fix: addresses PR comments

google-cloud-spanner/src/main/java/com/google/cloud/spanner/Backup.java

@@ -62,10 +62,6 @@ public Backup build() {
 
   /** Creates a backup on the server based on the source of this {@link Backup} instance. */
   public OperationFuture<Backup, CreateBackupMetadata> create() {
-    Preconditions.checkState(
-        getExpireTime() != null, "Cannot create a backup without an expire time");
-    Preconditions.checkState(
-        getDatabase() != null, "Cannot create a backup without a source database");
     return dbClient.createBackup(this);
   }
 

google-cloud-spanner/src/main/java/com/google/cloud/spanner/BackupInfo.java

@@ -219,8 +219,7 @@ public long getSize() {
 
   /**
    * Returns the {@link BackupEncryptionConfig} to encrypt the backup during its creation. Returns
-   * <code>
-   * null</code> if no customer-managed encryption key should be used.
+   * <code>null</code> if no customer-managed encryption key should be used.
    */
   public BackupEncryptionConfig getEncryptionConfig() {
     return encryptionConfig;

google-cloud-spanner/src/main/java/com/google/cloud/spanner/DatabaseAdminClientImpl.java

@@ -143,6 +143,11 @@ public OperationFuture<Backup, CreateBackupMetadata> createBackup(
   @Override
   public OperationFuture<Backup, CreateBackupMetadata> createBackup(Backup backupInfo)
       throws SpannerException {
+    Preconditions.checkArgument(
+        backupInfo.getExpireTime() != null, "Cannot create a backup without an expire time");
+    Preconditions.checkArgument(
+        backupInfo.getDatabase() != null, "Cannot create a backup without a source database");
+
     final OperationFuture<com.google.spanner.admin.database.v1.Backup, CreateBackupMetadata>
         rawOperationFuture = rpc.createBackup(backupInfo);
 

google-cloud-spanner/src/main/java/com/google/cloud/spanner/encryption/BackupEncryptionConfig.java

@@ -20,4 +20,4 @@
 
 /** Marker interface for encryption configurations that can be applied on backups. */
 @InternalApi
-public interface BackupEncryptionConfig extends EncryptionConfig {}
+public interface BackupEncryptionConfig {}

google-cloud-spanner/src/test/java/com/google/cloud/spanner/BackupTest.java

@@ -113,37 +113,6 @@ public void create() {
     verify(dbClient).createBackup(backup);
   }
 
-  @Test
-  public void createWithoutSource() {
-    Timestamp expireTime = Timestamp.now();
-    Backup backup =
-        dbClient
-            .newBackupBuilder(BackupId.of("test-project", "dest-instance", "backup-id"))
-            .setExpireTime(expireTime)
-            .build();
-    try {
-      backup.create();
-      fail("Expected exception");
-    } catch (IllegalStateException e) {
-      assertNotNull(e.getMessage());
-    }
-  }
-
-  @Test
-  public void createWithoutExpireTime() {
-    Backup backup =
-        dbClient
-            .newBackupBuilder(BackupId.of("test-project", "instance-id", "backup-id"))
-            .setDatabase(DatabaseId.of("test-project", "instance-id", "src-database"))
-            .build();
-    try {
-      backup.create();
-      fail("Expected exception");
-    } catch (IllegalStateException e) {
-      assertNotNull(e.getMessage());
-    }
-  }
-
   @Test
   public void createWithoutVersionTimeShouldSucceed() {
     final Timestamp expireTime = Timestamp.now();

google-cloud-spanner/src/test/java/com/google/cloud/spanner/DatabaseAdminClientImplTest.java

@@ -17,6 +17,7 @@
 package com.google.cloud.spanner;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.fail;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 import static org.mockito.MockitoAnnotations.initMocks;
@@ -261,7 +262,7 @@ public void listDatabasesError() {
             SpannerExceptionFactory.newSpannerException(ErrorCode.INVALID_ARGUMENT, "Test error"));
     try {
       client.listDatabases(INSTANCE_ID, Options.pageSize(1));
-      Assert.fail("Missing expected exception");
+      fail("Missing expected exception");
     } catch (SpannerException e) {
       assertThat(e.getMessage()).contains(INSTANCE_NAME);
       // Assert that the call was done without a page token.
@@ -279,7 +280,7 @@ public void listDatabaseErrorWithToken() {
             SpannerExceptionFactory.newSpannerException(ErrorCode.INVALID_ARGUMENT, "Test error"));
     try {
       Lists.newArrayList(client.listDatabases(INSTANCE_ID, Options.pageSize(1)).iterateAll());
-      Assert.fail("Missing expected exception");
+      fail("Missing expected exception");
     } catch (SpannerException e) {
       assertThat(e.getMessage()).contains(INSTANCE_NAME);
       // Assert that the call was done without a page token.
@@ -388,12 +389,6 @@ public void createBackupWithBackupObject() throws ExecutionException, Interrupte
     final Timestamp versionTime =
         Timestamp.ofTimeMicroseconds(
             TimeUnit.MILLISECONDS.toMicros(System.currentTimeMillis()) - TimeUnit.DAYS.toMicros(2));
-    final Backup expectedCallBackup =
-        Backup.newBuilder()
-            .setDatabase(DB_NAME)
-            .setExpireTime(expireTime.toProto())
-            .setVersionTime(versionTime.toProto())
-            .build();
     final com.google.cloud.spanner.Backup requestBackup =
         client
             .newBackupBuilder(BackupId.of(PROJECT_ID, INSTANCE_ID, BK_ID))
@@ -410,6 +405,28 @@ public void createBackupWithBackupObject() throws ExecutionException, Interrupte
     assertThat(op.get().getId().getName()).isEqualTo(BK_NAME);
   }
 
+  @Test(expected = IllegalArgumentException.class)
+  public void testCreateBackupNoExpireTime() {
+    final com.google.cloud.spanner.Backup requestBackup =
+        client
+            .newBackupBuilder(BackupId.of(PROJECT_ID, INSTANCE_ID, BK_ID))
+            .setDatabase(DatabaseId.of(PROJECT_ID, INSTANCE_ID, DB_ID))
+            .build();
+
+    client.createBackup(requestBackup);
+  }
+
+  @Test(expected = IllegalArgumentException.class)
+  public void testCreateBackupNoDatabase() {
+    final com.google.cloud.spanner.Backup requestBackup =
+        client
+            .newBackupBuilder(BackupId.of(PROJECT_ID, INSTANCE_ID, BK_ID))
+            .setExpireTime(Timestamp.now())
+            .build();
+
+    client.createBackup(requestBackup);
+  }
+
   @Test
   public void createEncryptedBackup() throws ExecutionException, InterruptedException {
     final OperationFuture<Backup, CreateBackupMetadata> rawOperationFuture =

google-cloud-spanner/src/test/java/com/google/cloud/spanner/encryption/EncryptionConfigProtoMapperTest.java

@@ -16,7 +16,6 @@
 package com.google.cloud.spanner.encryption;
 
 import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.fail;
 
 import com.google.spanner.admin.database.v1.CreateBackupEncryptionConfig;
 import com.google.spanner.admin.database.v1.EncryptionConfig;
@@ -85,7 +84,6 @@ public void testCreateBackupConfigUseDatabaseEncryption() {
   @Test(expected = IllegalArgumentException.class)
   public void testCreateBackupInvalidEncryption() {
     EncryptionConfigProtoMapper.createBackupEncryptionConfig(null);
-    fail("Create backup encryption config with invalid encryption should fail");
   }
 
   @Test
@@ -137,6 +135,5 @@ public void testRestoreDatabaseConfigUseBackupEncryption() {
   @Test(expected = IllegalArgumentException.class)
   public void testRestoreDatabaseConfigInvalidEncryption() {
     EncryptionConfigProtoMapper.restoreDatabaseEncryptionConfig(null);
-    fail("Restore database encryption config with invalid encryption should fail");
   }
 }

google-cloud-spanner/src/test/java/com/google/cloud/spanner/encryption/EncryptionConfigsTest.java

@@ -17,7 +17,6 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertSame;
-import static org.junit.Assert.fail;
 
 import org.junit.Test;
 
@@ -37,7 +36,6 @@ public void testCustomerManagedEncryption() {
   @Test(expected = IllegalArgumentException.class)
   public void testCustomerManagedEncryptionNullKeyName() {
     EncryptionConfigs.customerManagedEncryption(null);
-    fail("Customer managed encryption with null key should fail");
   }
 
   @Test