getF()+" "+f.getO()+" "+f.getV())
    .collect(Collectors.joining(" and","how to do this" , suffix));

for example, we are getting data from that object as

f.getF()--------"column name"

f.getO()--------"="

f.getV()--------"2"

so I want to add all the condition with and delimiter and produce a single String with all the conditions.

I think I can append "and" in the map() and later remove the last "and" from the string

something like this

columnname = 2 and columnnam1 > 1 and column2 = 6

I think you need to add an example with the values of `getF()`, `getO()`, `getV()`, and the expected output. — ernest_k, Jul 14 '20 at 08:09
Then all you need is a simple `Collectors.joining(" and ")`. No need for rocket science here. — Amongalen, Jul 14 '20 at 08:22
bro , I am still learning, sometimes you know the concept but missed on the side how to apply the same :) — sparsh610, Jul 14 '20 at 08:42
Does this answer your question? [Java: convert List to a String](https://stackoverflow.com/questions/1751844/java-convert-liststring-to-a-string) — Eklavya, Jul 14 '20 at 08:46

Vladimir Vagaytsev · Accepted Answer · 2020-07-14T08:22:54.980

If F is a column name, O is an operator and V is a value and if you need to get expression like column1 = 1 and column2 > 2 and ... then you just need to use Collectors.joining(" and "):

String query = data.stream() // stream1 - original stream of the data collection
    .filter(f -> f.getData().equalsIgnoreCase("yes")) // stream2 - contains only elements with "yes"
    .map(f -> f.getF()+" "+f.getO()+" "+f.getV()) // stream3 - stream of the strings produced by expression f.getF()+" "+f.getO()+" "+f.getV()
    .collect(Collectors.joining(" and "));

Some explanation. Here the .collect(Collectors.joining(...)) joins the elements of the stream produced by the previous .map(f -> f.getF()+" "+f.getO()+" "+f.getV()) call.

There are 3 different streams created:

String query = data.stream() // stream1 - original stream of the data collection
    .filter(f -> f.getData().equalsIgnoreCase("yes")) // stream2 - contains only elements with "yes"
    .map(f -> f.getF()+" "+f.getO()+" "+f.getV()) // stream3 - stream of the strings produced by expression f.getF()+" "+f.getO()+" "+f.getV()
    .collect(Collectors.joining(" and","how to do this" , suffix));

So, first you join f.getF(), f.getO() and f.getV() with " " joiner as follows:

.map(f -> f.getF()+" "+f.getO()+" "+f.getV())

You'll get the stream of the string like

"F1 O1 V1",
"F2 O2 V2",
...

Tis stream will be joined by .collect(Collectors.joining(...));. For example, using .collect(Collectors.joining(" and ")); will produce string like

"F1 O1 V1 and F2 O2 V2 and ..."

Updated the answer to make it more structured and to address the updated question. — Vladimir Vagaytsev, Jul 14 '20 at 08:25
Hi , just need little help, what is the best way to check if the f is not null, another filter before f->f.getData="yes" ? — sparsh610, Jul 14 '20 at 08:38
Hi, you can use `.filter(Objects::nonNull)` for that. If you need to check `getData()` result for nullability, then use `.filter(f -> f != null && f.getData() != null)` — Vladimir Vagaytsev, Jul 14 '20 at 08:42

Lino · Answer 2 · 2020-07-14T10:40:35.507

3

You were almost there:

String query = data.stream()
    .filter(f -> f.getData().equalsIgnoreCase("yes"))
    .map(f -> f.getF() + " " + f.getO() + " " + f.getV()))
    .collect(Collectors.joining(" and "));

Which will produce something like this:

columnName1 = 2 and columnName2 > 5

edited Jul 14 '20 at 10:40

answered Jul 14 '20 at 08:24

Lino

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How to join when using map with filter with stream

2 Answers2