Java Garbage Collection Interview Questions

One of the key areas of Java is garbage collection. Garbage collection enables apps to manage memory automatically. Interviewers frequently ask inquiries about garbage collection.

Q1: What is the purpose of garbage collection in Java?

Ans: Developers can programmatically recover the memory space that has been allotted to an object in programming languages like C and C++. The user is not in charge of controlling how much memory the objects in Java programs utilise. The collection and disposal routine is now a component of JVM and is in charge of locating and erasing items in memory that are not longer being used.

Q2: What are the disadvantages of trash collection?

Ans: Garbage Collection's primary flaw is that it halts all threads that are engaged when the memory recovery phase is taking place. Trash collection processes cannot be planned because the garbage collection techniques take seconds or minutes to complete.

Q3: Describe the Java Heap structure.

Ans: Java Heap refers to a section of memory that is shared by all threads. Three generations—the new generation, the old generation, and the PermGen space—are combined to make up the Java concept of a heap.

Q4: Describe the Java PermGen space.

Ans: The PermGen space contains a description of a Java classes that the JVM internally holds. The same manner in which the Youth of today and Old generation of the heap's other components were collected also applied to the garbage data in PermGen space.

Q5: Describe how a minor, major, & full garbage collection differ.

Ans: The distinction between major, minor, and full trash collection is not defined in any official literature or standard. Every one of them can be defined as follows:

1. On the leased space, complete waste collection is being done.
2. significant trash removal efforts in the survival space.
3. Mark-and-sweep procedures here on Eden space required minor rubbish collection labour.

Q6: How can you tell which Java trash collectors are major and minor?

Ans: Based on the results, we can determine which garbage collections are major and minor. The main collection prints "Full GC," while the minor collection prints "GC" whereas if garbage collection monitoring is enabled with the -XX:PrintGCDetails or verbose:gc options.

Q7: Distinguish between the garbage collectors of the ParNew and DefNew Young Generation.

Ans: New Generation garbage collectors include both ParNew and DefNew. A single-threaded garbage collector employed with the sequential garbage collector was referred as a DefNew young generation garbage collector, but a multi-threaded garbage collector used with the concurrent Mark Sweep is known as both a ParNew young generation garbage collector.

Q8: Describe how the trash collector's Finalize() method is utilised.

Ans: The garbage collector invokes the Finalize() function before collecting any item that qualifies for collection. This Finalize() method is employed to give the user one final opportunity to protest cleaning and release any leftover resources.

Q9: Is it possible to make the trash collector operate at any time?

Ans: No, we cannot enforce Java's garbage collection. Although we can phone the system and ask for it. Runtime, or its relative gc(). getRunitime(). get(). Calling these methods does not ensure that the GC will launch promptly.

Q10: Does JVM's permanent generating space experience garbage collection?

Ans: Garbage collection is possible in the PermGen area, yes. PermGen can activate Full Garbage Collector if it reaches a certain point or is already full.

Q11: At what point is a Java object ready for garbage collection?

When: An object is eligible for garbage collection.

1. It has a null marking.
2. It is no longer relevant.
3. if it is no longer employed by any non-null entities within an application.

Q12: What would you mean when you say "mark-and-sweep"?

Ans: The two states for waste collection are Mark and Sweep. JVM determines whether or not the object is still required during the Mark step. When an object is no longer required, it is designated for garbage collection.

Garbage collection and memory reclamation methods are carried out by JVM during the sweep step.

Q13: Describe the distinctions among Minor, Major, & Full waste collection?

Ans: There isn't any formal documentation that can explain the distinctions among Minor, Major, & Full garbage collection.

1. In Minor waste collection, the Eden space is used for the Mark and Sweep phases of rubbish collection.
2. In the surviving space, there are major garbage collection operations.
3. On the leased space, complete waste collection is performed.

Q14: Describe a memory leak and explain how it affects garbage collection?

Ans: Memory leaks occur when a garbage collector is unable to locate and delete an unneeded object from memory. The memory leak causes an increase in memory usage. JVM is compelled to make additional room for new objects after the memory consumption rises. Garbage collection's Mark and Sweep stages run more frequently. Each time these phases execute, less memory is released when there is no more heap space.

Q15: Is it possible to start trash collection from code?

Ans: Yes, we may ask the JVM to start trash collection by sending a request to it. To make the request, we employ the System.gc() command. There is no assurance that the JVM will reply to a request at any particular time.

Q16: Which portion of the space from the stack or heap is used by the garbage collection?

Ans: GC involves the memory's heap region.

Q17: Describe the duties of the GC?

Ans: Garbage collection's primary duty is to release unneeded memory. Although GC guarantees that the storage will be used effectively, it cannot promise that it will have sufficient space to run the program.

Q18: What is daemon thread, exactly? Is the GC thread a daemon?

Ans: A daemon thread is one that executes background tasks for the program while running in the background. Yes, the JVM starts the GC daemon thread.

Q19: What procedures must be followed to make an item eligible for GC once it is no longer required?

Ans: The three methods listed below can qualify an item for GC:

• By making all of the object references available point to null.

When an object's intended use is fulfilled, its reference can be changed to null, making it available to GC.

public class   {  
     public static void main (String [] args) {  
          String strData = "Example of javaLearner.";  
          strData = null;      
     }  
}  

• Using a reference variable to refer to a different object.

To make the object it was referencing to before being reassigned suitable for garbage collection, decouple its reference variable from it and set it to refer to another object.

public class GCExample1 {  
        public static void main (String [] args){  
        String strD1 = " Example Str1 by javaLearner.";  
        String strD2 = " Example Str2 by javaLearner.";  
        System.out.println(strD1); 
        strData1 = strData2;    
    }  
}  

• Islands of Isolation were built.

When two instance variables point to the same class's instance, they also refer to one another, and the object to which they refer lacks any other valid references, the two particles are eligible for garbage collection.

public class GCExample2 {  
     GCExample2 obj1;      
     public static void main(String [] str){  
          GCExample2 obj2 = new GCExample2();  
          GCExample2 obj3 = new GCExample2();  
          obj2.obj1 = obj3; 
          obj3.obj1 = obj2; 
          obj2 = null;  
          obj3 = null;  
     }  
} 

Q20: What benefit does the garbage collection provide?

Ans: The fundamental benefit of Java's automatic garbage collection is that it frees us from having to manually allocate and deallocate memory, allowing us to concentrate on problem-solving.

Q21:  Why would you want to override the finalise() method?

Ans: A method provided in java.lang is finalise(), also known as finalizer. The garbage collector will call right before collecting any item that qualifies for GC. Therefore, the finalise() method gives an object one final opportunity to clean up and liberate any leftover resources.

Q22: How may an object be declared eligible for GC if it is no longer required?

Ans: 1. Set all of the object references available to null.

2. Create a reference variable that points to a different object.

3. Creating Isolation Island

Q23:  How many times would the finalise() function for an object get called by the garbage collector?

Ans: Just once.

Q24: Does the garbage collector run in the foreground or background?

Ans: A daemon thread in the background is the garbage collector. The program that the JVM starts is backed by a daemon thread. When all foreground non-daemon threads have terminated, the thread does too.

Q25: What time is trash pickup?

Ans: Response and throughput are directly impacted by the suspension, which lengthens as more live items are discovered. The garbage-collection pause, also referred to as GC pause time, is a key concept in garbage collection that has an impact on how applications run.