Use Collections.isEmpty() instead of .size() == 0

Ensure that Collections.isEmpty() is used to check if there are no
elements in a collections. This is more explicit and can be faster than
calling .size().

Closes gh-4783

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/core/HintCommand.java

@@ -57,7 +57,7 @@ public class HintCommand extends AbstractCommand {
 			if (index == 0) {
 				showCommandHints(starting);
 			}
-			else if ((arguments.size() > 0) && (starting.length() > 0)) {
+			else if (!arguments.isEmpty() && (starting.length() > 0)) {
 				String command = arguments.remove(0);
 				showCommandOptionHints(command, Collections.unmodifiableList(arguments),
 						starting);

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/init/ProjectGenerationRequest.java

@@ -385,7 +385,7 @@ class ProjectGenerationRequest {
 			if (types.size() == 1) {
 				return types.values().iterator().next();
 			}
-			else if (types.size() == 0) {
+			else if (types.isEmpty()) {
 				throw new ReportableException("No type found with build '" + this.build
 						+ "' and format '" + this.format
 						+ "' check the service capabilities (--list)");

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/options/SourceOptions.java

@@ -96,7 +96,7 @@ public class SourceOptions {
 		}
 		this.args = Collections.unmodifiableList(
 				nonOptionArguments.subList(sourceArgCount, nonOptionArguments.size()));
-		Assert.isTrue(sources.size() > 0, "Please specify at least one file");
+		Assert.isTrue(!sources.isEmpty(), "Please specify at least one file");
 		this.sources = Collections.unmodifiableList(sources);
 	}
 

spring-boot-cli/src/main/java/org/springframework/boot/cli/compiler/ExtendedGroovyClassLoader.java

@@ -187,7 +187,7 @@ public class ExtendedGroovyClassLoader extends GroovyClassLoader {
 			if (urls.isEmpty()) {
 				findGroovyJarsFromClassPath(parent, urls);
 			}
-			Assert.state(urls.size() > 0, "Unable to find groovy JAR");
+			Assert.state(!urls.isEmpty(), "Unable to find groovy JAR");
 			return new ArrayList<URL>(urls).toArray(new URL[urls.size()]);
 		}