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Java / Java 17 Garbage Collection Interview Questions

What is the purpose of a garbage collector in the JVM?

The garbage collector exists to guarantee memory safety without requiring the developer to manually track object lifetimes.

Specifically, it eliminates dangling-pointer and use-after-free bugs by only reclaiming objects once nothing can reach them, it prevents unbounded memory growth (and eventual OutOfMemoryError) by continuously recycling dead objects, and in most collectors it also compacts the heap - sliding live objects together - so free space doesn't fragment into unusable slivers.

By taking on this bookkeeping, the JVM lets application code focus on logic rather than memory ownership rules, which is one of the core productivity and safety trade-offs of managed runtimes versus languages like C or C++.

The JVM's garbage collector exists mainly to prevent:
Besides reclaiming dead objects, many collectors also help by:

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What is garbage collection in Java? What is the purpose of a garbage collector in the JVM? What are the types of garbage collectors available in Java 17? What is the default garbage collector in Java 17? What is the young generation in the JVM heap? What is the old (tenured) generation in the JVM heap? What is Metaspace in Java 17? What is a minor GC? What is a major GC? What is a full GC? Define a stop-the-world pause? What is the Eden space? What are Survivor spaces used for? How do you enable GC logging in Java 17? What is the purpose of the -Xmx and -Xms flags? Why is G1 GC the default collector since Java 9? How does G1 GC divide the heap into regions? What is a humongous object in G1 GC? How does G1 GC decide which regions to collect first? What is a remembered set in G1 GC? What is a card table and how does it help garbage collection? How do you configure G1 GC's pause time goal? What is the difference between G1 GC and Parallel GC? When should you choose ZGC over G1 GC? When should you choose Shenandoah over G1 GC? What happens during a G1 GC mixed collection? Why doesn't Java 17 include the CMS garbage collector? How is Parallel GC different from Serial GC? What is the difference between throughput and low-latency garbage collectors? How do you troubleshoot frequent full GCs in a Java 17 application? What is string deduplication in G1 GC? How does the JVM decide when to trigger a GC cycle? What is promotion in the context of generational garbage collection? Why should you avoid explicit calls to System.gc()? What is the difference between soft, weak, and phantom references? Explain the internal working of G1 GC's concurrent marking cycle? Explain the execution flow of a ZGC collection cycle? How does ZGC achieve sub-millisecond pause times using colored pointers? Explain the lifecycle of an object through the JVM heap under G1 GC? How does Shenandoah's Brooks pointer (forwarding pointer) enable concurrent compaction? What is an evacuation failure in G1 GC and how do you handle it? How does garbage collection behave differently in a containerized environment with cgroup memory limits? How can you optimize G1 GC for large heaps (32GB+)? Explain the tricolor marking algorithm used in concurrent garbage collectors? What are write barriers and load barriers, and how do collectors use them? How do you analyze a GC log to identify a memory leak? What is the difference between G1 GC's young-only phase and space-reclamation phase? How does adaptive sizing (ergonomics) work in the JVM's garbage collectors? Explain how class unloading interacts with garbage collection and Metaspace? Which is better for a low-latency trading application - ZGC or Shenandoah - and why?
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