Ensuring Type Safety With Collections.checkedXXX() in Java

1. Introduction

In this article, we’ll explore the Collections.checkedXXX() methods, demonstrating how they can help catch type mismatches early, prevent bugs, and enhance code maintainability.

In Java, type safety is crucial to avoid runtime errors and ensure reliable code. These methods provide a way to enforce type safety at runtime for collections. We’ll delve into the various Collections.checkedXXX() methods and their benefits for using them effectively in our Java applications.

2. Understanding Type Safety in Java Collections

Type safety in Java collections is essential for preventing runtime errors and ensuring that a collection only contains elements of a specific type. Java generics, introduced in Java 5, provide compile-time type checking, enabling us to define collections with a particular type. For example, List<String> ensures that only strings can be added to the list.

However, we compromise type safety when dealing with raw types, unchecked operations, or legacy code that doesn’t use generics. This is where Collections.checkedXXX() methods come into play. These methods wrap a collection with a dynamic type check, enforcing type safety at runtime.

For instance, Collections.checkedList(new ArrayList(), String.class) returns a list that throws a ClassCastException if we add a non-string element. This additional layer of runtime checking complements compile-time checks, catching type mismatches early and making the code more robust.

We can use these methods especially when we expose collections through APIs or when working with collections populated by external sources. They help ensure that the elements in the collection adhere to the expected type, reducing the risk of bugs and simplifying debugging and maintenance.

Let’s learn about these methods now.

3. Understanding the Collections.checkedCollection() Method

First, let’s check the signature of this method:

public static <E> Collection<E> checkedCollection(Collection<E> c, Class<E> type)

The method returns a dynamically type-safe view of the specified collection. If we attempt to insert an element of the wrong type, a ClassCastException occurs immediately. Assuming that a collection contains no incorrectly typed elements before generating a dynamically type-safe view and that all subsequent access to the collection takes place through the view, it guarantees that the collection cannot contain an incorrectly typed element.

The Java language’s generics mechanism provides compile-time (static) type checking, but bypassing this mechanism with unchecked casts is possible. Typically, this isn’t an issue because the compiler warns about unchecked operations.

However, there are situations where we need more than static type checking. For instance, consider a scenario where we pass a collection to a third-party library, and the library code mustn’t corrupt the collection by inserting an element of the wrong type.

If we wrap the collection with a dynamically typesafe view, it allows for the quick identification of the source of the issue.

For instance, we have a declaration like this:

Collection<String> c = new Collection<>();

We can replace it with the following expression that wraps the original collection into the checked collection:

Collection<String> c = Collections.checkedCollection(new Collection(), String.class);

If we rerun the program, it fails when we insert an incorrectly typed element into the collection. This clearly shows where the issue is.

Using dynamically typesafe views also has benefits for debugging. For example, if a program encounters a ClassCastException, we must have added an incorrectly typed element into a parameterized collection. However, this exception can occur at any time after we insert an improper element, providing little information about the actual source of the problem.

4. Using the Collections.checkedCollection() Method

Let’s now understand how to use this method.

Suppose that we have a utility to verify data. Here is its implementation:

class DataProcessor {

    public boolean checkPrefix(Collection<?> data) {
        boolean result = true;
        if (data != null) {
            for (Object item : data) {
                if (item != null && !((String) item).startsWith("DATA_")) {
                    result = false;
                    break;
                }
            }
        }
        return result;
    }
}

The method checkPrefix() checks if the items in the collection start with the prefix “DATA_” or not. It expects that the items are not null and are of String type.

Let’s now test it:

@Test
void givenGenericCollection_whenInvalidTypeDataAdded_thenFailsAfterInvocation() {
    Collection data = new ArrayList<>();
    data.add("DATA_ONE");
    data.add("DATA_TWO");
    data.add(3); // should have failed here

    DataProcessor dataProcessor = new DataProcessor();

    assertThrows(ClassCastException.class, () -> dataProcessor.checkPrefix(data)); // but fails here
}

The test adds a String and an Integer to a generic collection, expecting a ClassCastException when processing. However, the error occurs in the checkPrefix() method, not during addition, since the collection isn’t type-checked.

Now let’s see how checkedCollection() helps us catch such errors earlier when we attempt to add an item of the wrong type to the collection:

@Test
void givenGenericCollection_whenInvalidTypeDataAdded_thenFailsAfterAdding() {
    Collection data = Collections.checkedCollection(new ArrayList<>(), String.class);
    data.add("DATA_ONE");
    data.add("DATA_TWO");

    assertThrows(ClassCastException.class, () -> {
      data.add(3); // fails here
    });

    DataProcessor dataProcessor = new DataProcessor();
    boolean result = dataProcessor.checkPrefix(data);
    assertTrue(result);
}

The test uses Collections.checkedCollection() to ensure we have added only strings to the collection. When attempting to add an integer, it throws a ClassCastException immediately, enforcing type safety before reaching the checkPrefix() method.

We cannot specify a type for this collection because doing so would break the contract and cause the IDE or syntax checker to raise errors.

The Collections class provides several checkedXXX methods, such as checkedList(), checkedMap(), checkedSet(), checkedQueue(), checkedNavigableMap(), checkedNavigableSet(), checkedSortedMap() and checkedSortedSet(). These methods enforce type safety at runtime for various collection types. These methods wrap collections with type checks, ensuring that we add only elements of the specified type, helping to prevent ClassCastException and maintain type integrity.

5. Notes About the Returned Collection

The returned collection doesn’t delegate the hashCode() and equals() operations to the backing collection. Instead, it relies on the Object‘s equals() and hashCode() methods. This approach ensures that the contracts of these operations are preserved, especially in cases where the backing collection is a set or a list.

Additionally, if the specified collection is serializable, the collection returned by the method will also be serializable.

It’s essential to note that because null is considered a value of any reference type, the collection returned by the method allows the insertion of null elements, as long as the backing collection allows it.

6. Conclusion

In this article, we explored the Collections.checkedXXX methods, demonstrating how they enforce runtime type safety in Java collections. We saw how checkedCollection() can prevent type errors by ensuring that we add only elements of a specified type.

Using these methods enhances code reliability and helps catch type mismatches early. By leveraging these tools, we can write safer, more robust code with better runtime type checks.