Add :format command to change format in interactive shell

src/main/java/kmql/CommandRegistry.java

@@ -7,6 +7,7 @@
 import java.util.concurrent.ConcurrentMap;
 
 import kmql.command.ExpireCommand;
+import kmql.command.FormatCommand;
 import kmql.command.HelpCommand;
 
 /**
@@ -18,6 +19,7 @@ public class CommandRegistry implements Iterable<Entry<String, Command>> {
     static {
         registerDefault("help", new HelpCommand());
         registerDefault("expire", new ExpireCommand());
+        registerDefault("format", new FormatCommand());
     }
 
     private final ConcurrentMap<String, Command> commands;

src/main/java/kmql/command/FormatCommand.java

@@ -0,0 +1,38 @@
+package kmql.command;
+
+import java.io.BufferedOutputStream;
+import java.io.PrintWriter;
+import java.util.List;
+
+import kmql.Command;
+import kmql.Engine;
+
+/**
+ * Expire table caches.
+ */
+public class FormatCommand implements Command {
+    @Override
+    public String help() {
+        return ":format FORMAT - Set output format";
+    }
+
+    @Override
+    public void execute(List<String> args, Engine engine, BufferedOutputStream output) {
+        PrintWriter pw = new PrintWriter(output);
+        try {
+            if (args.size() != 1) {
+                pw.println("Error usage: :format FORMAT");
+                return;
+            }
+            String format = args.get(0);
+            try {
+                engine.setOutputFormat(format);
+                pw.printf("Output format set to '%s'\n", format);
+            } catch (IllegalArgumentException ignored) {
+                pw.printf("Error: no such format '%s'\n", format);
+            }
+        } finally {
+            pw.flush();
+        }
+    }
+}

src/test/java/kmql/command/FormatCommandTest.java

@@ -0,0 +1,54 @@
+package kmql.command;
+
+import static java.util.Collections.emptyList;
+import static java.util.Collections.singletonList;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+import java.io.BufferedOutputStream;
+import java.sql.SQLException;
+import java.util.Arrays;
+
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnit;
+import org.mockito.junit.MockitoRule;
+
+import kmql.Database;
+import kmql.Engine;
+
+public class FormatCommandTest {
+    @Rule
+    public MockitoRule rule = MockitoJUnit.rule();
+    @Mock
+    private Database db;
+    @Mock
+    private Engine engine;
+    private final FormatCommand cmd = new FormatCommand();
+
+    @Before
+    public void setUp() {
+        doReturn(db).when(engine).db();
+    }
+
+    @Test
+    public void execute() throws SQLException {
+        cmd.execute(singletonList("json"), engine, mock(BufferedOutputStream.class));
+
+        verify(engine, times(1)).setOutputFormat("json");
+    }
+
+    @Test
+    public void executeInvalidArgs() throws SQLException {
+        cmd.execute(emptyList(), engine, mock(BufferedOutputStream.class));
+        cmd.execute(Arrays.asList("json", "table"), engine, mock(BufferedOutputStream.class));
+
+        verify(engine, never()).setOutputFormat(any(String.class));
+    }
+}