Polish some Collections API calls

See gh-17825

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jndi/JndiPropertiesHidingClassLoader.java

@@ -36,7 +36,7 @@ public class JndiPropertiesHidingClassLoader extends ClassLoader {
 	@Override
 	public Enumeration<URL> getResources(String name) throws IOException {
 		if ("jndi.properties".equals(name)) {
-			return Collections.enumeration(Collections.emptyList());
+			return Collections.emptyEnumeration();
 		}
 		return super.getResources(name);
 	}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/archive/ExplodedArchive.java

@@ -173,7 +173,7 @@ public class ExplodedArchive implements Archive {
 		private Iterator<File> listFiles(File file) {
 			File[] files = file.listFiles();
 			if (files == null) {
-				return Collections.<File>emptyList().iterator();
+				return Collections.emptyIterator();
 			}
 			Arrays.sort(files, this.entryComparator);
 			return Arrays.asList(files).iterator();

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/servlet/support/SpringBootServletInitializerTests.java

@@ -129,7 +129,7 @@ class SpringBootServletInitializerTests {
 		given(servletContext.getInitParameterNames())
 				.willReturn(Collections.enumeration(Collections.singletonList("spring.profiles.active")));
 		given(servletContext.getInitParameter("spring.profiles.active")).willReturn("from-servlet-context");
-		given(servletContext.getAttributeNames()).willReturn(Collections.enumeration(Collections.emptyList()));
+		given(servletContext.getAttributeNames()).willReturn(Collections.emptyEnumeration());
 		try (ConfigurableApplicationContext context = (ConfigurableApplicationContext) new PropertySourceVerifyingSpringBootServletInitializer()
 				.createRootApplicationContext(servletContext)) {
 			assertThat(context.getEnvironment().getActiveProfiles()).containsExactly("from-servlet-context");