Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is Garbage Collection in Java?
Garbage Collection is the automatic process of reclaiming the runtime unused memory by destroying the objects that are no longer reachable or referenced by the program.
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Ques 2. What is the role of the JVM (Java Virtual Machine) in garbage collection?
The JVM is responsible for managing the memory, including garbage collection. It runs the garbage collector to identify and reclaim unused memory.
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Ques 3. What is the purpose of the Young Generation in the Java heap?
The Young Generation is the part of the heap where new objects are created. It is designed for short-lived objects, and garbage collection occurs more frequently in this generation.
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Ques 4. What is the purpose of the 'finalize()' method in Java?
The 'finalize()' method is called by the garbage collector before an object is reclaimed. It allows an object to perform cleanup operations before being garbage collected.
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Ques 5. What is the role of the Old Generation in the Java heap?
The Old Generation is the part of the heap that holds long-lived objects. Objects that survive multiple garbage collection cycles in the Young Generation are eventually promoted to the Old Generation.
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Ques 6. What is the purpose of the 'System.gc()' method in Java?
The 'System.gc()' method is a hint to the JVM that suggests running the garbage collector. However, its actual execution is at the discretion of the JVM, and it may be ignored in some implementations.
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Ques 7. Explain the role of the '-XX:+UseG1GC' JVM option in enabling the G1 garbage collector.
The '-XX:+UseG1GC' option is used to enable the G1 garbage collector in Java. It instructs the JVM to use the G1 collector as the garbage collection algorithm.
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Intermediate / 1 to 5 years experienced level questions & answers
Ques 8. Explain the purpose of the finalize() method.
The finalize() method is called by the garbage collector before reclaiming the memory occupied by an object. It can be overridden to perform cleanup operations before an object is garbage collected.
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Ques 9. Name the different types of garbage collectors in Java.
The types of garbage collectors in Java include Serial Garbage Collector, Parallel Garbage Collector, CMS (Concurrent Mark-Sweep) Garbage Collector, and G1 (Garbage-First) Garbage Collector.
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Ques 10. Explain the difference between 'gc()' and 'System.gc()' in Java.
'gc()' is a hint to the garbage collector to run, while 'System.gc()' is a method that suggests to the JVM to run the garbage collector, but the actual execution is at the discretion of the JVM.
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Ques 11. Explain the concept of Generational Garbage Collection.
Generational Garbage Collection is based on the observation that most objects become unreachable shortly after they are created. It divides the heap into two main areas: the Young Generation and the Old Generation.
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Ques 12. What is the Eden Space in the Young Generation of the Java heap?
The Eden Space is the part of the Young Generation where new objects are initially allocated. Objects surviving garbage collection in Eden are moved to the Survivor Spaces.
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Ques 13. Explain the purpose of the Survivor Spaces in the Young Generation.
The Survivor Spaces (S0 and S1) in the Young Generation are used to hold objects that survive one garbage collection cycle in the Eden Space. Objects can be promoted to the Old Generation from the Survivor Spaces.
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Ques 14. What is the difference between garbage collection and memory leak?
Garbage collection is the process of automatically identifying and reclaiming unused memory, while a memory leak occurs when the application fails to release memory that is no longer needed, leading to increased memory consumption over time.
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Ques 15. What is the 'OutOfMemoryError' in Java, and how is it related to garbage collection?
The 'OutOfMemoryError' is an exception thrown when the Java Virtual Machine (JVM) runs out of memory. It can occur if the garbage collector is unable to free up enough memory to satisfy the memory allocation request.
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Ques 16. Explain the concept of garbage collection ergonomics in Java.
Garbage collection ergonomics involves the JVM automatically tuning garbage collection parameters based on the characteristics of the application, such as heap size, pause time goals, and allocation rates.
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Ques 17. What is the role of the '-Xmx' and '-Xms' JVM options in garbage collection?
The '-Xmx' option sets the maximum heap size, while the '-Xms' option sets the initial heap size. Properly configuring these options can impact garbage collection performance and overall application memory usage.
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Ques 18. Explain the concept of 'Garbage Collection Roots' in Java.
Garbage Collection Roots are objects that are considered to be reachable at all times and serve as starting points for the garbage collector. Examples include local variables, active threads, and static variables.
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Ques 19. How can you monitor and analyze garbage collection performance in Java?
Garbage collection performance can be monitored using tools like VisualVM, JConsole, and other profiling tools. Analyzing garbage collection logs and metrics helps identify issues and optimize performance.
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Ques 20. What is the 'CMSInitialMarkPause' phase in the CMS garbage collector?
The 'CMSInitialMarkPause' is the initial phase of the CMS (Concurrent Mark-Sweep) garbage collector. During this phase, the collector identifies and marks live objects in the Old Generation while briefly pausing application threads.
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Experienced / Expert level questions & answers
Ques 21. What is the PermGen space, and is it still used in Java 8 and later?
PermGen (Permanent Generation) space was used to store metadata related to classes. In Java 8 and later, PermGen space is replaced by Metaspace, which is more flexible and avoids memory leaks related to class metadata.
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Ques 22. Explain the concept of garbage collection tuning in Java.
Garbage collection tuning involves configuring the garbage collector to meet specific performance goals. It includes selecting the appropriate garbage collector algorithm and adjusting related parameters.
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Ques 23. Explain the concept of garbage collection pause times.
Garbage collection pause times refer to the periods during which application threads are stopped while the garbage collector performs its tasks. Minimizing pause times is essential for maintaining application responsiveness.
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Ques 24. Explain the concept of the 'garbage-first' (G1) garbage collector.
The G1 garbage collector is designed to provide high throughput and low-latency garbage collection. It divides the heap into regions and uses a series of Garbage-First (GF) queues to prioritize collection of regions with the most garbage.
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Ques 25. How does the Java Virtual Machine (JVM) handle circular references in garbage collection?
The JVM uses a reachability analysis to determine if objects are reachable or not. Circular references are handled by the garbage collector, which identifies and collects objects that are no longer reachable.
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Ques 26. What is the 'Reference' class in Java, and how is it used in garbage collection?
The 'Reference' class is part of the java.lang.ref package and provides a way to create and manipulate references to objects. It is often used in advanced memory management scenarios and garbage collection.
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Ques 27. Explain the concept of 'GC overhead limit exceeded' error in Java.
The 'GC overhead limit exceeded' error occurs when the JVM spends too much time on garbage collection, exceeding a specified threshold (98% by default) of the total time. It indicates a potential performance issue.
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Ques 28. Explain the concept of 'Card Table' in the context of the G1 garbage collector.
The 'Card Table' is a data structure used by the G1 garbage collector to track changes in the heap. It allows the collector to efficiently identify and collect only the regions of the heap containing modified objects.
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Ques 29. What is the 'CMSConcurrentAbortablePreclean' phase in the CMS garbage collector?
The 'CMSConcurrentAbortablePreclean' is a phase in the CMS (Concurrent Mark-Sweep) garbage collector that performs additional work on the Old Generation to prepare for the final mark phase. It is concurrent and aims to minimize pause times.
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Ques 30. Explain the concept of 'Shenandoah' garbage collector in Java.
Shenandoah is a low-pause-time garbage collector introduced in Java. It uses advanced techniques to perform garbage collection concurrently with application threads, minimizing pause times and improving overall responsiveness.
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