Implement methods in newly created type from an inherited sealed class.

- When a non-existent type is permitted by a sealed class, the newly
  created type should implement the necessary methods
- Related #1553

org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/corrections/proposals/NewCUProposal.java

@@ -48,6 +48,10 @@
 import org.eclipse.jdt.internal.core.manipulation.util.BasicElementLabels;
 import org.eclipse.jdt.internal.corext.dom.ASTNodes;
 import org.eclipse.jdt.internal.corext.dom.IASTSharedValues;
+import org.eclipse.jdt.internal.corext.fix.AddUnimplementedMethodsOperation;
+import org.eclipse.jdt.internal.corext.fix.CompilationUnitRewriteOperationsFixCore.CompilationUnitRewriteOperation;
+import org.eclipse.jdt.internal.corext.fix.IProposableFix;
+import org.eclipse.jdt.internal.corext.fix.UnimplementedCodeFixCore;
 import org.eclipse.jdt.internal.corext.util.JavaModelUtil;
 import org.eclipse.jdt.internal.corext.util.Messages;
 import org.eclipse.jdt.ls.core.internal.corext.refactoring.nls.changes.CreateFileChange;
@@ -276,6 +280,7 @@ protected Change createChange() throws CoreException {
 			CompositeChange change = new CompositeChange(fName);
 			change.add(new CreateFileChange(targetType.getResource().getRawLocation(), "", ""));
 			change.add(constructNewCUChange(parentCU));
+
 			return change;
 		} else {
 			return null;
@@ -353,6 +358,25 @@ private CompilationUnitChange constructNewCUChange(ICompilationUnit cu) throws C
 		String lineDelimiter = StubUtility.getLineDelimiterUsed(fCompilationUnit.getJavaProject());
 		String typeStub = constructTypeStub(cu, fTypeNameWithParameters, Flags.AccPublic, lineDelimiter);
 		String cuContent = constructCUContent(cu, typeStub, lineDelimiter);
+
+		IType cuType = fCompilationUnit.findPrimaryType();
+		String[] permittedNames = cuType.getPermittedSubtypeNames();
+		boolean isPermitted = Arrays.asList(permittedNames).stream().anyMatch(p -> fTypeNameWithParameters.equals(p));
+
+		if (isPermitted) {
+			cu.createType(typeStub, null, false, null);
+			ASTParser parser = ASTParser.newParser(IASTSharedValues.SHARED_AST_LEVEL);
+			parser.setSource(cu);
+			parser.setResolveBindings(true);
+			CompilationUnit cuRoot = (CompilationUnit) parser.createAST(null);
+
+			AddUnimplementedMethodsOperation operation = new AddUnimplementedMethodsOperation((ASTNode) cuRoot.types().get(0));
+			if (operation.getMethodsToImplement().length > 0) {
+				IProposableFix fix = new UnimplementedCodeFixCore(CorrectionMessages.UnimplementedMethodsCorrectionProposal_description, cuRoot, new CompilationUnitRewriteOperation[] { operation });
+				return fix.createChange(null);
+			}
+		}
+
 		CompilationUnitChange cuChange = new CompilationUnitChange("", cu);
 		cuChange.setEdit(new InsertEdit(0, cuContent));
 		return cuChange;

org.eclipse.jdt.ls.tests/src/org/eclipse/jdt/ls/core/internal/correction/UnresolvedTypesQuickFixTest.java

@@ -1415,15 +1415,23 @@ public void testTypeInSealedTypeDeclaration() throws Exception {
 		StringBuilder buf = new StringBuilder();
 		buf.append("package test1;\n");
 		buf.append("public sealed interface E permits F {\n");
-		buf.append("}\n");
+		buf.append("void methodE();\n");
+		buf.append("}");
 		ICompilationUnit cu = pack1.createCompilationUnit("E.java", buf.toString(), false, null);
 
 		buf = new StringBuilder();
 		buf.append("package test1;\n");
 		buf.append("\n");
 		buf.append("public final class F implements E {\n");
 		buf.append("\n");
+		buf.append("\t@Override\n");
+		buf.append("\tpublic void methodE() {\n");
+		buf.append("\t\t// TODO Auto-generated method stub\n");
+		buf.append("\t\t\n");
+		buf.append("\t}\n");
+		buf.append("\n");
 		buf.append("}\n");
+		buf.append("\n");
 		Expected e1 = new Expected("Create class 'F'", buf.toString());
 
 		assertCodeActions(cu, e1);