Fix use of default values

- Change parsing to extract and use option default value
  and also hook it with @ShellOption.
- Fixes #409

spring-shell-core/src/main/java/org/springframework/shell/command/CommandParser.java

@@ -18,6 +18,7 @@ package org.springframework.shell.command;
 import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.Comparator;
 import java.util.Deque;
 import java.util.List;
@@ -313,6 +314,21 @@ public interface CommandParser {
 						}
 					})
 					.collect(Collectors.toList());
+
+				// Check options which didn't get matched and add parser result
+				// for those having default value.
+				List<CommandOption> defaultValueOptionsToCheck = new ArrayList<>(options);
+				results.stream()
+					.forEach(pr -> {
+						if (pr.option != null) {
+							defaultValueOptionsToCheck.remove(pr.option);
+						}
+					});
+					defaultValueOptionsToCheck.stream()
+					.filter(co -> co.getDefaultValue() != null)
+					.forEach(co -> {
+						results.add(ParserResult.of(co, Collections.emptyList(), co.getDefaultValue(), null));
+					});
 				return ParserResults.of(results);
 			}
 

spring-shell-core/src/test/java/org/springframework/shell/command/CommandExecutionTests.java

@@ -421,6 +421,25 @@ public class CommandExecutionTests extends AbstractCommandTests {
 		assertThat(pojo1.method4Arg1).isNull();
 	}
 
+	@Test
+	public void testDefaultValue() {
+		CommandRegistration r1 = CommandRegistration.builder()
+			.command("command1")
+			.withOption()
+				.longNames("arg1")
+				.defaultValue("defaultValue1")
+				// .position(0)
+				// .arity(OptionArity.EXACTLY_ONE)
+				.and()
+			.withTarget()
+				.method(pojo1, "method4")
+				.and()
+			.build();
+		execution.evaluate(r1, new String[]{});
+		assertThat(pojo1.method4Count).isEqualTo(1);
+		assertThat(pojo1.method4Arg1).isEqualTo("defaultValue1");
+	}
+
 	@Test
 	public void testRequiredArg() {
 		CommandRegistration r1 = CommandRegistration.builder()