JVM Internals and Garbage Collection Interview Questions

Q1.  When are static variables loaded in memory ?

Ans. They are loaded at runtime when the respective Class is loaded. 

Q2.  What is a String Pool ?

Ans. String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

Q3.  how many objects are created with this code ?

String s =new String("abc"); 

Ans. Two objects will be created here. One object creates memory in heap with new operator and second in stack constant pool with "abc".

Q4.  Which are the different segments of memory ?

Ans. 

1. Stack Segment - contains local variables and Reference variables(variables that hold the address of an object in the heap)

2. Heap Segment - contains all created objects in runtime, objects only plus their object attributes (instance variables)

3. Code Segment -  The segment where the actual compiled Java bytecodes resides when loaded

Q5.  Which memory segment loads the java code ?

Ans. Code segment.

Q6.  Does garbage collection guarantee that a program will not run out of memory?

Ans. Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection

Q7.  Describe what happens when an object is created in Java ?

Ans. 

1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data.

2. The instance variables of the objects are initialized to their default values.

3. The constructor for the most derived class is invoked. The first thing a constructor does is call the constructor for its superclasses. This process continues until the constructor for java.lang.Object is called,

as java.lang.Object is the base class for all objects in java.

4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.

Q8.  Describe, in general, how java's garbage collector works ?

Ans. The Java runtime environment deletes objects when it determines that they are no longer being used. This process is known as garbage collection. The Java runtime environment supports a garbage collector that periodically frees the memory used by objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans Java's dynamic memory areas for objects, marking those that are referenced. After all possible paths to objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage and are collected.

Q9.  Can I import same package/class twice? Will the JVM load the package twice at runtime?

Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains wil complain about it. And the JVM will internally load the class only once no matter how many times you import the same class.

Q10.  Different types of memory used by JVM ?

Ans. Class , Heap , Stack , Register , Native Method Stack.

Q11.  What is a class loader ? What are the different class loaders used by JVM ?

Ans. Part of JVM which is used to load classes and interfaces.

Bootstrap , Extension and System are the class loaders used by JVM.

Q12.  Explain java.lang.OutOfMemoryError ?

Ans. This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector.

Q13.  Is JVM, a compiler or interpretor ?

Ans. Its an interpretor.

Q14.  Difference between loadClass and Class.forName ?

Ans. loadClass only loads the class but doesn't initialize the object whereas Class.forName initialize the object after loading it.

Q15.  Should we override finalize method ?

Ans. Finalize is used by Java for Garbage collection. It should not be done as we should leave the Garbage Collection to Java itself.

Q16.  Which kind of memory is used for storing object member variables and function local variables ?

Ans. Local variables are stored in stack whereas object variables are stored in heap.

Q17.  Why do member variables have default values whereas local variables don't have any default value ?

Ans. member variable are loaded into heap, so they are initialized with default values when an instance of a class is created. In case of local variables, they are stored in stack until they are being used.

Q18.  Why Java don't use pointers ?

Ans. Pointers are vulnerable and slight carelessness in their use may result in memory problems and hence Java intrinsically manage their use. 

Q19.  What are various types of Class loaders used by JVM ?

Ans. 

Bootstrap - Loads JDK internal classes, java.* packages.

Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE

System  - Loads classes from system classpath. 

Q20.  How are classes loaded by JVM ?

Ans. Class loaders are hierarchical. The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running.

Categories: Java interview Questions

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