Cleanup: extract Lists util class from duplicate code (#382)

* Add Lists class with firstValueOrEmpty and isNullOrEmpty
* Update KeyValueClient and SessionClient to use Lists class
* Change awkward assertions in CoordinateClientITest to use isNotNull

src/itest/java/org/kiwiproject/consul/CoordinateClientITest.java

@@ -1,6 +1,5 @@
 package org.kiwiproject.consul;
 
-import static java.util.Objects.isNull;
 import static org.assertj.core.api.Assertions.assertThat;
 
 import org.junit.jupiter.api.Test;
@@ -20,14 +19,14 @@ void shouldGetDatacenters() {
     @Test
     void shouldGetNodes() {
         List<Coordinate> nodes = client.coordinateClient().getNodes();
-        assertThat(isNull(nodes)).isFalse();
+        assertThat(nodes).isNotNull();
     }
 
     @Test
     void shouldGetNodesForDatacenter() {
         String datacenter = client.coordinateClient().getDatacenters().get(0).getDatacenter();
 
         List<Coordinate> nodes = client.coordinateClient().getNodes(datacenter);
-        assertThat(isNull(nodes)).isFalse();
+        assertThat(nodes).isNotNull();
     }
 }

src/main/java/org/kiwiproject/consul/KeyValueClient.java

@@ -3,6 +3,7 @@
 import static com.google.common.base.Preconditions.checkArgument;
 import static java.util.Objects.isNull;
 import static java.util.Objects.nonNull;
+import static org.kiwiproject.consul.util.Lists.firstValueOrEmpty;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.google.common.annotations.VisibleForTesting;
@@ -172,10 +173,6 @@ public void onFailure(Throwable throwable) {
         http.extractConsulResponse(api.getValue(Strings.trimLeadingSlash(key), queryOptions.toQuery()), wrapper, NOT_FOUND_404);
     }
 
-    private static Optional<Value> firstValueOrEmpty(List<Value> values) {
-        return nonNull(values) && !values.isEmpty() ? Optional.of(values.get(0)) : Optional.empty();
-    }
-
     private static <T> ConsulResponse<T> newConsulResponse(T value, ConsulResponse<List<Value>> response) {
         return new ConsulResponse<>(value,
                 response.getLastContact(),

src/main/java/org/kiwiproject/consul/SessionClient.java

@@ -1,7 +1,7 @@
 package org.kiwiproject.consul;
 
-import static java.util.Objects.isNull;
 import static org.kiwiproject.consul.ConsulClients.dcQuery;
+import static org.kiwiproject.consul.util.Lists.firstValueOrEmpty;
 
 import org.kiwiproject.consul.config.ClientConfig;
 import org.kiwiproject.consul.model.session.Session;
@@ -81,8 +81,7 @@ public Optional<SessionInfo> renewSession(final String dc, final String sessionI
         List<SessionInfo> sessionInfo = http.extract(api.renewSession(sessionId,
                 Map.of(), dcQuery(dc)));
 
-        return isNull(sessionInfo) || sessionInfo.isEmpty() ? Optional.empty() :
-                Optional.of(sessionInfo.get(0));
+        return firstValueOrEmpty(sessionInfo);
     }
 
     /**
@@ -132,8 +131,7 @@ public Optional<SessionInfo> getSessionInfo(final String sessionId) {
     public Optional<SessionInfo> getSessionInfo(final String sessionId, final String dc) {
         List<SessionInfo> sessionInfo = http.extract(api.getSessionInfo(sessionId, dcQuery(dc)));
 
-        return isNull(sessionInfo) || sessionInfo.isEmpty() ? Optional.empty() :
-                Optional.of(sessionInfo.get(0));
+        return firstValueOrEmpty(sessionInfo);
     }
 
     /**

src/main/java/org/kiwiproject/consul/util/Lists.java

@@ -0,0 +1,21 @@
+package org.kiwiproject.consul.util;
+
+import static java.util.Objects.isNull;
+
+import java.util.List;
+import java.util.Optional;
+
+public class Lists {
+
+    private Lists() {
+        // utility class
+    }
+
+    public static <T> Optional<T> firstValueOrEmpty(List<T> list) {
+        return isNullOrEmpty(list) ? Optional.empty() : Optional.of(list.get(0));
+    }
+
+    public static <T> boolean isNullOrEmpty(List<T> list) {
+        return isNull(list) || list.isEmpty();
+    }
+}

src/test/java/org/kiwiproject/consul/util/ListsTest.java

@@ -0,0 +1,56 @@
+package org.kiwiproject.consul.util;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.NullAndEmptySource;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import java.util.Collections;
+import java.util.List;
+
+@DisplayName("Lists")
+class ListsTest {
+
+    @Nested
+    class FirstValueOrEmpty {
+
+        @ParameterizedTest
+        @NullAndEmptySource
+        void shouldReturnEmptyOptional_WhenListIsNullOrEmpty(List<String> list) {
+            assertThat(Lists.firstValueOrEmpty(list)).isEmpty();
+        }
+
+        @Test
+        void shouldReturnOnlyValue_InSingleValuedList() {
+            var list = List.of(42);
+            assertThat(Lists.firstValueOrEmpty(list)).contains(42);
+        }
+
+        @Test
+        void shouldReturnFirstValue_InMultiValuedList() {
+            var list = List.of(10, 9, 8, 7);
+            assertThat(Lists.firstValueOrEmpty(list)).contains(10);
+        }
+    }
+
+    @Nested
+    class IsNullOrEmpty {
+
+        @ParameterizedTest
+        @NullAndEmptySource
+        void shouldReturnTrue_WhenListIsNullOrEmpty(List<String> list) {
+            assertThat(Lists.isNullOrEmpty(list)).isTrue();
+        }
+
+        @ParameterizedTest
+        @ValueSource(ints = { 1, 2, 3 })
+        void shouldReturnFalse_WhenListIsNotEmpty(int size) {
+            List<Integer> list = Collections.nCopies(size, 42);
+            assertThat(Lists.isNullOrEmpty(list)).isFalse();
+        }
+    }
+}