Is instanceof Really Slow in Java? A Benchmarking Study

Instanceof Performance in Java

When dealing with inheritance in Java, developers often encounter the question of whether using the instanceof operator impacts performance. While it's understood that object-oriented design strives to minimize instanceof usage, this query focuses solely on its performance implications.

In a typical scenario, a base class with multiple subclasses exists, and a function operating on the base class performs instanceof checks on each subclass. An alternative approach involves using a "type ID" integer primitive with a bitmask for subclass categorization. This approach raises the question of whether the JVM optimizes instanceof for superior speed.

Benchmarking Results

To evaluate different implementation options, a benchmark program was created. The options included:

• instanceof implementation (as a reference)
• Object-oriented approach with an abstract class and overridden test method
• Custom type implementation
• getClass() == _.class implementation

The benchmark measures the average execution time of each option over 10,000 iterations.

Performance Analysis

The results indicate that in Java 1.8, instanceof is the fastest approach, with getClass() being a close second. Here's a summary:

Operation	Runtime	Relative to instanceof
INSTANCEOF	39,598 ns/op	100.00%
GETCLASS	39,687 ns/op	100.22%
TYPE	46,295 ns/op	116.91%
OO	48,078 ns/op	121.42%

Conclusion

In Java 1.8, instanceof represents the fastest approach for checking class instances. While getClass() performs comparably, alternative approaches like TYPE and object-oriented inheritance incur a slight performance penalty. Developers seeking optimal performance should prioritize instanceof over the other options.

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