Simplify code by using Map computeIfAbsent

Closes gh-15543

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/TypeElementMembers.java

@@ -104,11 +104,7 @@ class TypeElementMembers {
 			else if (isSetter(method)) {
 				String propertyName = getAccessorName(name);
 				List<ExecutableElement> matchingSetters = this.publicSetters
-						.get(propertyName);
-				if (matchingSetters == null) {
-					matchingSetters = new ArrayList<>();
-					this.publicSetters.put(propertyName, matchingSetters);
-				}
+						.computeIfAbsent(propertyName, (k) -> new ArrayList<>());
 				TypeMirror paramType = method.getParameters().get(0).asType();
 				if (getMatchingSetter(matchingSetters, paramType) == null) {
 					matchingSetters.add(method);

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/metadata/ConfigurationMetadata.java

@@ -136,11 +136,7 @@ public class ConfigurationMetadata {
 	}
 
 	private <K, V> void add(Map<K, List<V>> map, K key, V value) {
-		List<V> values = map.get(key);
-		if (values == null) {
-			values = new ArrayList<>();
-			map.put(key, values);
-		}
+		List<V> values = map.computeIfAbsent(key, (k) -> new ArrayList<>());
 		values.add(value);
 	}