# Tutorial: Types and the class hierarchy¶

## Overview¶

The standard QL library for Java represents Java types by means of the Type class and its various subclasses.

In particular, class PrimitiveType represents primitive types that are built into the Java language (such as boolean and int), whereas RefType and its subclasses represent reference types, that is classes, interfaces, array types, and so on. This includes both types from the Java standard library (like java.lang.Object) and types defined by non-library code.

Class RefType also models the class hierarchy: member predicates getASupertype and getASubtype allow you to find a reference type’s immediate super types and sub types. For example, consider the following Java program:

class A {}

interface I {}

class B extends A implements I {}


Here, class A has exactly one immediate super type (java.lang.Object) and exactly one immediate sub type (B); the same is true of interface I. Class B, on the other hand, has two immediate super types (A and I), and no immediate sub types.

To determine ancestor types (including immediate super types, and also their super types, etc.), we can use transitive closure. For example, to find all ancestors of B in the example above, we could use the following query:

import java

from Class B
where B.hasName("B")
select B.getASupertype+()


See this in the query console. If this query were run on the example snippet above, the query would return A, I, and java.lang.Object.

Tip

If you want to see the location of B as well as A, you can replace B.getASupertype+() with B.getASupertype*() and re-run the query.

Besides class hierarchy modeling, RefType also provides member predicate getAMember for accessing members (that is, fields, constructors, and methods) declared in the type, and predicate inherits(Method m) for checking whether the type either declares or inherits a method m.

## Example: Finding problematic array casts¶

As an example of how to use the class hierarchy API, we can write a query that finds downcasts on arrays, that is, cases where an expression e of some type A[] is converted to type B[], such that B is a (not necessarily immediate) subtype of A.

This kind of cast is problematic, since downcasting an array results in a runtime exception, even if every individual array element could be downcast. For example, the following code throws a ClassCastException:

Object[] o = new Object[] { "Hello", "world" };
String[] s = (String[])o;


If the expression e happens to actually evaluate to a B[] array, on the other hand, the cast will succeed:

Object[] o = new String[] { "Hello", "world" };
String[] s = (String[])o;


In this tutorial, we do not try to distinguish these two cases. Our query should simply look for cast expressions ce that cast from some type source to another type target, such that:

• Both source and target are array types.
• The element type of source is a transitive super type of the element type of target.

This recipe is not too difficult to translate into a QL query:

import java

from CastExpr ce, Array source, Array target
where source = ce.getExpr().getType() and
target = ce.getType() and
target.getElementType().(RefType).getASupertype+() = source.getElementType()
select ce, "Potentially problematic array downcast."


See this in the query console. Many projects return results for this query.

Note that by casting target.getElementType() to a RefType, we eliminate all cases where the element type is a primitive type, that is, target is an array of primitive type: the problem we are looking for cannot arise in that case. Unlike in Java, a cast in QL never fails: if an expression cannot be cast to the desired type, it is simply excluded from the query results, which is exactly what we want.

### Improvements¶

Running this query on old Java code, before version 5, often returns many false positive results arising from uses of the method Collection.toArray(T[]), which converts a collection into an array of type T[].

In code that does not use generics, this method is often used in the following way:

List l = new ArrayList();
// add some elements of type A to l
A[] as = (A[])l.toArray(new A[0]);


Here, l has the raw type List, so l.toArray has return type Object[], independent of the type of its argument array. Hence the cast goes from Object[] to A[] and will be flagged as problematic by our query, although at runtime this cast can never go wrong.

To identify these cases, we can create two QL classes that represent, respectively, the Collection.toArray class, and calls to this method or any method that overrides it:

/** class representing java.util.Collection.toArray(T[]) */
class CollectionToArray extends Method {
CollectionToArray() {
this.getDeclaringType().hasQualifiedName("java.util", "Collection") and
this.hasName("toArray") and
this.getNumberOfParameters() = 1
}
}

/** class representing calls to java.util.Collection.toArray(T[]) */
class CollectionToArrayCall extends MethodAccess {
CollectionToArrayCall() {
exists(CollectionToArray m |
this.getMethod().getSourceDeclaration().overrides*(m)
)
}

/** the call's actual return type, as determined from its argument */
Array getActualReturnType() {
result = this.getArgument(0).getType()
}
}


Notice the use of getSourceDeclaration and overrides in the constructor of CollectionToArrayCall: we want to find calls to Collection.toArray and to any method that overrides it, as well as any parameterized instances of these methods. In our example above, for instance, the call l.toArray resolves to method toArray in the raw class ArrayList. Its source declaration is methodtoArray in the generic class ArrayList, which overrides AbstractCollection.toArray, which in turn overrides Collection.toArray.

Using these new classes we can extend our query to exclude calls to toArray on an argument of type A[] which are then cast to A[]:

import java

// Insert the class definitions from above

from CastExpr ce, Array source, Array target
where source = ce.getExpr().getType() and
target = ce.getType() and
target.getElementType().(RefType).getASupertype+() = source.getElementType() and
not ce.getExpr().(CollectionToArrayCall).getActualReturnType() = target
select ce, "Potentially problematic array downcast."


See this in the query console. Notice that fewer results are found by this improved query.

## Example: Finding mismatched contains checks¶

As another example, we develop a query that finds uses of Collection.contains where the type of the queried element is unrelated to the element type of the collection, thus guaranteeing that the test will always return false.

For example, Apache Zookeeper used to have a snippet of code similar to the following in class QuorumPeerConfig:

Map<Object, Object> zkProp;

// ...

if (zkProp.entrySet().contains("dynamicConfigFile")){
// ...
}


Since zkProp is a map from Object to Object, zkProp.entrySet returns a collection of type Set<Entry<Object, Object>>. Such a set cannot possibly contain an element of type String. (The code has since been fixed to use zkProp.containsKey.)

In general, we want to find calls to Collection.contains (or any of its overriding methods in any parameterized instance of Collection), such that the type E of collection elements and the type A of the argument to contains are unrelated, that is, they have no common subtype.

We start by creating a QL class that describes java.util.Collection:

class JavaUtilCollection extends GenericInterface {
JavaUtilCollection() {
this.hasQualifiedName("java.util", "Collection")
}
}


To make sure we have not mistyped anything, we can run a simple test query:

from JavaUtilCollection juc
select juc


This query should return precisely one result.

Next, we can create a QL class that describes java.util.Collection.contains:

class JavaUtilCollectionContains extends Method {
JavaUtilCollectionContains() {
this.getDeclaringType() instanceof JavaUtilCollection and
this.hasStringSignature("contains(Object)")
}
}


Notice that we use hasStringSignature to check that:

• The method in question has name contains.
• It has exactly one argument.
• The type of the argument is Object.

Alternatively, we could have implemented these three checks more verbosely using hasName, getNumberOfParameters, and getParameter(0).getType() instanceof TypeObject.

As before, it is a good idea to test the new QL class by running a simple query to select all instances of JavaUtilCollectionContains; again there should only be a single result.

Now we want to identify all calls to Collection.contains, including any methods that override it, and considering all parameterized instances of Collection and its subclasses. That is, we are looking for method accesses where the source declaration of the invoked method (reflexively or transitively) overrides Collection.contains. We encode this in a QL class JavaUtilCollectionContainsCall:

class JavaUtilCollectionContainsCall extends MethodAccess {
JavaUtilCollectionContainsCall() {
exists(JavaUtilCollectionContains jucc |
this.getMethod().getSourceDeclaration().overrides*(jucc)
)
}
}


This definition is slightly subtle, so you should run a short query to test that JavaUtilCollectionContainsCall correctly identifies calls to Collection.contains.

For every call to contains, we are interested in two things: the type of the argument, and the element type of the collection on which it is invoked. So we need to add two member predicates getArgumentType and getCollectionElementType to class JavaUtilCollectionContainsCall to compute this information.

The former is easy:

Type getArgumentType() {
result = this.getArgument(0).getType()
}


For the latter, we proceed as follows:

• Find the declaring type D of the contains method being invoked.
• Find a (reflexive or transitive) super type S of D that is a parameterized instance of java.util.Collection.
• Return the (only) type argument of S.

We encode this in QL as follows:

Type getCollectionElementType() {
exists(RefType D, ParameterizedInterface S |
D = this.getMethod().getDeclaringType() and
D.hasSupertype*(S) and S.getSourceDeclaration() instanceof JavaUtilCollection and
result = S.getTypeArgument(0)
)
}


Having added these two member predicates to JavaUtilCollectionContainsCall, we need to write a predicate that checks whether two given reference types have a common subtype:

predicate haveCommonDescendant(RefType tp1, RefType tp2) {
exists(RefType commondesc | commondesc.hasSupertype*(tp1) and commondesc.hasSupertype*(tp2))
}


Now we are ready to write a first version of our query:

import java

// Insert the class definitions from above

from JavaUtilCollectionContainsCall juccc, Type collEltType, Type argType
where collEltType = juccc.getCollectionElementType() and argType = juccc.getArgumentType() and
not haveCommonDescendant(collEltType, argType)
select juccc, "Element type " + collEltType + " is incompatible with argument type " + argType


### Improvements¶

For many programs, this query yields a large number of false positive results due to type variables and wild cards: if the collection element type is some type variable E and the argument type is String, for example, QL will consider that the two have no common subtype, and our query will flag the call. An easy way to exclude such false positive results is to simply require that neither collEltType nor argType are instances of TypeVariable.

Another source of false positives is autoboxing of primitive types: if, for example, the collection’s element type is Integer and the argument is of type int, predicate haveCommonDescendant will fail, since int is not a RefType. Thus, our query should check that collEltType is not the boxed type of argType.

Finally, null is special because its type (known as <nulltype> in QL) is compatible with every reference type, so we should exclude it from consideration.

Adding these three improvements, our final query becomes:

import java

// Insert the class definitions from above

from JavaUtilCollectionContainsCall juccc, Type collEltType, Type argType
where collEltType = juccc.getCollectionElementType() and argType = juccc.getArgumentType() and
not haveCommonDescendant(collEltType, argType) and
not collEltType instanceof TypeVariable and not argType instanceof TypeVariable and
not collEltType = argType.(PrimitiveType).getBoxedType() and
not argType.hasName("<nulltype>")
select juccc, "Element type " + collEltType + " is incompatible with argument type " + argType