Java 8 Applying stream filter based on a condition

Question

In Java 8, is there a way to apply the filter on a stream based on a condition,

example

I have this stream

if (isAccessDisplayEnabled) {
     src = (List < Source > ) sourceMeta.getAllSources.parallelStream()
         .filter(k - > isAccessDisplayEnabled((Source) k))
         .filter(k - > containsAll((Source) k, substrings, searchString))
         .collect(Collectors.toList());
 } else {
     src = (List < Source > ) sourceMeta.getAllSources.parallelStream()
         .filter(k - > containsAll((Source) k, substrings, searchString))
         .collect(Collectors.toList());
 }

I am adding the filter

.filter(k - > isAccessDisplayEnabled((Source) k)))

on the stream based on the if-else condition. Is there a way to avoid that if-else, since if there are more filters coming up,then it will be hard to maintain.

Please let me know

Holger · Accepted Answer · 2017-11-29 15:06:18Z

54

One way to do it is

Stream<Source> stream = sourceMeta.getAllSources.parallelStream().map(x -> (Source)x);
if(isAccessDisplayEnabled) stream = stream.filter(s -> isAccessDisplayEnabled(s));
src = stream.filter(s - > containsAll(s, substrings, searchString))
            .collect(Collectors.toList());

another

 src = sourceMeta.getAllSources.parallelStream().map(x -> (Source)x)
     .filter(isAccessDisplayEnabled? s - > isAccessDisplayEnabled(s): s -> true)
     .filter(s - > containsAll(s, substrings, searchString))
     .collect(Collectors.toList());

In either case, note how performing one type cast at the beginning simplifies the entire stream pipline.

Both solutions avoid re-evaluating isAccessDisplayEnabled for every stream element, however, the second relies on the JVM’s capability of inlining s -> true when this code turns out to be performance critical.

edited Nov 29, 2017 at 15:06

answered Feb 23, 2017 at 20:29

Holger

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8 Comments

Umar Over a year ago

I am not able to understand this line, isAccessDisplayEnabled? s -> true: s - > isAccessDisplayEnabled(s), please can you explain a bit more or to a blog that explains it.

Holger Over a year ago

Try reading it as isAccessDisplayEnabled? (s -> true): (s -> isAccessDisplayEnabled(s)). If isAccessDisplayEnabled is true, use the lambda expression s -> true, which is effectively like having no filter, otherwise, use s - > isAccessDisplayEnabled(s). It’s a combination of the ternary operator with two alternative lambda expressions.

sandwood Over a year ago

@Holder : ok for ternary but order is wrong , no ? I guess if you have a boolean isAccessDisplayEnabled then you want to filter, otherwise if false you do not filter. ` isAccessDisplayEnabled? (s -> isAccessDisplayEnabled(s)) : (s -> true) `

Cherry Over a year ago

Why not to use filter(!isAccessDisplayEnabled || isAccessDisplayEnabled(s)) ?

Holger Over a year ago

@Cherry the syntax would be filter(s -> !isAccessDisplayEnabled || isAccessDisplayEnabled(s)) and it would work, but as said in the answer, both solutions [of the answer] avoid re-evaluating isAccessDisplayEnabled for every stream element. Since isAccessDisplayEnabled doesn’t change throughout the entire operation, it would be a waste of CPU cycles to check this over and over again.

|

jchitel · Accepted Answer · 2017-02-23 19:19:30Z

5

Your condition has the same name as your method. I'm going to assume you meant for those to be different, so let's say it was this:

if (someCondition) {
    src = (List < Source > ) sourceMeta.getAllSources.parallelStream()
        .filter(k - > isAccessDisplayEnabled((Source) k))
        .filter(k - > containsAll((Source) k, substrings, searchString))
        .collect(Collectors.toList());
} else {
    src = (List < Source > ) sourceMeta.getAllSources.parallelStream()
        .filter(k - > containsAll((Source) k, substrings, searchString))
        .collect(Collectors.toList());
}

If you want to remove the if/else, you can instead perform the check in the first filter:

src = (List < Source > ) sourceMeta.getAllSources.parallelStream()
    .filter(k - > !someCondition || isAccessDisplayEnabled((Source) k))
    .filter(k - > containsAll((Source) k, substrings, searchString))
    .collect(Collectors.toList());

In the else case, you take everything and remove the isAccessDisplayEnabled() method call, so the condition is effectively "if someCondition is false or isAccessDisplayEnabled(k)". If someCondition comes out false, then the isAccessDisplayEnabled() check is skipped.

answered Feb 23, 2017 at 19:19

jchitel

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4 Comments

Holger Over a year ago

…but someCondition may get reevaluated for every element.

David Conrad Over a year ago

A variable and a method can have the same name in Java. Java is a Lisp 2 in that sense. ;)

jchitel Over a year ago

@DavidConrad I'm sure that's true, however I don't think he meant it that way. I just changed the variable name to clear things up.

Umar Over a year ago

Yes, you are correct, i miss typed variable and method have different names

Daniel Alder · Accepted Answer · 2024-08-29 13:10:50Z

A totally different approach compared to the other anwsers: You can split the fluent syntax of java stream to different parts, and do whatever you need to construct you own perfect pipeline. You can save any part of the stream definition in variables, it is not executed before you call the collect method.

Unverified code (you'll have to fix the type declaration):

Stream<whatever> unfinishedStream = sourceMeta.getAllSources.parallelStream();
if (isAccessDisplayEnabled) {
    unfinishedStream = unfinishedStream.filter(k - > isAccessDisplayEnabled((Source) k));
}
src = (List < Source > ) unfinishedStream 
     .filter(k - > containsAll((Source) k, substrings, searchString))
     .collect(Collectors.toList());

At first, this looks like the most optimal solution. But I heard that java does quite extreme compile-time stream optimization, by transforming them to simple loops. If you start constructing your pipeline dynamically as shown above, Java will likely fall back to simply executing what you defined, with a lot of method calls and internal structure. But I didn't do any benchmarks.

We use this kind of programming a lot for dynamic SQL statements with configurable filters, but there's no such compile-time optimization.

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