diff --git a/querydsl-core/src/main/java/com/mysema/query/JoinExpression.java b/querydsl-core/src/main/java/com/mysema/query/JoinExpression.java
index 4a844fce5..1b2c7341e 100644
--- a/querydsl-core/src/main/java/com/mysema/query/JoinExpression.java
+++ b/querydsl-core/src/main/java/com/mysema/query/JoinExpression.java
@@ -5,7 +5,10 @@
  */
 package com.mysema.query;
 
+import org.apache.commons.lang.builder.EqualsBuilder;
+
 import com.mysema.query.grammar.types.Expr;
+import com.mysema.query.util.Assert;
 
 /**
  * JoinExpression represents a join element in a Query instance
@@ -21,13 +24,13 @@ public class JoinExpression<T> {
     
     public JoinExpression(JoinType type, Expr<?> target) {
         this.type = type;
-        this.target = target;
+        this.target = Assert.notNull(target);
         this.metadata = null;
     }
     
     public JoinExpression(JoinType type, Expr<?> target, T metadata) {
         this.type = type;
-        this.target = target;
+        this.target = Assert.notNull(target);
         this.metadata = metadata;
     }
     
@@ -55,4 +58,23 @@ public class JoinExpression<T> {
         return type + " " + target;
     }
     
+    @Override
+    public int hashCode(){
+        return target.hashCode();
+    }
+    
+    @Override
+    public boolean equals(Object o){
+        if (o instanceof JoinExpression){
+            JoinExpression<?> j = (JoinExpression<?>)o;
+            return new EqualsBuilder()
+                .append(condition, j.condition)
+                .append(metadata, j.metadata)
+                .append(target, j.target)
+                .append(type, j.type).isEquals();
+        }else{
+            return false;
+        }
+    }
+    
 }
\ No newline at end of file
diff --git a/querydsl-core/src/main/java/com/mysema/query/grammar/types/Expr.java b/querydsl-core/src/main/java/com/mysema/query/grammar/types/Expr.java
index eba61c8a3..34f5f4977 100644
--- a/querydsl-core/src/main/java/com/mysema/query/grammar/types/Expr.java
+++ b/querydsl-core/src/main/java/com/mysema/query/grammar/types/Expr.java
@@ -24,7 +24,7 @@ import com.mysema.query.util.NumberUtil;
  * @author tiwe
  * @version $Id$
  */
-public abstract class Expr<D> {
+public abstract class Expr<D>{
         
     private final Class<? extends D> type;
     private String toString;
diff --git a/querydsl-core/src/main/java/com/mysema/query/grammar/types/Operation.java b/querydsl-core/src/main/java/com/mysema/query/grammar/types/Operation.java
index 84f9a54e7..2b813b96f 100644
--- a/querydsl-core/src/main/java/com/mysema/query/grammar/types/Operation.java
+++ b/querydsl-core/src/main/java/com/mysema/query/grammar/types/Operation.java
@@ -37,7 +37,9 @@ public interface Operation<OP,RT> {
         }
         public Expr<?>[] getArgs() {return args;}
         public Op<Boolean> getOperator() {return op;}  
-
+        public OBoolean clone(){
+            return new OBoolean(op, args.clone());
+        }
     }    
     
     /**
@@ -56,7 +58,10 @@ public interface Operation<OP,RT> {
             this(null, op, args);
         }
         public Expr<?>[] getArgs() {return args;}
-        public Op<OpType> getOperator() {return op;}         
+        public Op<OpType> getOperator() {return op;}
+        public OComparable<OpType,D> clone(){
+            return new OComparable<OpType,D>((Class<D>)getType(), op, args.clone());
+        }
     }
         
     /**
@@ -75,7 +80,10 @@ public interface Operation<OP,RT> {
             this(null, op, args);
         }
         public Expr<?>[] getArgs() {return args;}
-        public Op<OpType> getOperator() {return op;}       
+        public Op<OpType> getOperator() {return op;}    
+        public ONumber<OpType,D> clone(){
+            return new ONumber<OpType,D>((Class<D>)getType(), op, args.clone());
+        }
     }
     
     /**
@@ -90,6 +98,9 @@ public interface Operation<OP,RT> {
         }
         public Expr<?>[] getArgs() {return args;}
         public Op<String> getOperator() {return op;}    
+        public OString clone(){
+            return new OString(op, args.clone());
+        }
     }
     
     public static class OStringArray extends Expr<String[]> implements Operation<String,String[]>{
@@ -102,6 +113,9 @@ public interface Operation<OP,RT> {
         }
         public Expr<?>[] getArgs() {return args;}
         public Op<String> getOperator() {return op;}
+        public OStringArray clone(){
+            return new OStringArray(op, args.clone());
+        }
     }
     
 }