Refactored and added unit tests for listp

src/function/builtin/LISTP.java

@@ -1,32 +1,21 @@
 package function.builtin;
 
-import function.LispFunction;
+import function.*;
 import sexpression.*;
 
-/**
- * <code>LISTP</code> represents the LISTP function in Lisp.
- */
 public class LISTP extends LispFunction {
 
-    // The number of arguments that LISTP takes.
-    private static final int NUM_ARGS = 1;
+    private ArgumentValidator argumentValidator;
 
-    public SExpression call(Cons argList) {
-        // retrieve the number of arguments passed to LISTP
-        int argListLength = LENGTH.getLength(argList);
-
-        // make sure we have received the proper number of arguments
-        if (argListLength != NUM_ARGS) {
-            Cons originalSExpr = new Cons(new Symbol("LISTP"), argList);
-            String errMsg = "too " + ((argListLength > NUM_ARGS) ? "many" : "few") + " arguments given to LISTP: "
-                            + originalSExpr;
-
-            throw new RuntimeException(errMsg);
+    public LISTP() {
+        this.argumentValidator = new ArgumentValidator("LISTP");
+        this.argumentValidator.setExactNumberOfArguments(1);
     }
 
-        SExpression arg = argList.getCar();
+    public SExpression call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
 
-        return (arg.listp() ? Symbol.T : Nil.getInstance());
+        return argumentList.getCar().listp() ? Symbol.T : Nil.getInstance();
     }
 
 }

test/function/builtin/LISTPTester.java

@@ -0,0 +1,39 @@
+package function.builtin;
+
+import static testutil.TestUtilities.*;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.*;
+
+public class LISTPTester {
+
+    @Test
+    public void testListpWithList_ReturnsT() {
+        String input = "(listp '(1))";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("T"));
+    }
+
+    @Test
+    public void testListWithNonList_ReturnsNil() {
+        String input = "(listp 1)";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("NIL"));
+    }
+
+    @Test(expected = TooFewArgumentsException.class)
+    public void testListpWithTooFewArguments() {
+        String input = "(listp)";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("NIL"));
+    }
+
+    @Test(expected = TooManyArgumentsException.class)
+    public void testListpWithTooManyArguments() {
+        String input = "(listp '() '())";
+
+        assertSExpressionsMatch(evaluateString(input), parseString("NIL"));
+    }
+
+}