Refactored the ErrorManager and added unit tests

src/error/ErrorManager.java

@@ -1,6 +1,7 @@
 package error;
 
 import java.text.MessageFormat;
+import java.util.function.Consumer;
 
 /**
  * Prints error messages and potentially terminates the application.
@@ -14,15 +15,23 @@ public class ErrorManager {
     public static final String ANSI_YELLOW = "\u001B[33m";
     public static final String ANSI_PURPLE = "\u001B[35m";
 
-    public static void generateError(LispException lispException) {
+    private Runnable systemTerminatingFunction;
+    private Consumer<String> outputFunction;
+
+    public ErrorManager(Runnable systemTerminatingFunction, Consumer<String> outputFunction) {
+        this.systemTerminatingFunction = systemTerminatingFunction;
+        this.outputFunction = outputFunction;
+    }
+
+    public void generateError(LispException lispException) {
         String color = (lispException.getSeverity() >= CRITICAL_LEVEL) ? ANSI_PURPLE : ANSI_RED;
-        String formattedMessage = MessageFormat.format("{0}error: {1}{2}", color, lispException.getMessage(),
+        String formattedMessage = MessageFormat.format("{0}error: {1}{2}\n", color, lispException.getMessage(),
                                                        ANSI_RESET);
 
-        System.out.println(formattedMessage);
+        outputFunction.accept(formattedMessage);
 
         if (lispException.getSeverity() >= CRITICAL_LEVEL) {
-            System.exit(1);
+            systemTerminatingFunction.run();
         }
     }
 

src/eval/LispFunction.java

@@ -23,10 +23,8 @@ public abstract class LispFunction {
 
     /**
      * Determine if the arguments passed to this Lisp function should be evaluated. A subclass
-     * should override this method to return <code>false</code> if it does not want its arguments to
-     * be evaluated prior to being passed.
-     *
-     * @return <code>true</code>
+     * should override this method to return false if it does not want its arguments to be evaluated
+     * prior to being passed.
      */
     public boolean evaluateArguments() {
         return true;

src/main/LispInterpreter.java

@@ -6,14 +6,15 @@
 
 package main;
 
-import parser.*;
-import sexpression.SExpression;
-import eval.*;
+import java.io.FileInputStream;
+import java.io.FileNotFoundException;
+import java.text.MessageFormat;
+
 import error.ErrorManager;
 import error.LispException;
-
-import java.io.*;
-import java.text.MessageFormat;
+import eval.EVAL;
+import parser.LispParser;
+import sexpression.SExpression;
 
 /**
  * This is an interpreter for the Lisp programming language. It takes the name of a file as a
@@ -36,6 +37,7 @@ public class LispInterpreter {
      */
     public static void main(String[] args) {
         LispParser parser = null;
+        ErrorManager errorManager = new ErrorManager(new TerminateInterpreter(), System.out::print);
         boolean interactive = false;
 
         if (args.length > 0) {
@@ -44,7 +46,7 @@ public class LispInterpreter {
             try {
                 parser = new LispParser(new FileInputStream(args[0]), args[0]);
             } catch (FileNotFoundException e) {
-                ErrorManager.generateError(new LispException() {
+                errorManager.generateError(new LispException() {
 
                     private static final long serialVersionUID = 1L;
 
@@ -78,10 +80,10 @@ public class LispInterpreter {
                 System.out.println(result);
             } catch (LispException e) {
                 LispInterpreter.erasePrompt(interactive);
-                ErrorManager.generateError(e);
+                errorManager.generateError(e);
             } catch (RuntimeException e) {
                 LispInterpreter.erasePrompt(interactive);
-                ErrorManager.generateError(new LispException() {
+                errorManager.generateError(new LispException() {
 
                     private static final long serialVersionUID = 1L;
 
@@ -113,4 +115,12 @@ public class LispInterpreter {
         }
     }
 
+    private static class TerminateInterpreter implements Runnable {
+
+        @Override
+        public void run() {
+            System.exit(1);
+        }
+    }
+
 }

test/error/ErrorManagerTester.java

@@ -0,0 +1,101 @@
+package error;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import java.util.HashSet;
+import java.util.Set;
+import java.util.function.Consumer;
+
+import org.junit.Test;
+
+public class ErrorManagerTester {
+
+    private static final String TERMINATED = "terminated";
+    private static final String MESSAGE = "message";
+
+    private ErrorManager createErrorManagerWithIndicators(Set<String> indicatorSet) {
+        Runnable terminationFunction = new Runnable() {
+
+            @Override
+            public void run() {
+                indicatorSet.add(TERMINATED);
+            }
+        };
+
+        Consumer<String> outputFunction = new Consumer<String>() {
+
+            @Override
+            public void accept(String t) {
+                indicatorSet.add(MESSAGE);
+            }
+        };
+
+        return new ErrorManager(terminationFunction, outputFunction);
+    }
+
+    private LispException createLispException(int severity) {
+        return new LispException() {
+
+            private static final long serialVersionUID = 1L;
+
+            @Override
+            public int getSeverity() {
+                return severity;
+            }
+
+            @Override
+            public String getMessage() {
+                return MESSAGE;
+            }
+        };
+    }
+
+    @Test
+    public void givenCriticalExceptionSeverity_RunsProvidedTerminationFunction() {
+        Set<String> indicatorSet = new HashSet<>();
+        ErrorManager errorManager = createErrorManagerWithIndicators(indicatorSet);
+
+        errorManager.generateError(createLispException(ErrorManager.CRITICAL_LEVEL));
+        assertTrue(indicatorSet.contains(TERMINATED));
+    }
+
+    @Test
+    public void givenNonCriticalExceptionSeverity_DoesNotRunProvidedTerminationFunction() {
+        Set<String> indicatorSet = new HashSet<>();
+        ErrorManager errorManager = createErrorManagerWithIndicators(indicatorSet);
+
+        errorManager.generateError(createLispException(0));
+        assertFalse(indicatorSet.contains(TERMINATED));
+    }
+
+    @Test
+    public void noMessageDisplayedBeforeError() {
+        Set<String> indicatorSet = new HashSet<>();
+        createErrorManagerWithIndicators(indicatorSet);
+
+        assertFalse(indicatorSet.contains(TERMINATED));
+        assertFalse(indicatorSet.contains(MESSAGE));
+    }
+
+    @Test
+    public void usesOutputFunctionToDisplayMessages_NoTermination() {
+        Set<String> indicatorSet = new HashSet<>();
+        ErrorManager errorManager = createErrorManagerWithIndicators(indicatorSet);
+
+        errorManager.generateError(createLispException(0));
+        assertFalse(indicatorSet.contains(TERMINATED));
+        assertTrue(indicatorSet.contains(MESSAGE));
+    }
+
+    @Test
+    public void usesOutputFunctionToDisplayMessages_WithTermination() {
+        Set<String> indicatorSet = new HashSet<>();
+        ErrorManager errorManager = createErrorManagerWithIndicators(indicatorSet);
+
+        errorManager.generateError(createLispException(ErrorManager.CRITICAL_LEVEL));
+        assertTrue(indicatorSet.contains(TERMINATED));
+        assertTrue(indicatorSet.contains(MESSAGE));
+    }
+
+}