add missing constructor validator

langserver/java/validater.js

@@ -89,6 +89,7 @@ function validate(mod, androidLibrary) {
         require('./validation/bad-implements'),
         require('./validation/non-implemented-interfaces'),
         require('./validation/bad-overrides'),
+        require('./validation/missing-constructor'),
     ];
     let problems = [
         module_validaters.map(v => v(mod, imports, source_types)),

langserver/java/validation/missing-constructor.js

@@ -0,0 +1,41 @@
+const { ModuleBlock, TypeDeclBlock } = require('../parser9');
+const ParseProblem = require('../parsetypes/parse-problem');
+const {SourceType, SourceConstructor} = require('../source-type');
+
+/**
+ * @param {SourceType} source_type 
+ * @param {ParseProblem[]} probs 
+ */
+function checkConstructor(source_type, probs) {
+    if (source_type.typeKind !== 'class') {
+        return;
+    }
+    if (source_type.constructors[0] instanceof SourceConstructor) {
+        return;
+    }
+    const superclass = source_type.supers.find(s => s.typeKind === 'class');
+    if (!superclass) {
+        // if there's no superclass, the class must inherit from an interface
+        // - which means the inherited class is Object (and a default constructor exists)
+        return;
+    }
+    if (!superclass.constructors.find(c => c.parameterCount === 0)) {
+        // the source type has no declared constructors, but the superclass
+        // does not include a default (parameterless) constructor
+        probs.push(ParseProblem.Error(source_type._decl.name_token, `Type ${superclass.fullyDottedRawName} requires a constructor to be declared because the inherited class '${superclass.fullyDottedRawName}' does not define a default constructor.`));
+    }
+}
+
+/**
+ * @param {ModuleBlock} mod 
+ * @param {*} imports
+ * @param {SourceType[]} source_types
+ */
+module.exports = function(mod, imports, source_types) {
+    /** @type {ParseProblem[]} */
+    const probs = [];
+      
+    source_types.forEach(type => checkConstructor(type, probs));
+      
+    return probs;
+}