fix(pacmak): _ is not treated as a keyword in Java (#4094)

Since Java 9, `_` is considered a keyword. Although we are technically
still targeting Java 8 with our source, if we want our source to
be parseable by more modern tools or are considering raising the
threshold at some point, it would be good to have this in place already.

This might technically be considered breaking in case someone is
using `_` as an identifier in a public API, since in newer releases
it will be renamed. But  given that most jsii library authors will be
targeting users that will be running on recent Java versions, and Java 9
is ~6 years old, the chance of that happening in practice is vanishingly
small.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

packages/@scope/jsii-calc-lib/lib/index.ts

@@ -127,6 +127,15 @@ export class BaseFor2647 {
   }
 }
 
+/**
+ * For Java, test that an identifier that is just a _ is handled
+ */
+export class FunctionWithUnderscoreArgument {
+  public foo(_: string) {
+    return _;
+  }
+}
+
 export * as submodule from './submodule';
 export * from './duplicate-inherited-prop';
 export * as deprecationRemoval from './deprecation-removal';

packages/@scope/jsii-calc-lib/test/assembly.jsii

@@ -95,14 +95,14 @@
     "@scope/jsii-calc-lib.deprecationRemoval": {
       "locationInModule": {
         "filename": "lib/index.ts",
-        "line": 132
+        "line": 141
       },
       "symbolId": "lib/deprecation-removal:"
     },
     "@scope/jsii-calc-lib.submodule": {
       "locationInModule": {
         "filename": "lib/index.ts",
-        "line": 130
+        "line": 139
       },
       "readme": {
         "markdown": "# Submodule Readme\n\nThis is a submodule readme.\n"
@@ -333,6 +333,51 @@
       "name": "EnumFromScopedModule",
       "symbolId": "lib/index:EnumFromScopedModule"
     },
+    "@scope/jsii-calc-lib.FunctionWithUnderscoreArgument": {
+      "assembly": "@scope/jsii-calc-lib",
+      "docs": {
+        "stability": "deprecated",
+        "summary": "For Java, test that an identifier that is just a _ is handled."
+      },
+      "fqn": "@scope/jsii-calc-lib.FunctionWithUnderscoreArgument",
+      "initializer": {
+        "docs": {
+          "stability": "deprecated"
+        }
+      },
+      "kind": "class",
+      "locationInModule": {
+        "filename": "lib/index.ts",
+        "line": 133
+      },
+      "methods": [
+        {
+          "docs": {
+            "stability": "deprecated"
+          },
+          "locationInModule": {
+            "filename": "lib/index.ts",
+            "line": 134
+          },
+          "name": "foo",
+          "parameters": [
+            {
+              "name": "_",
+              "type": {
+                "primitive": "string"
+              }
+            }
+          ],
+          "returns": {
+            "type": {
+              "primitive": "string"
+            }
+          }
+        }
+      ],
+      "name": "FunctionWithUnderscoreArgument",
+      "symbolId": "lib/index:FunctionWithUnderscoreArgument"
+    },
     "@scope/jsii-calc-lib.IDoublable": {
       "assembly": "@scope/jsii-calc-lib",
       "docs": {
@@ -1022,5 +1067,5 @@
     }
   },
   "version": "0.0.0",
-  "fingerprint": "XDMAZYhhgc09X8VS8hpn3ch21YxKDn+HB0w82IofsRM="
+  "fingerprint": "yrpQ+/ynmcWEtNZgCAeq2yp4JpEpln2V/fRhUc5uVUs="
 }

packages/jsii-pacmak/lib/targets/java.ts

@@ -576,6 +576,7 @@ class JavaGenerator extends Generator {
     'void',
     'volatile',
     'while',
+    '_',
   ];
 
   /**
@@ -587,6 +588,11 @@ class JavaGenerator extends Generator {
       return propertyName;
     }
 
+    if (propertyName === '_') {
+      // Slightly different pattern for this one
+      return '__';
+    }
+
     if (JavaGenerator.RESERVED_KEYWORDS.includes(propertyName)) {
       return `${propertyName}Value`;
     }
@@ -602,6 +608,11 @@ class JavaGenerator extends Generator {
       return methodName;
     }
 
+    if (methodName === '_') {
+      // Different pattern for this one. Also this should never happen, who names a function '_' ??
+      return 'doIt';
+    }
+
     if (JavaGenerator.RESERVED_KEYWORDS.includes(methodName)) {
       return `do${jsiiToPascalCase(methodName)}`;
     }