Resolve ErrorProne warnings

base/src/main/java/io/spine/base/Environment.java

@@ -115,7 +115,7 @@
 public final class Environment {
 
     private static final ImmutableList<EnvironmentType> BASE_TYPES =
-            ImmutableList.of(Tests.type(), Production.type());
+            ImmutableList.of(Tests.ENVIRONMENT, Production.ENVIRONMENT);
 
     private static final Environment INSTANCE = new Environment();
 
@@ -224,7 +224,7 @@ private void determineCurrentType() {
      */
     @Deprecated
     public boolean isTests() {
-        return is(Tests.type());
+        return is(Tests.ENVIRONMENT);
     }
 
     /**
@@ -271,7 +271,7 @@ public void setTo(EnvironmentType type) {
     @Deprecated
     @VisibleForTesting
     public void setToTests() {
-        this.currentEnvType = Tests.type();
+        this.currentEnvType = Tests.ENVIRONMENT;
         Tests.enable();
     }
 
@@ -285,7 +285,7 @@ public void setToTests() {
     @Deprecated
     @VisibleForTesting
     public void setToProduction() {
-        this.currentEnvType = Production.type();
+        this.currentEnvType = Production.ENVIRONMENT;
         Tests.clearTestingEnvVariable();
     }
 

base/src/main/java/io/spine/base/EnvironmentType.java

@@ -20,7 +20,7 @@
 
 package io.spine.base;
 
-import com.google.common.base.Objects;
+import com.google.errorprone.annotations.Immutable;
 
 /**
  * A type of environment.
@@ -31,7 +31,8 @@
  *         that their API is consistent with the env types provided by the {@code base} library:
  *         {@link Production}, {@link Tests}.
  */
-public abstract class EnvironmentType {
+@Immutable
+public interface EnvironmentType {
 
     /**
      * Returns {@code true} if the underlying system is currently in this environment type.
@@ -40,28 +41,5 @@ public abstract class EnvironmentType {
      * variable may be set for every virtual machine. Application developer may use this type of
      * knowledge to determine the current environment.
      */
-    protected abstract boolean enabled();
-
-    /**
-     * @inheritDoc
-     *
-     * <p>By default, environments types are compared based on their classes.
-     */
-    @Override
-    public boolean equals(Object obj) {
-        return this.getClass()
-                   .equals(obj.getClass());
-    }
-
-    /**
-     * @inheritDoc
-     *
-     * <p>By default, adheres to the {@code equals} and {@code hashCode} contract, assuming that
-     * the implementation of the {@code equals} is the {@linkplain EnvironmentType#equals(Object)
-     * default one}.
-     */
-    @Override
-    public int hashCode() {
-        return Objects.hashCode(getClass());
-    }
+    boolean enabled();
 }

base/src/main/java/io/spine/base/Production.java

@@ -28,32 +28,12 @@
  * <p>If the system is not in the {@link Tests} environment, it is in the production environment.
  */
 @Immutable
-public final class Production extends EnvironmentType {
+public enum Production implements EnvironmentType {
 
-    @Override
-    protected boolean enabled() {
-        return !Tests.type()
-                     .enabled();
-    }
-
-    /**
-     * Returns the singleton instance.
-     */
-    public static Production type() {
-        return Singleton.INSTANCE.production;
-    }
+    ENVIRONMENT;
 
-    private enum Singleton {
-
-        INSTANCE;
-
-        @SuppressWarnings({
-                "NonSerializableFieldInSerializableClass",
-                "PMD.SingularField" /* this field cannot be local */})
-        private final Production production;
-
-        Singleton() {
-            this.production = new Production();
-        }
+    @Override
+    public boolean enabled() {
+        return !Tests.ENVIRONMENT.enabled();
     }
 }

base/src/main/java/io/spine/base/Tests.java

@@ -36,7 +36,9 @@
  */
 @Immutable
 @SuppressWarnings("AccessOfSystemProperties" /* is necessary for this class to function */)
-public final class Tests extends EnvironmentType {
+public enum Tests implements EnvironmentType {
+
+    ENVIRONMENT;
 
     /**
      * The key name of the system property which tells if a code runs under a testing framework.
@@ -71,7 +73,7 @@ public final class Tests extends EnvironmentType {
      *         the system property explicitly.
      */
     @Override
-    protected boolean enabled() {
+    public boolean enabled() {
         String testProp = System.getProperty(ENV_KEY_TESTS);
         if (testProp != null) {
             testProp = TEST_PROP_PATTERN.matcher(testProp)
@@ -99,25 +101,4 @@ static void clearTestingEnvVariable() {
     static void enable() {
         System.setProperty(ENV_KEY_TESTS, String.valueOf(true));
     }
-
-    /**
-     * Returns the singleton instance of this class.
-     */
-    public static Tests type() {
-        return Singleton.INSTANCE.tests;
-    }
-
-    private enum Singleton {
-
-        INSTANCE;
-
-        @SuppressWarnings({
-                "NonSerializableFieldInSerializableClass",
-                "PMD.SingularField" /* this field cannot be local */})
-        private final Tests tests;
-
-        Singleton() {
-            this.tests = new Tests();
-        }
-    }
 }