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Posted on Jun 1, 2023

How to Avoid Unchecked Casts in Java Programs

Unchecked cast refers to the process of converting a variable of one data type to another data type without checks by the Java compiler.

This operation is unchecked because the compiler does not verify if the operation is valid or safe. Unchecked casts can lead to runtime errors, such as ClassCastException, when the program tries to assign an object to a variable of an incompatible type.

Hence, it is important to avoid unchecked casts in Java programs to prevent potential errors and ensure the program's reliability.

Consequences of Unchecked Casts

In Java programs, unchecked casts can lead to several issues. The most common problem is a ClassCastException at runtime, which occurs when we try to cast an object to a wrong type. This can cause the program to crash or behave unexpectedly.

Unchecked casts also violate the type safety of the Java language, which can lead to bugs that are difficult to detect and debug. Additionally, unchecked casts can make the code less readable and maintainable, as they hide the true type of objects and dependencies between components.

Therefore, it is important to avoid unchecked casts and use other mechanisms, such as generics or polymorphism, to ensure type safety and code quality in Java programs.

How Unchecked Casts Occur

Unchecked casts in Java programs occur when an object of one type is assigned to a reference of another type without proper type checking. This can happen when a programmer assumes that a reference to a superclass is actually a reference to its subclass and tries to cast it into that subclass. If the assumption is incorrect, the cast will result in a ClassCastException at runtime.

Unchecked casts can also occur when dealing with raw types, which are generic types without any type parameters specified. In such cases, the compiler cannot perform type checking and the programmer must ensure that the proper type conversions are made. Failing to do so can result in unchecked casts and potential runtime errors.

Why unchecked casts are problematic

In Java, unchecked casts allow a programmer to cast any object reference to any other reference type without providing any type information at compile-time. While this flexibility may seem useful, it can lead to serious run-time errors. If the object being casted is not actually of the type specified, a ClassCastException will occur at run-time.

Unchecked casts can cause difficult-to-debug errors in large and complex codebases, as it may not be immediately clear where the error originated. Additionally, unchecked casts can undermine Java's type system, creating code that is harder to read, maintain, and reason about. As a result, avoiding unchecked casts should be a priority when writing Java programs.

Examples of Unchecked Casts in Java

Unchecked casts are a common source of Java program errors. Here are some examples of unchecked casts:

This cast statement above can result in a class cast exception if the object referred to by obj is not a List.

In this case, the cast could fail at runtime if the array contains objects of a type other than String.

Finally, this cast could fail if the object referred to by obj is not a Map.

Using Generics to Avoid Unchecked Casts in Java

In Java, Generics is a powerful feature that allows you to write classes and methods that are parameterized by one or more types. Generics are a way of making your code more type-safe and reusable. With generics, you can define classes and methods that work on a variety of types, without having to write separate code for each type.

Using generics in Java programs has several advantages. It enables type safety at compile-time, which can prevent ClassCastException errors at runtime. With generics, the compiler can detect type mismatches and prevent them from happening, which leads to more robust and reliable code. It also allows for code reuse without sacrificing type safety and improve performance by avoiding unnecessary casting and allowing for more efficient code generation.

Generics allow Java developers to create classes and methods that can work with different data types. For example, a List can be defined to hold any type of object using generics. Here's an example:

In this example, we create a List that holds String objects. We can add String objects to the list and iterate over them using a for-each loop. The use of generics allows us to ensure type safety and avoid unchecked casts. Another example is the Map interface, which can be used to map keys to values of any data type using generics.

Using the instanceof operator to Avoid Unchecked Casts in Java

The instanceof operator is a built-in operator in Java that is used to check whether an object is an instance of a particular class or interface. The operator returns a boolean value - true if the object is an instance of the specified class or interface, and false otherwise.

The instanceof operator is defined as follows:

where object is the object that is being checked, and class/interface is the class or interface that is being tested against.

The instanceof operator can be useful in situations where we need to perform different operations based on the type of an object. It provides a way to check the type of an object at runtime, which can help prevent errors that can occur when performing unchecked casts.

Here are some examples of using the instanceof operator:

In this example, we use the instanceof operator to check whether the object obj is an instance of the String class. If it is, we perform an explicit cast to convert the object to a String and call the toUpperCase() method on it.

In this example, we use the instanceof operator to check whether the List object passed as a parameter is an instance of the ArrayList or LinkedList classes. If it is, we perform an explicit cast to convert the List to the appropriate class and perform different operations on it depending on its type.

Overall, using the instanceof operator can help us write more robust and flexible code. However, it should be used judiciously as it can also make code harder to read and understand.

Using Polymorphism to Avoid Unchecked Casts in Java

Polymorphism is a fundamental concept in object-oriented programming. It refers to the ability of an object or method to take on multiple forms. It allows us to write code that can work with objects of different classes as long as they inherit from a common superclass or implement a common interface. This helps to reduce code duplication and makes our programs more modular and extensible.

Some of the advantages of using polymorphism are:

• Code reusability: We can write code that can work with multiple objects without having to rewrite it for each specific class.
• Flexibility: Polymorphism allows us to write code that can adapt to different types of objects at runtime.
• Ease of maintenance: By using polymorphism, changes made to a superclass or interface are automatically propagated to all its subclasses.

Here are a few examples of how you can use polymorphism to avoid unchecked casts in Java:

Example 1: Shape Hierarchy

In this example, the abstract class Shape defines the common behavior draw(), which is implemented by the concrete classes Circle and Rectangle. By using the Shape reference type, we can invoke the draw() method on different objects without the need for unchecked casts.

Example 2: Polymorphic Method Parameter

In this example, the makeAnimalSound() method accepts an Animal parameter. We can pass different Animal objects, such as Dog or Cat, without the need for unchecked casts. The appropriate implementation of the makeSound() method will be invoked based on the dynamic type of the object.

By utilizing polymorphism in these examples, we achieve type safety and avoid unchecked casts, allowing for cleaner and more flexible code.

Tips to Avoid Unchecked Casts in Java Programs

Unchecked casts in Java programs can introduce runtime errors and compromise type safety. Fortunately, there are several techniques and best practices you can employ to avoid unchecked casts and ensure a more robust codebase. Here are some effective tips to help you write Java programs that are type-safe and free from unchecked cast exceptions.

• Use generic classes, interfaces, and methods to ensure that your code handles compatible types without relying on casting.
• Embrace polymorphism by utilizing abstract classes and interfaces, define common behavior and interact with objects through their common type.
• Check the type of an object using the instanceof operator. This allows you to verify that an object is of the expected type before proceeding with the cast.
• Favor composition over inheritance, where classes contain references to other classes as instance variables.
• Familiarize yourself with design patterns that promote type safety and avoid unchecked casts. Patterns such as Factory Method, Builder, and Strategy provide alternative approaches to object creation and behavior, often eliminating the need for explicit casting.
• Clearly define the contracts and preconditions for your methods. A well-defined contract helps ensure that the method is called with appropriate types, improving overall code safety.
• Refactor your code and improve its overall design. Look for opportunities to apply the aforementioned tips, such as utilizing generics, polymorphism, or design patterns.

Unchecked casts in Java programs can introduce runtime errors and undermine type safety. By adopting practices like using generics, leveraging polymorphism, checking types with instanceof, favoring composition over inheritance, reviewing design patterns, employing design by contract, and improving code design, you can avoid unchecked casts and enhance the robustness of your Java programs. Prioritizing type safety will result in more reliable code and a smoother development process.

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• 13 May 2020

Using varargs when working with generics

By the way, do you remember what varargs is.

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Mutable, unmodifiable, and immutable empty List in Java

This article will discuss different ways to create a mutable, unmodifiable, immutable, and fixed-length empty list in Java.

Mutable lists supports modification operations such as add, remove, and clear on it. Unmodifiable lists are “read-only” wrappers over other lists. They do not support add, remove, and clear operations, but we can modify their underlying list. Lists that guarantee that no change in the list will ever be visible (even if their underlying list is modified) are referred to as immutable .

It is worth noting that making a list final will not make it unmodifiable or immutable. We can still add elements or remove elements from it. Only the reference to the list is final.

1. Mutable Empty List

⮚ Using Plain Java

We can simply use the ArrayList constructor, which constructs an empty resizable-array implementation of the List interface. The following code takes advantage of the new “diamond” syntax introduced in Java SE 7.

⮚ Using Guava

Guava’s Lists.newArrayList() creates a mutable, empty ArrayList instance. This method is deprecated in Java 7 and above. We should call the ArrayList constructor directly.

⮚ Java 8

We can use the Java 8 Stream to construct empty collections by combining stream factory methods and collectors. Collectors.toList() returns a Collector that accumulates the input elements into a new List. To create an empty list, we can pass an empty array.

2. Unmodifiable Empty List

⮚ Using Collections

Collections unmodifiableList() returns an unmodifiable view of the specified empty list.

⮚ Using Apache Collections: ListUtils Class

Apache Commons Collections ListUtils.unmodifiableList() returns an unmodifiable list backed by the specified empty list.

  Both the above methods throw a NullPointerException if the specified list is null. If we try to modify the returned list, the list will throw an UnsupportedOperationException . However, any changes in the original mutable list will be visible in the unmodifiable list.

3. Immutable Empty List

There are several ways to create an immutable empty list in Java. The list will throw an UnsupportedOperationException if any modification operation is performed on it.

⮚ Using Arrays.asList()

Arrays.asList() method returns a fixed-size list backed by the specified array. If we don’t specify an array, this will return an immutable empty list.

  The list will throw an UnsupportedOperationException if an add or remove operation is performed on it and an ArrayIndexOutOfBoundsException if any update operation is performed (for instance, via the List.set(int, Object) method).

We can use Collections.EMPTY_LIST that returns a serializable and immutable empty list.

  The above method might throw an unchecked assignment warning. The following example demonstrates the type-safe way to obtain an empty list:

⮚ In Java 8

We could adapt the Collectors.toList() collector discussed earlier to always produce an immutable empty list, as shown below:

Guava provides several static utility methods that can be used to obtain an immutable empty list.

1. ImmutableList.copyOf returns an immutable empty list if the specified list is empty.

  The method will throw a NullPointerException if the specified list is null, and any changes in the underlying mutable list will not be visible in the immutable list.

  2. Guava also provides a builder that can create an immutable empty list instance similarly.

  3. ImmutableList.of() can also be used to return an immutable empty list.

This list behaves and performs comparably to Collections.emptyList() , and is preferable mainly for consistency and maintainability of code. The list is internally cast to any type as it will never hold any elements.

⮚ Using Apache Collections

Apache Commons Collections provides ListUtils.EMPTY_LIST that returns an immutable empty list.

⮚ In Java 9

Java 9 comes with static factory methods on the List interface that can create compact, unmodifiable instances. We can use the List.of() to create an immutable empty list. The list will throw an UnsupportedOperationException if any modification operation is performed on it.

4. Fixed Sized Empty List

There is another type of empty list possible in Java apart from mutable, unmodifiable, and immutable lists called fixed-sized list.

Apache Commons Collections ListUtils provides fixedSizeList() method that can return a fixed-sized empty list backed by the specified empty list. We cannot add elements to the returned list, and it will throw an UnsupportedOperationException if any resize operation is performed on it.

That’s all about mutable, unmodifiable, and immutable empty List in Java.

Mutable, unmodifiable, and immutable empty Map in Java

Mutable, unmodifiable, and immutable empty Set in Java

Immutable List in Java

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Unchecked assignment: ‘java.util.List’ to ‘java.util.Collection’

I am having an adapter where i have two lists one list is for InvestorsList where it comes with the list of investors and the other list is called investorListFull which is used to filter results when searching.

Below is how i have declared the lists

Below is how the lists are assigned in my recyclerview adapter constructor

Below is how i am filtering results in investors list

I am getting Unchecked assignment error in publish results investorList.addAll((List) filterResults.values);

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I am getting Unchecked cast error in publish results investorList.addAll((List) filterResults.values);

That’s because you’re doing an unchecked cast. Actually, you’re doing both a checked and an unchecked cast there.

(List) filterResults.values is a checked cast. This means that the Java compiler will insert a checkcast instruction into the bytecode to ensure that filterResults.values (an Object ) really is an instance of List .

However, investorList.addAll expects a List<Investor> , not a List . List is a raw type . You can pass a raw-typed List to a method expecting a List<Something> ; but this is flagged as unsafe because the compiler can’t guarantee that it really is a List<Something> – there is nothing about the List that makes it a “list of Something “, because of type erasure. The compiler can insert a checkcast instruction to ensure it’s a List ; but not one to ensure it’s a List<Something> : this fact is unchecked .

What it’s saying with the warning is “there may be something wrong here; I just can’t prove it’s wrong”. If you know for sure – by construction – that filterResults.values really is a List<Investor> , casting it to List<Investor> is safe.

You should write the line as:

Note that this will still give you an unchecked cast warning, because it’s still an unchecked cast – you just avoid the use of raw types as well.

If you feel confident in suppressing the warning, declare a variable, so you can suppress the warning specifically on that variable, rather than having to add the suppression to the method or class; and document why it’s safe to suppress there:

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Improved Compiler Warnings and Errors When Using Non-Reifiable Formal Parameters with Varargs Methods

This page covers the following topics:

Heap Pollution

Variable arguments methods and non-reifiable formal parameters, potential vulnerabilities of varargs methods with non-reifiable formal parameters, suppressing warnings from varargs methods with non-reifiable formal parameters.

Most parameterized types, such as ArrayList<Number> and List<String> , are non-reifiable types . A non-reifiable type is a type that is not completely available at runtime. At compile time, non-reifiable types undergo a process called type erasure during which the compiler removes information related to type parameters and type arguments. This ensures binary compatibility with Java libraries and applications that were created before generics. Because type erasure removes information from parameterized types at compile-time, these types are non-reifiable.

Heap pollution occurs when a variable of a parameterized type refers to an object that is not of that parameterized type. This situation can only occur if the program performed some operation that would give rise to an unchecked warning at compile-time. An unchecked warning is generated if, either at compile-time (within the limits of the compile-time type checking rules) or at runtime, the correctness of an operation involving a parameterized type (for example, a cast or method call) cannot be verified.

Consider the following example:

During type erasure, the types ArrayList<Number> and List<String> become ArrayList and List , respectively.

The variable ls has the parameterized type List<String> . When the List referenced by l is assigned to ls , the compiler generates an unchecked warning; the compiler is unable to determine at compile time, and moreover knows that the JVM will not be able to determine at runtime, if l refers to a List<String> type; it does not. Consequently, heap pollution occurs.

As a result, at compile time, the compiler generates another unchecked warning at the add statement. The compiler is unable to determine if the variable l refers to a List<String> type or a List<Integer> type (and another heap pollution situation occurs). However, the compiler does not generate a warning or error at the get statement. This statement is valid; it is calling the List<String>.get method to retrieve a String object. Instead, at runtime, the get statement throws a ClassCastException .

In detail, a heap pollution situation occurs when the List object l , whose static type is List<Number> , is assigned to another List object, ls , that has a different static type, List<String> . However, the compiler still allows this assignment. It must allow this assignment to preserve backwards compatibility with versions of Java SE that do not support generics. Because of type erasure, List<Number> and List<String> both become List . Consequently, the compiler allows the assignment of the object l , which has a raw type of List , to the object ls .

Furthermore, a heap pollution situation occurs when the l.add method is called. The static type second formal parameter of the add method is String , but this method is called with an actual parameter of a different type, Integer . However, the compiler still allows this method call. Because of type erasure, the type of the second formal parameter of the add method (which is defined as List<E>.add(int,E) ) becomes Object . Consequently, the compiler allows this method call because, after type erasure, the l.add method can add any object of type Object , including an object of Integer type, which is a subtype of Object .

Consider the method ArrayBuilder.addToList in the following example. It is a variable arguments (also known as varargs) method that adds the objects of type T contained in the elements varargs formal parameter to the List listArg :

The Java SE 7 compiler generates the following warning for the definition of the method ArrayBuilder.addToList :

When the compiler encounters a varargs method, it translates the varargs formal parameter into an array. However, the Java programming language does not permit the creation of arrays of parameterized types. In the method ArrayBuilder.addToList , the compiler translates the varargs formal parameter T... elements to the formal parameter T[] elements , an array. However, because of type erasure, the compiler converts the varargs formal parameter to Object[] elements . Consequently, there is a possibility of heap pollution. See the next section, Potential Vulnerabilities of Varargs Methods with Non-Reifiable Formal Parameters , for more information.

Note : The Java SE 5 and 6 compilers generate this warning when the ArrayBuilder.addToList is called; in this example, the warning is generated for the class HeapPollutionExample . These compilers do not generate the warning at the declaration site. However, the Java SE 7 generates the warning at both the declaration site and the call site (unless the warnings are preempted with annotations; see Suppressing Warnings from Varargs Methods with Non-Reifiable Formal Parameters for more information). The advantage of generating a warning when a compiler encounters a varargs method that has a non-reifiable varargs formal parameter at the declaration site as opposed to the call site is that there is only one declaration site; there are potentially many call sites.

The method ArrayBuilder.faultyMethod shows why the compiler warns you about these kinds of methods. The first statement of this method assigns the varargs formal parameter l to the Object array objectArgs :

This statement can potentially introduce heap pollution. A value that does match the parameterized type of the varargs formal parameter l can be assigned to the variable objectArray , and thus can be assigned to l . However, the compiler does not generate an unchecked warning at this statement. The compiler has already generated a warning when it translated the varargs formal parameter List<String>... l to the formal parameter List[] l . This statement is valid; the variable l has the type List[] , which is a subtype of Object[] .

Consequently, the compiler does not issue a warning or error if you assign a List object of any type to any array component of the objectArray array as shown by this statement:

This statement assigns to the first array component of the objectArray array with a List object that contains one object of type Integer .

Suppose you call the ArrayBuilder.makeArray method with the following statement:

At runtime, the JVM throws a ClassCastException at the following statement:

The object stored in the first array component of the variable l has the type List<Integer> , but this statement is expecting an object of type List<String> .

If you declare a varargs method that has parameterized parameters, and you ensure that the body of the method does not throw a ClassCastException or other similar exception due to improper handling of the varargs formal parameter (as shown in the ArrayBuilder.faultyMethod method), you can suppress the warning that the compiler generates for these kinds of varargs methods by using one of the following options:

Add the following annotation to static and non-constructor method declarations:

Unlike the @SuppressWarnings annotation, the @SafeVarargs annotation is a documented part of the method's contract; this annotation asserts that the implementation of the method will not improperly handle the varargs formal parameter.

Add the following annotation to the method declaration:

Unlike the @SafeVarargs annotation, the @SuppressWarnings("varargs") does not suppress warnings generated from the method's call site.

Use the compiler option -Xlint:varargs .

For example, the following version of the ArrayBuilder class has two additional methods, addToList2 and addToList3 :

The Java compiler generates the following warnings for this example:

• At the method's declaration: [unchecked] Possible heap pollution from parameterized vararg type T
• When the method is called: [unchecked] unchecked generic array creation for varargs parameter of type List<String>[]
• addToList2 : When the method is called (no warning is generated at the method's declaration): [unchecked] unchecked generic array creation for varargs parameter of type List<String>[]
• addToList3 : No warnings are generated either at the method's declaration or when it is called.

Note : In Java SE 5 and 6, it is the responsibility of the programmer who calls a varargs method that has a non-reifiable varargs formal parameter to determine whether heap pollution would occur. However, if this programmer did not write such a method, he or she cannot easily determine this. In Java SE 7, it is the responsibility of the programmer who writes these kinds of varargs methods to ensure that they properly handle the varargs formal parameter and ensure heap pollution does not occur.

• Unchecked Cast in Java
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What Is the Unchecked Cast Warning in Java

Understanding unchecked cast warnings in java, preventing unchecked cast warnings, best practices for preventing unchecked cast warnings in java.

Java is a programming language that enforces type safety, which means that we should always specify the type of data that we are going to store or use and cannot store incompatible types in them.

Discover how to prevent unchecked cast warnings in Java. Explore the causes, solutions, and best practices for ensuring type safety and reliability in your code.

An unchecked cast warning in Java occurs when the compiler cannot ensure type safety during a casting operation. It warns the developer about potential runtime errors, such as ClassCastException , that may occur due to type mismatches.

Unchecked cast warnings typically arise when casting from a generic type to a specific type or when casting to a parameterized type without proper type checking. Addressing these warnings is crucial to ensure code reliability and prevent unexpected runtime errors.

In Java programming, unchecked cast warnings are common occurrences that indicate potential type safety issues in your code. Let’s delve into two significant causes of unchecked cast warnings:

Understanding these causes is crucial for maintaining code quality and preventing unexpected runtime errors. Let’s explore solutions to address unchecked cast warnings in Java code.

In Java programming, unchecked cast warnings signify potential type safety issues that can lead to runtime errors if not addressed properly. Direct casting from raw types and casting without type checking are common scenarios where unchecked cast warnings occur.

Understanding how to prevent these warnings is crucial for maintaining code integrity and preventing unexpected runtime errors. In this section, we’ll explore effective techniques to prevent unchecked cast warnings in Java by addressing direct casting from raw types and casting without proper type checking.

Code Example:

The code example provided illustrates two common scenarios in Java where unchecked cast warnings can arise: direct casting from raw types and casting without proper type checking.

In the first scenario, a raw ArrayList named rawList is instantiated, and a String ( Hello ) is added to it. Initially, an attempt is made to directly cast rawList to a parameterized type List<String> .

Such direct casting from a raw type can trigger unchecked cast warnings as it bypasses type safety checks. To address this, we adopt a safer approach by creating a new ArrayList , stringList1 .

By passing rawList as a parameter to its constructor, we ensure that stringList1 maintains the correct generic type. This action effectively prevents unchecked cast warnings, ensuring type safety throughout the code.

In the second scenario, an Object ( obj ) is assigned the value World . There is an initial attempt to cast obj directly to List<String> without performing proper type checking.

Such casting without type checking can lead to unchecked cast warnings as it lacks verification of type compatibility. To mitigate this risk, we instantiate a new ArrayList ( stringList2 ) and add obj after performing necessary type checking and casting.

By ensuring that the object being added to stringList2 is indeed a String type, we maintain type safety and avoid unchecked cast warnings.

By employing these techniques, developers can effectively prevent unchecked cast warnings in their Java code, thereby enhancing type safety and reducing the likelihood of unexpected runtime errors.

The code will produce the following output:

By following the demonstrated techniques, developers can effectively prevent unchecked cast warnings in Java code, ensuring type safety and reducing the risk of unexpected runtime errors. Understanding and implementing these practices are essential for maintaining code reliability and integrity in Java applications.

Use Generics Consistently

Utilize parameterized types (generics) consistently throughout your code to ensure type safety and prevent unchecked cast warnings.

Perform Type Checking Before Casting

Always perform proper type checking before casting objects to parameterized types to ensure compatibility and prevent unchecked cast warnings.

Avoid Raw Types

Minimize the use of raw types and prefer parameterized types whenever possible to maintain type safety and prevent unchecked cast warnings.

Consider Type Inference

Leverage type inference where applicable to automatically determine generic types and reduce the likelihood of unchecked cast warnings.

Review and Test Code

Regularly review and test your code to identify and address any instances of unchecked cast warnings, ensuring robustness and reliability.

Unchecked cast warnings in Java signal potential type safety issues arising from direct casting from raw types and casting without proper type checking. The causes include direct casting from raw types and casting without type checking.

Solutions involve creating new parameterized types instead of directly casting from raw types and performing type checking before casting. Best practices include using generics consistently, avoiding raw types, considering type inference, and regularly reviewing and testing code.

By implementing these strategies, developers can ensure type safety and prevent unchecked cast warnings in Java programs.

Rupam Saini is an android developer, who also works sometimes as a web developer., He likes to read books and write about various things.