Polish

1. Removed unnecessary null checks
2. Combined collection initialization and modification
3. Minor collections related optimizations

Closes gh-15837

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/domain/EntityScanPackages.java

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -155,8 +155,7 @@ public class EntityScanPackages {
 			String[] basePackages = attributes.getStringArray("basePackages");
 			Class<?>[] basePackageClasses = attributes
 					.getClassArray("basePackageClasses");
-			Set<String> packagesToScan = new LinkedHashSet<>();
-			packagesToScan.addAll(Arrays.asList(basePackages));
+			Set<String> packagesToScan = new LinkedHashSet<>(Arrays.asList(basePackages));
 			for (Class<?> basePackageClass : basePackageClasses) {
 				packagesToScan.add(ClassUtils.getPackageName(basePackageClass));
 			}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -1185,8 +1185,7 @@ public class SpringApplication {
 	 */
 	public void setInitializers(
 			Collection<? extends ApplicationContextInitializer<?>> initializers) {
-		this.initializers = new ArrayList<>();
-		this.initializers.addAll(initializers);
+		this.initializers = new ArrayList<>(initializers);
 	}
 
 	/**
@@ -1213,8 +1212,7 @@ public class SpringApplication {
 	 * @param listeners the listeners to set
 	 */
 	public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
-		this.listeners = new ArrayList<>();
-		this.listeners.addAll(listeners);
+		this.listeners = new ArrayList<>(listeners);
 	}
 
 	/**

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/SpringIterableConfigurationPropertySource.java

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -132,9 +132,6 @@ class SpringIterableConfigurationPropertySource extends SpringConfigurationPrope
 
 	private Cache getCache() {
 		CacheKey cacheKey = CacheKey.get(getPropertySource());
-		if (cacheKey == null) {
-			return null;
-		}
 		if (ObjectUtils.nullSafeEquals(cacheKey, this.cacheKey)) {
 			return this.cache;
 		}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/ServletComponentScanRegistrar.java

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -77,8 +77,7 @@ class ServletComponentScanRegistrar implements ImportBeanDefinitionRegistrar {
 				metadata.getAnnotationAttributes(ServletComponentScan.class.getName()));
 		String[] basePackages = attributes.getStringArray("basePackages");
 		Class<?>[] basePackageClasses = attributes.getClassArray("basePackageClasses");
-		Set<String> packagesToScan = new LinkedHashSet<>();
-		packagesToScan.addAll(Arrays.asList(basePackages));
+		Set<String> packagesToScan = new LinkedHashSet<>(Arrays.asList(basePackages));
 		for (Class<?> basePackageClass : basePackageClasses) {
 			packagesToScan.add(ClassUtils.getPackageName(basePackageClass));
 		}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/ServletContextInitializerBeans.java

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -238,8 +238,7 @@ public class ServletContextInitializerBeans
 				}
 			}
 		}
-		List<Entry<String, T>> beans = new ArrayList<>();
-		beans.addAll(map.entrySet());
+		List<Entry<String, T>> beans = new ArrayList<>(map.entrySet());
 		beans.sort((o1, o2) -> AnnotationAwareOrderComparator.INSTANCE
 				.compare(o1.getValue(), o2.getValue()));
 		return beans;

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationTests.java

@@ -16,7 +16,6 @@
 
 package org.springframework.boot;
 
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Iterator;
@@ -347,7 +346,7 @@ public class SpringApplicationTests {
 		SpringApplication application = new SpringApplication(ExampleConfig.class);
 		application.setWebApplicationType(WebApplicationType.NONE);
 		final AtomicReference<ApplicationContext> reference = new AtomicReference<>();
-		application.setInitializers(Arrays.asList(
+		application.setInitializers(Collections.singletonList(
 				(ApplicationContextInitializer<ConfigurableApplicationContext>) reference::set));
 		this.context = application.run("--foo=bar");
 		assertThat(this.context).isSameAs(reference.get());
@@ -387,7 +386,7 @@ public class SpringApplicationTests {
 			}
 
 		}
-		application.setListeners(Arrays.asList(new InitializerListener()));
+		application.setListeners(Collections.singletonList(new InitializerListener()));
 		this.context = application.run("--foo=bar");
 		assertThat(this.context).isSameAs(reference.get());
 		// Custom initializers do not switch off the defaults

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/annotation/ConfigurationsTests.java

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,6 +21,7 @@ import java.io.OutputStream;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Comparator;
 import java.util.Set;
 
@@ -59,7 +60,8 @@ public class ConfigurationsTests {
 	public void getClassesShouldMergeByClassAndSort() {
 		Configurations c1 = new TestSortedConfigurations(
 				Arrays.asList(OutputStream.class, InputStream.class));
-		Configurations c2 = new TestConfigurations(Arrays.asList(Short.class));
+		Configurations c2 = new TestConfigurations(
+				Collections.singletonList(Short.class));
 		Configurations c3 = new TestSortedConfigurations(
 				Arrays.asList(String.class, Integer.class));
 		Configurations c4 = new TestConfigurations(Arrays.asList(Long.class, Byte.class));

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/config/ConfigFileApplicationListenerTests.java

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,7 +20,6 @@ import java.io.File;
 import java.io.FileOutputStream;
 import java.io.OutputStream;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
@@ -1054,8 +1053,8 @@ public class ConfigFileApplicationListenerTests {
 
 		@Override
 		List<EnvironmentPostProcessor> loadPostProcessors() {
-			return new ArrayList<>(
-					Arrays.asList(new LowestPrecedenceEnvironmentPostProcessor()));
+			return new ArrayList<>(Collections
+					.singletonList(new LowestPrecedenceEnvironmentPostProcessor()));
 		}
 
 	}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/jetty/JettyServletWebServerFactoryTests.java

@@ -21,6 +21,7 @@ import java.nio.charset.Charset;
 import java.time.Duration;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Locale;
 import java.util.Map;
 
@@ -209,7 +210,7 @@ public class JettyServletWebServerFactoryTests
 	@Test
 	public void wrappedHandlers() throws Exception {
 		JettyServletWebServerFactory factory = getFactory();
-		factory.setServerCustomizers(Arrays.asList((server) -> {
+		factory.setServerCustomizers(Collections.singletonList((server) -> {
 			Handler handler = server.getHandler();
 			HandlerWrapper wrapper = new HandlerWrapper();
 			wrapper.setHandler(handler);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/server/MimeMappingsTests.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -77,8 +77,7 @@ public class MimeMappingsTests {
 		MimeMappings mappings = new MimeMappings();
 		mappings.add("foo", "bar");
 		mappings.add("baz", "boo");
-		List<MimeMappings.Mapping> mappingList = new ArrayList<>();
-		mappingList.addAll(mappings.getAll());
+		List<MimeMappings.Mapping> mappingList = new ArrayList<>(mappings.getAll());
 		assertThat(mappingList.get(0).getExtension()).isEqualTo("foo");
 		assertThat(mappingList.get(0).getMimeType()).isEqualTo("bar");
 		assertThat(mappingList.get(1).getExtension()).isEqualTo("baz");

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

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -144,7 +144,7 @@ public abstract class AbstractFilterRegistrationBeanTests {
 		AbstractFilterRegistrationBean<?> bean = createFilterRegistrationBean(
 				mockServletRegistration("a"));
 		bean.setServletRegistrationBeans(new LinkedHashSet<ServletRegistrationBean<?>>(
-				Arrays.asList(mockServletRegistration("b"))));
+				Collections.singletonList(mockServletRegistration("b"))));
 		bean.onStartup(this.servletContext);
 		verify(this.registration).addMappingForServletNames(
 				EnumSet.of(DispatcherType.REQUEST), false, "b");

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

 /*
- * Copyright 2012-2018 the original author or authors.
+ * Copyright 2012-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,7 +16,6 @@
 
 package org.springframework.boot.web.servlet.support;
 
-import java.util.Arrays;
 import java.util.Collections;
 
 import javax.servlet.ServletContext;
@@ -135,8 +134,8 @@ public class SpringBootServletInitializerTests {
 	@Test
 	public void servletContextPropertySourceIsAvailablePriorToRefresh() {
 		ServletContext servletContext = mock(ServletContext.class);
-		given(servletContext.getInitParameterNames()).willReturn(
-				Collections.enumeration(Arrays.asList("spring.profiles.active")));
+		given(servletContext.getInitParameterNames()).willReturn(Collections
+				.enumeration(Collections.singletonList("spring.profiles.active")));
 		given(servletContext.getInitParameter("spring.profiles.active"))
 				.willReturn("from-servlet-context");
 		given(servletContext.getAttributeNames())