Super simple start

src/main/java/net/minecraft/commands/CommandDispatcher.java

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+package net.minecraft.commands;
+
+import com.google.common.collect.Maps;
+import net.minecraft.commands.builder.CommandBuilder;
+import net.minecraft.commands.exceptions.CommandException;
+import net.minecraft.commands.exceptions.UnknownCommandException;
+
+import java.util.Map;
+
+public class CommandDispatcher {
+    private final Map<String, Runnable> commands = Maps.newHashMap();
+
+    public CommandBuilder createCommand(final String name) {
+        return new CommandBuilder() {
+            @Override
+            public void onFinish() {
+                if (commands.containsKey(name)) {
+                    throw new IllegalArgumentException("New command " + name + " conflicts with existing command " + name);
+                }
+                commands.put(name, getCommandExecutor());
+            }
+        };
+    }
+
+    public void execute(String command) throws CommandException {
+        Runnable runnable = commands.get(command);
+        if (runnable == null) {
+            throw new UnknownCommandException();
+        }
+
+        runnable.run();
+    }
+}

src/main/java/net/minecraft/commands/builder/CommandBuilder.java

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+package net.minecraft.commands.builder;
+
+public abstract class CommandBuilder {
+    private boolean finished;
+    private Runnable commandExecutor;
+
+    public void finish() {
+        if (finished) {
+            throw new IllegalStateException("Cannot finish() multiple times!");
+        }
+        if (commandExecutor == null) {
+            throw new IllegalStateException("Cannot finish() without a command executor!");
+        }
+        onFinish();
+        finished = true;
+    }
+
+    protected abstract void onFinish();
+
+    public CommandBuilder executes(Runnable runnable) {
+        this.commandExecutor = runnable;
+        return this;
+    }
+
+    public Runnable getCommandExecutor() {
+        return commandExecutor;
+    }
+}

src/main/java/net/minecraft/commands/builder/package-info.java

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+@ParametersAreNonnullByDefault
+package net.minecraft.commands.builder;
+
+import javax.annotation.ParametersAreNonnullByDefault;

src/main/java/net/minecraft/commands/exceptions/CommandException.java

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+package net.minecraft.commands.exceptions;
+
+public class CommandException extends Exception {
+}

src/main/java/net/minecraft/commands/exceptions/UnknownCommandException.java

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+package net.minecraft.commands.exceptions;
+
+public class UnknownCommandException extends CommandException {
+}

src/main/java/net/minecraft/commands/exceptions/package-info.java

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+@ParametersAreNonnullByDefault
+package net.minecraft.commands.exceptions;
+
+import javax.annotation.ParametersAreNonnullByDefault;

src/main/java/net/minecraft/commands/package-info.java

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+@ParametersAreNonnullByDefault
+package net.minecraft.commands;
+
+import javax.annotation.ParametersAreNonnullByDefault;

src/main/test/net/minecraft/commands/CommandDispatcherTest.java

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+package net.minecraft.commands;
+
+import net.minecraft.commands.exceptions.UnknownCommandException;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+import org.mockito.runners.MockitoJUnitRunner;
+
+@RunWith(MockitoJUnitRunner.class)
+public class CommandDispatcherTest {
+    CommandDispatcher subject;
+    @Mock Runnable runnable;
+
+    @Before
+    public void setUp() throws Exception {
+        subject = new CommandDispatcher();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testDuplicateCommand() throws Exception {
+        subject.createCommand("foo").executes(runnable).finish();
+        subject.createCommand("foo").executes(runnable).finish();
+    }
+
+    @Test
+    public void testCreateAndExecuteCommand() throws Exception {
+        subject.createCommand("foo").executes(runnable).finish();
+
+        subject.execute("foo");
+        Mockito.verify(runnable).run();
+    }
+
+    @Test(expected = UnknownCommandException.class)
+    public void testExecuteUnknownCommand() throws Exception {
+        subject.execute("foo");
+    }
+}

src/main/test/net/minecraft/commands/builder/CommandBuilderTest.java

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+package net.minecraft.commands.builder;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Answers;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+import org.mockito.runners.MockitoJUnitRunner;
+
+@RunWith(MockitoJUnitRunner.class)
+public class CommandBuilderTest {
+    @Mock(answer = Answers.CALLS_REAL_METHODS) CommandBuilder builder;
+    @Mock Runnable commandExecutor;
+
+    @Before
+    public void setUp() throws Exception {
+        Mockito.doNothing().when(builder).onFinish();
+    }
+
+    @Test
+    public void testFinish() throws Exception {
+        builder.executes(commandExecutor);
+        builder.finish();
+
+        Mockito.verify(builder).onFinish();
+    }
+
+    @Test(expected = IllegalStateException.class)
+    public void testFinishTwice() throws Exception {
+        builder.executes(commandExecutor);
+        builder.finish();
+        builder.finish();
+    }
+
+    @Test(expected = IllegalStateException.class)
+    public void testFinishWithoutCommandExecutor() throws Exception {
+        builder.finish();
+    }
+}