How to use method references in Java

Let’s consider a situation where a class overrides a method from its superclass and wants to refer to the superclass’s method version using a method reference:

class Base {
    public void show() {
        System.out.println("Base class show method.");
    }
}

public class Derived extends Base {
    @Override
    public void show() {
        System.out.println("Derived class show method.");
    }

    public void performShow(Runnable action) {
        action.run();
    }

    public void executeShow() {
        // Using 'super' with method reference to refer to the superclass's method
        performShow(super::show);
    }

    public static void main(String[] args) {
        new Derived().executeShow();
    }
}

In this example:

• super::show is a method reference that points to the show method of the superclass Base from the subclass Derived.
• The method executeShow uses this method reference to ensure that the superclass method is used, even though it is overridden in the current class.

Practical applications of method references

You can use method references wherever functional interfaces are applicable. They are particularly useful in stream operations, event listener setups, and scenarios involving methods from functional programming interfaces like Function, Consumer, Predicate, and Supplier. Here are some examples:

• Stream operations: In the Java Streams API, method references are commonly used in map, filter, forEach, and other operations. For example, stream.map(String::toUpperCase) uses a method reference to transform each element of a stream to uppercase.
• Event listeners: In GUI applications or event-driven programming, method references simplify the attachment of behavior to events. As an example, button.addActionListener(this::methodName) binds the method directly as an event listener.
• Constructors and factories: When using Java Streams and other APIs that require object generation, constructor references provide a shorthand for object creation, such as Stream.generate(ClassName::new).

Advantages and disadvantages of using method references

Method references enhance the expressive power of Java, allowing developers to write more functional-style programming. Their integration with Java’s existing features like streams and lambdas helps developers write clean, maintainable code. Features like method references are part of Java’s ongoing integration of functional programming patterns.

Using method references has a few advantages:

• Readability: Code written using method references is clearer and more succinct.
• Efficiency: Method references help reduce boilerplate code compared to traditional anonymous class implementations.
• Maintainability: Method references are easier than lambdas to understand and modify.

While method references can simplify and clarify code, they also have potential downsides that can lead to less maintainable code:

• Overuse in complex scenarios: Avoid using method references for methods with complex operations or when transformations on parameters are needed before method application. For instance, if a lambda involves modifying parameters before a method call, replacing it with a method reference might obscure what’s happening, as in stream.map(x -> process(x).save()) versus stream.map(MyClass::save).
• Reduced readability: Method references should not be used if they will make your code harder to understand. This can happen if the method being referenced is not self-explanatory or involves side effects that are not immediately clear from the context of use.
• Debugging challenges: Debugging might be slightly more challenging with method references because they do not provide as much inline detail as lambdas. When something goes wrong, it might be harder to pinpoint if a method reference is passing unexpected values.

By keeping these best practices and pitfalls in mind, developers can effectively use method references to write cleaner, more efficient Java code while avoiding common traps that might lead to code that is hard to read and maintain.

Are method references better than lambdas?

Method references can yield better performance than lambda expressions, especially in certain contexts. Here are the key points to consider regarding the performance benefits of method references:

1. Reduced code overhead: Method references often result in cleaner and more concise code. They do not require the boilerplate code that lambdas might (such as explicit parameter declaration), which can make the resulting bytecode slightly more efficient. This reduction in code complexity can make optimization by the Java virtual machine easier, potentially leading to better performance.
2. JVM optimization: The JVM has optimizations specifically tailored for method references, such as invoking invokevirtual or invokeinterface directly without the additional overhead of the lambda wrapper. These optimizations can potentially make method references faster or at least as fast as equivalent lambda expressions.
3. Reuse of method reference instances: Unlike lambda expressions, method references do not capture values from the enclosing context unless explicitly required. This often allows the JVM to cache and reuse instances of method references more effectively than lambda expressions, which might capture different values and thus may require separate instances each time they are created.
4. Specific scenarios: The performance advantage of method references can be more noticeable in specific scenarios where the same method is reused multiple times across a large dataset, such as in streams or repetitive method calls within loops. In these cases, the reduced overhead and potential caching benefits can contribute to better performance.
5. Benchmarking is key: While theoretical advantages exist, the actual performance impact can vary depending on the specific use case, the complexity of the lambda expression, and the JVM implementation. It’s generally a good practice to benchmark performance if it is a critical factor.