Make inspecting collections a bit more friendly

Fixes #3474

lib/logstash/java_integration.rb

@@ -92,4 +92,8 @@ module java::util::Collection
     duped.addAll(other)
     duped
   end
+
+  def inspect
+    "<#{self.class.name}:#{self.hashCode} #{self.to_a(&:inspect)}>"
+  end
 end

spec/lib/logstash/java_integration_spec.rb

@@ -82,6 +82,23 @@ describe "Java integration" do
   context "Java::JavaUtil::Collection" do
     subject{Java::JavaUtil::ArrayList.new(initial_array)}
 
+    context "when inspecting" do
+      let(:items) { [:a, {:b => :c}] }
+      subject { java.util.ArrayList.new(items) }
+
+      it "should include the contents of the Collection" do
+        expect(subject.inspect).to include(items.inspect)
+      end
+
+      it "should include the class name" do
+        expect(subject.inspect).to include("ArrayList")
+      end
+
+      it "should include the hash code of the collection" do
+        expect(subject.inspect).to include(subject.hashCode.to_s)
+      end
+    end
+
     context "when deleting a unique instance" do
       let(:initial_array) {["foo", "bar"]}