Missing required option doesn't throw

This is a proposed fix for: airlift#22

Tests still need to be fixed.

src/main/java/io/airlift/airline/ParseResult.java

@@ -0,0 +1,22 @@
+package io.airlift.airline;
+
+import com.google.common.collect.ImmutableList;
+
+import java.util.Collections;
+import java.util.List;
+
+public class ParseResult {
+    private List<ParseException> errors = Collections.emptyList();
+
+    public void addError(ParseException error) {
+        errors.add(error);
+    }
+
+    public boolean hasErrors() {
+        return errors.size() != 0;
+    }
+
+    public List<ParseException> getErrors() {
+        return ImmutableList.copyOf(errors);
+    }
+}

src/main/java/io/airlift/airline/SingleCommand.java

@@ -57,12 +57,31 @@ public C parse(String... args)
     }
 
     public C parse(Iterable<String> args)
+    {
+        ParseResult parseResult = new ParseResult();
+        C command = parse(parseResult, args);
+
+        // For backward compatibility we fail fast here. Given that exceptions aren't a great way to handle user errors
+        // since there can be multiple we may consider deprecating this approach.
+        if (parseResult.hasErrors()) {
+            throw parseResult.getErrors().get(0);
+        }
+
+        return command;
+    }
+
+    public C parse(ParseResult parseResult, String... args)
+    {
+        return parse(parseResult, ImmutableList.copyOf(args));
+    }
+
+    public C parse(ParseResult parseResult, Iterable<String> args)
     {
         checkNotNull(args, "args is null");
-        
+
         Parser parser = new Parser();
         ParseState state = parser.parseCommand(commandMetadata, args);
-        validate(state);
+        validate(state, parseResult);
 
         CommandMetadata command = state.getCommand();
 
@@ -75,35 +94,35 @@ public C parse(Iterable<String> args)
                 ImmutableMap.<Class<?>, Object>of(CommandMetadata.class, commandMetadata));
     }
 
-    private void validate(ParseState state)
+    private void validate(ParseState state, ParseResult parseResult)
     {
         CommandMetadata command = state.getCommand();
         if (command == null) {
             List<String> unparsedInput = state.getUnparsedInput();
             if (unparsedInput.isEmpty()) {
-                throw new ParseCommandMissingException();
+                parseResult.addError(new ParseCommandMissingException());
             }
             else {
-                throw new ParseCommandUnrecognizedException(unparsedInput);
+                parseResult.addError(new ParseCommandUnrecognizedException(unparsedInput));
             }
         }
 
         ArgumentsMetadata arguments = command.getArguments();
         if (state.getParsedArguments().isEmpty() && arguments != null && arguments.isRequired()) {
-            throw new ParseArgumentsMissingException(arguments.getTitle());
+            parseResult.addError(new ParseArgumentsMissingException(arguments.getTitle()));
         }
 
         if (!state.getUnparsedInput().isEmpty()) {
-            throw new ParseArgumentsUnexpectedException(state.getUnparsedInput());
+            parseResult.addError(new ParseArgumentsUnexpectedException(state.getUnparsedInput()));
         }
 
         if (state.getLocation() == Context.OPTION) {
-            throw new ParseOptionMissingValueException(state.getCurrentOption().getTitle());
+            parseResult.addError(new ParseOptionMissingValueException(state.getCurrentOption().getTitle()));
         }
 
         for (OptionMetadata option : command.getAllOptions()) {
             if (option.isRequired() && !state.getParsedOptions().containsKey(option)) {
-                throw new ParseOptionMissingException(option.getOptions().iterator().next());
+                parseResult.addError(new ParseOptionMissingException(option.getOptions().iterator().next()));
             }
         }
     }

src/test/java/io/airlift/airline/SingleCommandTest.java

@@ -49,6 +49,7 @@
 
 import static com.google.common.base.Predicates.compose;
 import static com.google.common.base.Predicates.equalTo;
+import static com.google.common.base.Predicates.instanceOf;
 import static com.google.common.collect.Iterables.find;
 import static io.airlift.airline.SingleCommand.singleCommand;
 import static org.testng.Assert.assertTrue;
@@ -171,6 +172,14 @@ public void multipleUnparsedFail()
         singleCommand(ArgsMultipleUnparsed.class).parse();
     }
 
+    @Test
+    public void multipleUnparsedFailWithResult()
+    {
+        ParseResult result = new ParseResult();
+        singleCommand(ArgsMultipleUnparsed.class).parse(result);
+        assertParseResultHasErrorOfType(result, IllegalArgumentException.class);
+    }
+
     public void privateArgs()
     {
         ArgsPrivate args = singleCommand(ArgsPrivate.class).parse("-verbose", "3");
@@ -244,12 +253,27 @@ public void requiredMainParameters()
         singleCommand(ArgsRequired.class).parse();
     }
 
+    @Test
+    public void requiredMainParamertersWithParseResult() {
+        ParseResult result = new ParseResult();
+        singleCommand(ArgsRequired.class).parse(result);
+        assertParseResultHasErrorOfType(result, ParseException.class);
+    }
+
     @Test(expectedExceptions = ParseException.class, expectedExceptionsMessageRegExp = ".*option.*missing.*")
     public void requiredOptions()
     {
         singleCommand(OptionsRequired.class).parse();
     }
 
+    @Test
+    public void requiredOptionsWithParseResult()
+    {
+        ParseResult result = new ParseResult();
+        singleCommand(OptionsRequired.class).parse();
+        assertParseResultHasErrorOfType(result, ParseException.class);
+    }
+
     @Test
     public void ignoresOptionalOptions()
     {
@@ -388,4 +412,10 @@ public static class CommandTest
         public Boolean interactive = false;
 
     }
+
+    private void assertParseResultHasErrorOfType(ParseResult result, Class clazz) {
+        Assert.assertEquals(result.hasErrors(), true);
+        String errorMessage = String.format("Expected ParseResult to contain at least one instance of: '%s'", clazz);
+        Assert.assertNotNull(find(result.getErrors(), instanceOf(IllegalArgumentException.class)), errorMessage);
+    }
 }