[SPARK-53814] Use Java List.of instead of Collections.(empty|singleton)List

### What changes were proposed in this pull request?

This PR aims to use Java `List.of` instead of `Collections.emptyList` or `Collections.singletonList`.

### Why are the changes needed?

This PR aims to use Java 9+ `List.of` API instead of `Collections.emptyList` or `Collections.singletonList` because it's more concise and modern APIs.

```java
- HealthProbe healthyProbe =
-   new HealthProbe(Collections.singletonList(operator), Collections.emptyList());
  HealthProbe healthyProbe = new HealthProbe(List.of(operator), List.of());
```

### Does this PR introduce _any_ user-facing change?

No behavior change.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #366 from dongjoon-hyun/SPARK-53814.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

spark-operator/src/main/java/org/apache/spark/k8s/operator/metrics/PrometheusPullModelHandler.java

@@ -24,7 +24,6 @@
 
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Properties;
@@ -94,15 +93,15 @@ public void handle(HttpExchange exchange) throws IOException {
           exchange,
           HTTP_OK,
           formatMetricsSnapshot(),
-          Map.of("Content-Type", Collections.singletonList("text/plain;version=0.0.4")));
+          Map.of("Content-Type", List.of("text/plain;version=0.0.4")));
     } else {
       HttpServletRequest httpServletRequest = null;
       String value = getMetricsSnapshot(httpServletRequest);
       sendMessage(
           exchange,
           HTTP_OK,
           String.join("\n", filterNonEmptyRecords(value)),
-          Map.of("Content-Type", Collections.singletonList("text/plain;version=0.0.4")));
+          Map.of("Content-Type", List.of("text/plain;version=0.0.4")));
     }
   }
 

spark-operator/src/main/java/org/apache/spark/k8s/operator/reconciler/SparkAppReconciler.java

@@ -25,7 +25,6 @@
 import static org.apache.spark.k8s.operator.utils.Utils.commonResourceLabelsStr;
 
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 
 import io.fabric8.kubernetes.api.model.Pod;
@@ -178,20 +177,16 @@ protected List<AppReconcileStep> getReconcileSteps(final SparkApplication app) {
         steps.add(
             new AppResourceObserveStep(
                 List.of(new AppDriverStartObserver(), new AppDriverReadyObserver())));
-        steps.add(
-            new AppResourceObserveStep(Collections.singletonList(new AppDriverRunningObserver())));
-        steps.add(
-            new AppResourceObserveStep(Collections.singletonList(new AppDriverTimeoutObserver())));
+        steps.add(new AppResourceObserveStep(List.of(new AppDriverRunningObserver())));
+        steps.add(new AppResourceObserveStep(List.of(new AppDriverTimeoutObserver())));
       }
       case DriverReady,
           InitializedBelowThresholdExecutors,
           RunningHealthy,
           RunningWithBelowThresholdExecutors -> {
         steps.add(new AppRunningStep());
-        steps.add(
-            new AppResourceObserveStep(Collections.singletonList(new AppDriverRunningObserver())));
-        steps.add(
-            new AppResourceObserveStep(Collections.singletonList(new AppDriverTimeoutObserver())));
+        steps.add(new AppResourceObserveStep(List.of(new AppDriverRunningObserver())));
+        steps.add(new AppResourceObserveStep(List.of(new AppDriverTimeoutObserver())));
       }
       default -> steps.add(new AppUnknownStateStep());
     }

spark-operator/src/main/java/org/apache/spark/k8s/operator/reconciler/reconcilesteps/AppRunningStep.java

@@ -22,7 +22,7 @@
 import static org.apache.spark.k8s.operator.Constants.*;
 import static org.apache.spark.k8s.operator.reconciler.ReconcileProgress.completeAndDefaultRequeue;
 
-import java.util.Collections;
+import java.util.List;
 import java.util.Set;
 
 import io.fabric8.kubernetes.api.model.Pod;
@@ -85,8 +85,7 @@ public ReconcileProgress reconcile(
       }
     }
     if (proposedStateSummary.equals(prevStateSummary)) {
-      return observeDriver(
-          context, statusRecorder, Collections.singletonList(new AppDriverRunningObserver()));
+      return observeDriver(context, statusRecorder, List.of(new AppDriverRunningObserver()));
     } else {
       ApplicationStatus updatedStatus =
           context

spark-operator/src/test/java/org/apache/spark/k8s/operator/metrics/source/KubernetesMetricsInterceptorTest.java

@@ -22,7 +22,6 @@
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -66,7 +65,7 @@ void cleanUp() {
   @Order(1)
   void testMetricsEnabled() {
     KubernetesMetricsInterceptor metricsInterceptor = new KubernetesMetricsInterceptor();
-    List<Interceptor> interceptors = Collections.singletonList(metricsInterceptor);
+    List<Interceptor> interceptors = List.of(metricsInterceptor);
     try (KubernetesClient client =
         KubernetesClientFactory.buildKubernetesClient(
             interceptors, kubernetesClient.getConfiguration())) {
@@ -106,7 +105,7 @@ void testMetricsEnabled() {
   @Order(2)
   void testWhenKubernetesServerNotWorking() {
     KubernetesMetricsInterceptor metricsInterceptor = new KubernetesMetricsInterceptor();
-    List<Interceptor> interceptors = Collections.singletonList(metricsInterceptor);
+    List<Interceptor> interceptors = List.of(metricsInterceptor);
     try (KubernetesClient client =
         KubernetesClientFactory.buildKubernetesClient(
             interceptors, kubernetesClient.getConfiguration())) {

spark-operator/src/test/java/org/apache/spark/k8s/operator/probe/HealthProbeTest.java

@@ -25,7 +25,6 @@
 import static org.mockito.Mockito.when;
 
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -108,7 +107,7 @@ public boolean isStarted() {
 
   @Test
   void testHealthProbeWithInformerHealthWithMultiOperators() {
-    HealthProbe healthyProbe = new HealthProbe(operators, Collections.emptyList());
+    HealthProbe healthyProbe = new HealthProbe(operators, List.of());
     isRunning.set(true);
     assertFalse(
         healthyProbe.isHealthy(),
@@ -128,8 +127,7 @@ void testHealthProbeWithInformerHealthWithMultiOperators() {
 
   @Test
   void testHealthProbeWithInformerHealthWithSingleOperator() {
-    HealthProbe healthyProbe =
-        new HealthProbe(Collections.singletonList(operator), Collections.emptyList());
+    HealthProbe healthyProbe = new HealthProbe(List.of(operator), List.of());
     assertFalse(healthyProbe.isHealthy(), "Health Probe should fail when operator is not running");
     isRunning.set(true);
     unhealthyEventSources.put(
@@ -143,8 +141,7 @@ void testHealthProbeWithInformerHealthWithSingleOperator() {
   @Test
   void testHealthProbeWithSentinelHealthWithMultiOperators() {
     var sentinelManager = mock(SentinelManager.class);
-    HealthProbe healthyProbe =
-        new HealthProbe(operators, Collections.singletonList(sentinelManager));
+    HealthProbe healthyProbe = new HealthProbe(operators, List.of(sentinelManager));
     isRunning.set(true);
     isRunning2.set(true);
     when(sentinelManager.allSentinelsAreHealthy()).thenReturn(false);

spark-operator/src/test/java/org/apache/spark/k8s/operator/probe/ProbeServiceTest.java

@@ -31,7 +31,6 @@
 import java.net.HttpURLConnection;
 import java.net.MalformedURLException;
 import java.net.URL;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 
@@ -56,9 +55,7 @@ void testHealthProbeEndpointWithStaticProperties() throws Exception {
     when(runtimeInfo.unhealthyInformerWrappingEventSourceHealthIndicator())
         .thenReturn(new HashMap<>());
     when(sentinelManager.allSentinelsAreHealthy()).thenReturn(true);
-    ProbeService probeService =
-        new ProbeService(
-            Collections.singletonList(operator), Collections.singletonList(sentinelManager), null);
+    ProbeService probeService = new ProbeService(List.of(operator), List.of(sentinelManager), null);
     probeService.start();
     hitHealthyEndpoint();
     probeService.stop();
@@ -83,8 +80,7 @@ void testHealthProbeEndpointWithDynamicProperties() throws Exception {
         .thenReturn(new HashMap<>());
     when(sentinelManager.allSentinelsAreHealthy()).thenReturn(true);
     ProbeService probeService =
-        new ProbeService(
-            List.of(operator, operator1), Collections.singletonList(sentinelManager), null);
+        new ProbeService(List.of(operator, operator1), List.of(sentinelManager), null);
     probeService.start();
     hitHealthyEndpoint();
     probeService.stop();
@@ -110,8 +106,7 @@ void testReadinessProbeEndpointWithDynamicProperties() throws Exception {
         .thenReturn(new HashMap<>());
     when(operator1.getKubernetesClient()).thenReturn(client);
     ProbeService probeService =
-        new ProbeService(
-            List.of(operator, operator1), Collections.singletonList(sentinelManager), null);
+        new ProbeService(List.of(operator, operator1), List.of(sentinelManager), null);
     probeService.start();
     hitStartedUpEndpoint();
     probeService.stop();

spark-operator/src/test/java/org/apache/spark/k8s/operator/reconciler/SparkAppReconcilerTest.java

@@ -29,7 +29,7 @@
 import static org.mockito.Mockito.mockConstruction;
 import static org.mockito.Mockito.when;
 
-import java.util.Collections;
+import java.util.List;
 import java.util.Optional;
 
 import io.fabric8.kubernetes.api.model.Pod;
@@ -96,8 +96,8 @@ void testCleanupRunningApp() {
                   when(mock.getClient()).thenReturn(mockClient);
                   when(mock.getDriverPod()).thenReturn(Optional.of(mockDriver));
                   when(mock.getDriverPodSpec()).thenReturn(mockDriver);
-                  when(mock.getDriverPreResourcesSpec()).thenReturn(Collections.emptyList());
-                  when(mock.getDriverResourcesSpec()).thenReturn(Collections.emptyList());
+                  when(mock.getDriverPreResourcesSpec()).thenReturn(List.of());
+                  when(mock.getDriverResourcesSpec()).thenReturn(List.of());
                 });
         MockedStatic<ReconcilerUtils> utils = Mockito.mockStatic(ReconcilerUtils.class)) {
       // delete running app
@@ -128,8 +128,8 @@ void testCleanupAppTerminatedWithoutReleaseResources() {
                   when(mock.getClient()).thenReturn(mockClient);
                   when(mock.getDriverPod()).thenReturn(Optional.of(mockDriver));
                   when(mock.getDriverPodSpec()).thenReturn(mockDriver);
-                  when(mock.getDriverPreResourcesSpec()).thenReturn(Collections.emptyList());
-                  when(mock.getDriverResourcesSpec()).thenReturn(Collections.emptyList());
+                  when(mock.getDriverPreResourcesSpec()).thenReturn(List.of());
+                  when(mock.getDriverResourcesSpec()).thenReturn(List.of());
                 });
         MockedStatic<ReconcilerUtils> utils = Mockito.mockStatic(ReconcilerUtils.class)) {
       // delete app
@@ -160,8 +160,8 @@ void testCleanupAppTerminatedResourceReleased() {
                 (mock, context) -> {
                   when(mock.getResource()).thenReturn(app);
                   when(mock.getClient()).thenReturn(mockClient);
-                  when(mock.getDriverPreResourcesSpec()).thenReturn(Collections.emptyList());
-                  when(mock.getDriverResourcesSpec()).thenReturn(Collections.emptyList());
+                  when(mock.getDriverPreResourcesSpec()).thenReturn(List.of());
+                  when(mock.getDriverResourcesSpec()).thenReturn(List.of());
                 });
         MockedStatic<ReconcilerUtils> utils = Mockito.mockStatic(ReconcilerUtils.class)) {
       // delete app

spark-operator/src/test/java/org/apache/spark/k8s/operator/reconciler/reconcilesteps/AppCleanUpStepTest.java

@@ -31,7 +31,6 @@
 
 import java.time.Duration;
 import java.time.Instant;
-import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 
@@ -170,8 +169,8 @@ void onDemandCleanupForRunningAppExpectDelete() {
         when(mockAppContext.getClient()).thenReturn(mockClient);
         Pod driverPod = mock(Pod.class);
         when(mockAppContext.getDriverPod()).thenReturn(Optional.of(driverPod));
-        when(mockAppContext.getDriverPreResourcesSpec()).thenReturn(Collections.emptyList());
-        when(mockAppContext.getDriverResourcesSpec()).thenReturn(Collections.emptyList());
+        when(mockAppContext.getDriverPreResourcesSpec()).thenReturn(List.of());
+        when(mockAppContext.getDriverResourcesSpec()).thenReturn(List.of());
         when(mockRecorder.persistStatus(eq(mockAppContext), any())).thenReturn(true);
         when(mockRecorder.appendNewStateAndPersist(eq(mockAppContext), any())).thenReturn(true);
 
@@ -260,8 +259,8 @@ void onDemandCleanupForTerminatedAppExpectDelete() {
     when(mockAppContext.getClient()).thenReturn(mockClient);
     Pod driverPod = mock(Pod.class);
     when(mockAppContext.getDriverPod()).thenReturn(Optional.of(driverPod));
-    when(mockAppContext.getDriverPreResourcesSpec()).thenReturn(Collections.emptyList());
-    when(mockAppContext.getDriverResourcesSpec()).thenReturn(Collections.emptyList());
+    when(mockAppContext.getDriverPreResourcesSpec()).thenReturn(List.of());
+    when(mockAppContext.getDriverResourcesSpec()).thenReturn(List.of());
     when(mockRecorder.persistStatus(eq(mockAppContext), any())).thenReturn(true);
     when(mockRecorder.appendNewStateAndPersist(eq(mockAppContext), any())).thenReturn(true);
 
@@ -314,14 +313,12 @@ void cleanupForAppExpectDeleteWithRecompute() {
     ConfigMap resource2 = mock(ConfigMap.class);
     when(mockAppContext1.getDriverPod()).thenReturn(Optional.of(driverPod));
     when(mockAppContext1.getDriverPodSpec()).thenReturn(driverPodSpec);
-    when(mockAppContext1.getDriverPreResourcesSpec())
-        .thenReturn(Collections.singletonList(resource1));
-    when(mockAppContext1.getDriverResourcesSpec()).thenReturn(Collections.singletonList(resource2));
+    when(mockAppContext1.getDriverPreResourcesSpec()).thenReturn(List.of(resource1));
+    when(mockAppContext1.getDriverResourcesSpec()).thenReturn(List.of(resource2));
     when(mockAppContext2.getDriverPod()).thenReturn(Optional.of(driverPod));
     when(mockAppContext2.getDriverPodSpec()).thenReturn(driverPodSpec);
-    when(mockAppContext2.getDriverPreResourcesSpec())
-        .thenReturn(Collections.singletonList(resource1));
-    when(mockAppContext2.getDriverResourcesSpec()).thenReturn(Collections.singletonList(resource2));
+    when(mockAppContext2.getDriverPreResourcesSpec()).thenReturn(List.of(resource1));
+    when(mockAppContext2.getDriverResourcesSpec()).thenReturn(List.of(resource2));
     when(mockRecorder.persistStatus(any(), any())).thenReturn(true);
     when(mockRecorder.appendNewStateAndPersist(any(), any())).thenReturn(true);
 

spark-operator/src/test/java/org/apache/spark/k8s/operator/reconciler/reconcilesteps/AppInitStepTest.java

@@ -25,7 +25,6 @@
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 
@@ -103,10 +102,9 @@ void driverResourcesHaveOwnerReferencesToDriver() {
     SparkApplication application = new SparkApplication();
     application.setMetadata(applicationMetadata);
     when(mocksparkAppContext.getResource()).thenReturn(application);
-    when(mocksparkAppContext.getDriverPreResourcesSpec()).thenReturn(Collections.emptyList());
+    when(mocksparkAppContext.getDriverPreResourcesSpec()).thenReturn(List.of());
     when(mocksparkAppContext.getDriverPodSpec()).thenReturn(driverPodSpec);
-    when(mocksparkAppContext.getDriverResourcesSpec())
-        .thenReturn(Collections.singletonList(resourceConfigMapSpec));
+    when(mocksparkAppContext.getDriverResourcesSpec()).thenReturn(List.of(resourceConfigMapSpec));
     when(mocksparkAppContext.getClient()).thenReturn(kubernetesClient);
     when(recorder.appendNewStateAndPersist(any(), any())).thenReturn(true);
     when(recorder.persistStatus(any(), any())).thenReturn(true);
@@ -137,9 +135,9 @@ void createdPreResourcesPatchedWithOwnerReferencesToDriver() {
     application.setMetadata(applicationMetadata);
     when(mocksparkAppContext.getResource()).thenReturn(application);
     when(mocksparkAppContext.getDriverPreResourcesSpec())
-        .thenReturn(Collections.singletonList(preResourceConfigMapSpec));
+        .thenReturn(List.of(preResourceConfigMapSpec));
     when(mocksparkAppContext.getDriverPodSpec()).thenReturn(driverPodSpec);
-    when(mocksparkAppContext.getDriverResourcesSpec()).thenReturn(Collections.emptyList());
+    when(mocksparkAppContext.getDriverResourcesSpec()).thenReturn(List.of());
     when(recorder.appendNewStateAndPersist(any(), any())).thenReturn(true);
     when(recorder.persistStatus(any(), any())).thenReturn(true);
 
@@ -201,10 +199,9 @@ void appInitStepShouldBeIdempotentWhenStatusUpdateFails() {
     SparkApplication application = new SparkApplication();
     application.setMetadata(applicationMetadata);
     when(mocksparkAppContext.getResource()).thenReturn(application);
-    when(mocksparkAppContext.getDriverPreResourcesSpec()).thenReturn(Collections.emptyList());
+    when(mocksparkAppContext.getDriverPreResourcesSpec()).thenReturn(List.of());
     when(mocksparkAppContext.getDriverPodSpec()).thenReturn(driverPodSpec);
-    when(mocksparkAppContext.getDriverResourcesSpec())
-        .thenReturn(Collections.singletonList(resourceConfigMapSpec));
+    when(mocksparkAppContext.getDriverResourcesSpec()).thenReturn(List.of(resourceConfigMapSpec));
     when(mocksparkAppContext.getClient()).thenReturn(kubernetesClient);
     when(recorder.appendNewStateAndPersist(any(), any())).thenReturn(false, true);
     when(recorder.persistStatus(any(), any())).thenReturn(false, true);

spark-submission-worker/src/main/java/org/apache/spark/k8s/operator/SparkAppResourceSpec.java

@@ -21,7 +21,6 @@
 
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -170,7 +169,7 @@ private SparkPod addConfigMap(SparkPod pod, Map<String, String> confFilesMap) {
   private List<HasMetadata> configureDriverServerIngress(
       SparkPod pod, List<DriverServiceIngressSpec> driverServiceIngressList) {
     if (driverServiceIngressList == null || driverServiceIngressList.isEmpty()) {
-      return Collections.emptyList();
+      return List.of();
     }
     return driverServiceIngressList.stream()
         .map(spec -> DriverServiceIngressUtils.buildIngressService(spec, pod.pod().getMetadata()))