This PR adds memoization to Java **toModelName()** method. (#12909)

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This PR greatly reduces the time taken for Java code generation.
**toModelName()** method is invoked numerous times and it was accumulating a lot of time.

This has reduced the Java SDK generation time for our rather large open API spec from ~3h30m hours to 4 mins.

The generated code is the same as before.

spec: https://cdn.intersight.com/components/an-apidocs/1.0.11-7546/model/intersight-openapi-v3-1.0.11-7546.yaml

@wing328

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java

@@ -130,6 +130,8 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
     protected boolean implicitHeaders = false;
     protected String implicitHeadersRegex = null;
 
+    private Map<String, String> schemaKeyToModelNameCache = new HashMap<>();
+
     public AbstractJavaCodegen() {
         super();
 
@@ -830,6 +832,12 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
             return schemaMapping.get(name);
         }
 
+        // memoization
+        String origName = name;
+        if (schemaKeyToModelNameCache.containsKey(origName)) {
+            return schemaKeyToModelNameCache.get(origName);
+        }
+
         final String sanitizedName = sanitizeName(name);
 
         String nameWithPrefixSuffix = sanitizedName;
@@ -850,6 +858,7 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
         // model name cannot use reserved keyword, e.g. return
         if (isReservedWord(camelizedName)) {
             final String modelName = "Model" + camelizedName;
+            schemaKeyToModelNameCache.put(origName, modelName);
             LOGGER.warn("{} (reserved word) cannot be used as model name. Renamed to {}", camelizedName, modelName);
             return modelName;
         }
@@ -857,11 +866,14 @@ public abstract class AbstractJavaCodegen extends DefaultCodegen implements Code
         // model name starts with number
         if (camelizedName.matches("^\\d.*")) {
             final String modelName = "Model" + camelizedName; // e.g. 200Response => Model200Response (after camelize)
+            schemaKeyToModelNameCache.put(origName, modelName);
             LOGGER.warn("{} (model name starts with number) cannot be used as model name. Renamed to {}", name,
                     modelName);
             return modelName;
         }
 
+        schemaKeyToModelNameCache.put(origName, camelizedName);
+
         return camelizedName;
     }