add test for compact and compact! in java_integrations

Fixes #3772

lib/logstash/java_integration.rb

@@ -81,12 +81,18 @@ module java::util::Collection
   def compact
     duped = Java::JavaUtil::ArrayList.new(self)
     duped.compact!
+    duped
   end
 
   def compact!
+    size_before = self.size
     self.removeAll(java::util::Collections.singleton(nil))
+    if size_before == self.size
+      nil
+    else
       self
     end
+  end
 
   # support the Ruby intersection method on Java Collection
   def &(other)

rakelib/test.rake

@@ -29,6 +29,11 @@ namespace "test" do
     exit(Spec::Core::Runner.run(["--fail-fast", Rake::FileList["spec/**/*_spec.rb"]]))
   end
 
+  desc "run core specs on a single file"
+  task "core-single-file", [:specfile] => ["setup"] do |t,args|
+    exit(RSpec::Core::Runner.run([Rake::FileList[args.specfile]]))
+  end
+
   desc "run all installed plugins specs"
   task "plugins" => ["setup"] do
     # grab all spec files using the live plugins gem specs. this allows correclty also running the specs

spec/lib/logstash/java_integration_spec.rb

@@ -211,6 +211,36 @@ describe "Java integration" do
         end
       end
     end
+
+    context "when compacting" do
+      context "#compact with nils" do
+        let(:initial_array) { [1,2,3,nil,nil,6] }
+        it "should remove nil values from a copy" do
+          expect(subject.compact).to eq([1,2,3,6])
+          expect(subject).to eq([1,2,3,nil,nil,6])
+        end
+      end
+
+      context "#compact! with nils" do
+        let(:initial_array) { [1,2,3,nil,nil,6] }
+        it "should remove nil values" do
+          expect(subject.compact!).to eq([1,2,3,6])
+          expect(subject).to eq([1,2,3,6])
+        end
+
+        it "should return the original" do
+          expect(subject.compact!.object_id).to eq(subject.object_id)
+        end
+      end
+
+      context "#compact! without nils" do
+        let(:initial_array) { [1,2,3,6] }
+        it "should return nil" do
+          expect(subject.compact!).to be nil
+          expect(subject).to eq([1,2,3,6])
+        end
+      end
+    end
   end
 
   context "Enumerable implementation" do