some minor changes

Author: wumpz

src/main/java/net/sf/jsqlparser/util/cnfexpression/CloneHelper.java

@@ -40,7 +40,7 @@
  * @author messfish
  *
  */
-public class CloneHelper {
+class CloneHelper {
 
     /**
      * This method is used for changing the logical structure of the tree.

src/main/java/net/sf/jsqlparser/util/cnfexpression/MultiAndExpression.java

@@ -30,7 +30,7 @@
  * @author messfish
  *
  */
-public class MultiAndExpression extends MultipleExpression {
+public final class MultiAndExpression extends MultipleExpression {
 
     public MultiAndExpression(List<Expression> childlist) {
         super(childlist);

src/main/java/net/sf/jsqlparser/util/cnfexpression/MultiOrExpression.java

@@ -30,7 +30,7 @@
  * @author messfish
  *
  */
-public class MultiOrExpression extends MultipleExpression {
+public final class MultiOrExpression extends MultipleExpression {
 
     public MultiOrExpression(List<Expression> childlist) {
         super(childlist);

src/main/java/net/sf/jsqlparser/util/cnfexpression/MultipleExpression.java

@@ -34,7 +34,7 @@
  */
 public abstract class MultipleExpression implements Expression {
 
-    private List<Expression> childlist;
+    private final List<Expression> childlist;
 
     public MultipleExpression(List<Expression> childlist) {
         this.childlist = childlist;
@@ -82,7 +82,7 @@ public String toString() {
         for(int i=0; i<size(); i++) {
             sb.append(getChild(i));
             if(i!=size() - 1) {
-                sb.append(" " + getStringExpression() + " ");
+                sb.append(" ").append(getStringExpression()).append(" ");
             }
         }
         sb.append(")");