Core Java Interview Questions -4

Core Java Interview Questions -4

1.What must be the order of catch blocks when catching more than one exception?   

The sub classes must come first. Otherwise it will give a compile time error.

2.What is de-serialization?

De-serialization is the process of restoring the state of an object.

3. How can you force garbage collection in java?   

You cannot force Garbage Collection, but you can request for it by calling the method System.gc().  But it doesn’t mean that Garbage Collection will start immediately. The garbage collection is a low priority thread of JVM.
4. How can you call a constructor from another constructor ?

By using this() reference.

5. How can you call the constructor of super class ?

By using super() syntax.

6. What’s the difference between normal methods and constructors?
Constructors must have the same name of the class and can not have a return type. They are called only once,  while regular methods can be called whenever required. We cannot explicitly call a constructor.

7. What is the use of packages in java ?   

Packages are a way to organize files in java when a project consists of more than one module. It helps in resolving name conflicts when different modules have classes with the same names.

8.Why threads block or enters to waiting state on I/O?
Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are performed.

9.How can we call a method or variable of the super class from child class ?   
We can use super.method() or super.variable syntax for this purpose.

10. If you are overriding equals() method of a class, what other methods you might need to override ?


11. what is a the difference between System.err and System.out?
We can redirect System.out to another file but we cannot redirect System.err stream

12.What is the difference between yield() and sleep()?
When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.

13.What is the difference between synchronized block and synchronized method ?

Synchronized blocks place locks for the specified block where as synchronized methods place locks for the entire method.

14.What is externalizable ?   
It is an interface that extends Serializable. It is having two different methods writeExternal() and readExternal. This interface allows us to customize the output.

15.What is serialization ?   
Serialization is the process of saving the state of an object.

16. What are  the differences between an abstract class and an interface?
An abstract class can have concrete method, which is not allowed in an interface. Abstract class can have private or protected methods and variables and only public methods and variables are allowed in interface. We can implement more than one interface , but we can extend only one abstract class. Interfaces provides loose coupling where as abstract class provides tight coupling.

17.How Observer and Observable are used?
Subclass of Observable class maintain a list of observers. Whenever an Observable object is updated, it invokes the update() method of each of its observers to notify the observers that it has a changed state. An observer is  any object that implements the interface Observer.

18.What is an abstract method ?
An abstract method is a method that don’t have a body. It is declared with modifier abstract.

19.What is a Vector?

Vector is a grow able array of objects.

20.How can you create your own exception ?   
Our class must extend either Exception or its sub class

21.What is List interface ?

List is an ordered collection of objects.

22.What are the differences between boolean & operator and  & operator
When an expression containing the & operator is evaluated, both operands are evaluated. And the & operator is applied to the operand. When an expression containing && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then only the second operand is evaluated otherwise the second part will not get executed.  && is also called short cut and.

23.What are Wrapper Classes ?
They are wrappers to primitive data types. They allow us to access primitives as objects.

24.What are transient variables in java?
Transient variables are variable that cannot be serialized.

25.What is the use of the finally block?
Finally is the block of code that executes always. The code in finally block will execute even if an exception is occurred. finally will not execute when the user calls System.exit().

26.What is the difference between  time slicing and  preemptive scheduling ?
In preemptive scheduling, highest priority task continues execution till it enters a not running state or a higher priority task comes into existence. In time slicing, the task continues its execution for a predefined period of time and reenters the pool of ready tasks.

27.Can we declare an anonymous class as both extending a class and implementing an interface?
No. An anonymous class can extend a class or implement an interface, but it cannot be declared to do both

28.Can we call finalize() method ?
Yes.  Nobody will stop us to call any method , if it is accessible in our class. But a garbage collector cannot call an object’s finalize method if that object is reachable.

29.What is synchronization?
Synchronization is the ability to control the access of multiple threads to shared resources. Synchronization stops multithreading. With synchronization , at  a time only one thread will be able to access a shared resource.

30.What is the initial state of a thread when it is created and started?
The thread is in ready state

31. What is the difference between a continue statement and a break statement?

Break statement results in the immediate termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

32 What is the difference between String and StringBuffer class ?

Strings are immutable (constant), their values cannot be changed after they are created. StringBuffer supports mutable objects.

33 How many times may an object’s finalize() method be invoked by the garbage collector?
Only once.

34 What is thread priority?
Thread Priority is an integer value that identifies the relative order in which it should be executed with respect to others. The thread priority values ranging from 1- 10 and the default value is 5. But if a thread have higher priority doesn’t means that it will execute first. The thread scheduling depends on the OS.

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

Garbage collection does not guarantee that a program will not run out of memory. It is also possible for programs to create objects that are not subject to garbage collection. And there is no guarantee that Garbage Collection thread will be executed.

36 What are the different states of a thread ?

The different thread states are ready, running, waiting and dead.

37 What must a class do to implement an interface?
It must identify the interface in its implements clause. Also it must provide definition for all the methods in the interface otherwise it must be declared abstract.

38What is the difference between static and non static inner class ?
A non-static inner class can have an object instances that are associated with instances of the class’s outer class. A static inner class can not have any object instances.
39 What is an abstract class?

An abstract class is an incomplete class. It is declared with the modifier abstract. We cannot create objects of the abstract class. It is used to specify a common behavioral protocol for all its child classes.

40 What does wait method do ?
It causes current thread to wait until either another thread invokes notify or notifyAll method of the current object, or a specified amount of time has elapsed.

41 What is a native method?

A native method is a method that is implemented in a language other than Jav

42Why we cannot override static methods?
Static means they are associated with a class. In static methods , the binding mechanism is static binding. So it must be available at the compile time.

43 Can we catch an error in our java program ?

Yes. We can . We can catch anything that is derived from Throwable. Since Error is a sub class of Throwable we can catch an error also.

44 What is static ?

static means one per class.  static variables are created when the class loads. They are associated with the class.  In order to access a static we don’t need objects. We can directly access static methods and variable by calling classname.variablename.

45 Which is the base class for all classes ?


46 What is the difference between readers and streams?
Readers are character oriented where streams are byte oriented.  The readers are having full support for Unicode dat

47 What are the restrictions placed on overloading a method ?

Overloading methods must differ in their parameter list, or number of parameters.

48 What are the different primitive data type in java ?
There are 8 primitive types in jav boolean , char, byte, short, int long, float, double.

49 What will happen if the results were not caught ?
An uncaught exception results in the uncaughtException() method of the thread’s ThreadGroup, which results in the termination of the program.

50 What are different type of exceptions in Java?
There are two types of exceptions in jav Checked exceptions and Unchecked exceptions. Any exception that is is derived from Throwable and Exception is called checked exception except RuntimeException and its sub classes. The compiler will check whether the exception is caught or not at compile time. We need to catch the checked exception or declare in the throws clause.  Any exception that is derived from Error and RuntimeException is called unchecked exception. We don’t need to explicitly catch a unchecked exception.

51 What is constructor chaining ?

When a constructor of a class is executed it will automatically call the default constructor of the super class (if no explicit call to any of the super class constructor) till the root of the hierarchy.

52.What is a void return type ?
A void indicates that the method will not return anything.

53 What is the difference between static and non static variables ?

A static variable is associated with the class as a whole rather than with specific instances of a class. There will be only one value for static variable for all instances of that class.  Non-static variables take on unique values with each object instance.

54 What are the restrictions placed on overriding a method ?

The overridden method have the exact signature of the super class method, including the return type. The access specified cannot be less restrictive than the super class method. We cannot throw any new exceptions in overridden method.

55 What is the difference between notify and notifyAll method ?

notify wakes up a single thread that is waiting for object’s monitor. If any threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. notifyAll Wakes up all threads that are waiting on this object’s monitor. A thread waits on an object’s monitor by calling one of the wait methods.

56 What is casting ?

Casting means converting one type to another. There are mainly two types of casting. Casting between primitive types and casting between object references. Casting between primitive numeric types is used to convert larger data types to smaller data types. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

57 What is a ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to create the program’s appearance to the particular locale in which it is being run.

58 When does a compiler supplies a default constructor for a class?

If there is no other constructor exist in a class, the compiler will supply a default constructor.

59.What is the difference between == and equals ?

The equals method can be considered to perform a deep comparison of the value of an object, whereas the == operator performs a shallow comparison.  If we are not overriding the equals method both will give the same result. == will is used to compare the object references. It is used to check whether two objects are points to the same reference.

60. What are the different ways in which a thread can enter into waiting state?

There are three ways for a thread to enter into waiting state. By invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object’s lock, or by invoking an object’s wait() method.

61. What is the difference between the prefix and postfix forms of the ++ operator?

The prefix form first performs the increment operation and then returns the value of the increment operation. The postfix form first returns the current value of the expression and then performs the increment operation on that value.

62. What is numeric promotion?

Numeric promotion is the conversion of a smaller numeric type to a larger numeric type. In numerical promotion, byte, char, and short values are converted to int  values. The int, long and float values are converted to the desired types if required.

63.What is the difference between RandomAccessFile and File?
The File class contains information  the files and directories of the local file system. The RandomAccessFile class contains  the methods needed to directly access data contained in any part of a file.

64.What is the difference between a switch statement and an if statement?

If statement is used to select from two alternatives. It uses a boolean expression to decide which alternative should be executed. The expression in if must be a boolean value.  The switch statement is used to select from multiple alternatives.  The case values must be promoted to an  to int value.

65.What is hashCode?

The hashcode of a Java Object is simply a number (32-bit signed int) that allows an object to be managed by a hash-based data structure. A hashcode should be, equal for equal object (this is mandatory!) ,  fast to compute based on all or most of the internal state of an object, use all or most of the space of 32-bit integers in a fairly uniform way , and likely to be different even for objects that are very similar. If you are overriding hashCode you need to override equals method also.

66.What is an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

67.What is final ?

A final is a keyword in jav If final keyword is applied to a variable, then the variable will become a constant. If it applied to method, sub classes cannot override the method. If final keyword is applied to a class we cannot extend from that class.

68.How can we create a thread?

A thread can be created by extending Thread class or by implementing Runnable interface. Then we need to override the method public void run().

69.What is class loader in java ?

A class loader is a class that is responsible for loading the class. All JVM contains one class loader called primordial class loader.

70 What is the difference among JVM Spec, JVM Implementation, JVM Runtime ?

The JVM spec is the blueprint for the JVM generated and owned by Sun. The JVM implementation is the actual implementation of the spec by a vendor and the JVM runtime is the actual running instance of a JVM implementation

71.What are synchronized methods and synchronized statements?
Synchronized methods are methods that are declared with the keyword synchronized. A thread executes a synchronized method only after it has acquired the lock for the method’s object or class. Synchronized statements are similar to synchronized methods. It is a block of code declared with synchronized keyword. A synchronized statement can be executed only after a thread has acquired the lock for the object or class referenced in the synchronized statement.

72.What do you mean by immutable ? How to create an immutable object ?

Immutability means an object cannot be modified after it has been initialized.  There will not be any setter methods in an immutable class. And normally these classes will be final.

73.What is the difference between Vector and ArrayList ?
Vector is synchronized, ArrayList is not. Vector is having a constructor to specify the incremental capacity. But ArrayList don’t have. By default Vector grows by 100% but ArrayList grows by 50% only.

74.What is the difference between Hashtable and HashMap ?

Hashtable is synchronized . but HashMap is not synchronized.  Hashtable does not allow null values , but HashMap allows null values.

75.Why java is said to be pass-by-value ?

When assigning an object to a variable, we are actually assigning the memory address of that object to the variable. So the value passed is actually the memory location of the object. This results in object aliasing, meaning you can have many variables referring to the same object on the heap.

76.How is the difference between thread and process?

A process runs in its own address space. No two processes share their address space. Threads will run in the same address space of the process that owns them.

77.What is object pooling?

Creating a large number of identical short lived objects is called object pooling. This helps to minimize the need of garbage collection and makes the memory use more effective.

78.What is object cloning?
It is the process of duplicating an object so that two identical objects will exist in the memory at the same time.

79.What are the access modifiers available in Java ?
Access modifier specify where a method or attribute can be used. Public is accessible from anywhere. Protected is accessible from the same class and its subclasses. Package/Default are accessible from the same package. Private is only accessible from within the class.

80.What is garbage collection?

Garbage collection is the process of releasing memory used by unreferenced objects. It relieves the programmer from the process of manually releasing the memory used by objects .

81.What is a weak reference ?

A weak reference is the one that does nor prevent the referenced object from being garbage collected. The weak reference will not keep the object that it refers to alive. A weak reference is not counted as a reference in garbage collection. This will make the memory use more effective.

82.What is a Dictionary?

Dictionary is a parent class for any class that maps keys to values., In a dictionary every key is associated with at most one value.

83.What is the disadvantage of garbage collection?

It adds an overhead that can affect performance. Additionally there is no guarantee that the object will be garbage collected.

84.What is a marker interface ?
An interface that contains no methods. Eg: Serializable, Cloneable, SingleThreadModel etc. It is used to just mark java classes that support certain capability.

85.What are tag interfaces?

Tag interface is an alternate name for marker interface.

86.What are the restrictions placed on static method ?
We cannot override static methods. We cannot access any object variables inside static method. Also the this reference also not available in static methods.

87.What is JIT?

JIT stands for Just In Time compiler. It compiles java byte code to native code.

88.What is java byte code?
Byte code is an sort of intermediate code. The byte code is processed by virtual machine.

89.What is method overloading?

Method overloading is the process of creating a new method with the same name and different signature.

90.What is method overriding?

Method overriding is the process of giving a new definition for an existing method in its child class.

91.What is finalize() ?

Finalize is a protected method in jav When the garbage collector is executes , it will first call finalize( ), and on the next garbage-collection it reclaim the objects memory. So finalize( ), gives you the chance to perform some cleanup operation at the time of garbage collection.

92.What is JAR file ?

JAR stands for Java Archive. This is a file format that enables you to bundle multiple files into a single archive file.  A jar file will contains a file inside META-INF folder that describes the version and other features of jar file.

93.What is deadlock?
Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource which is held by the other waiting thread.

94.What is internationalization?

Internationalization is the process of designing an application so that it can be adapted to various languages and regions without changes.

95.What is classpath?

Classpath is the path where Java looks for loading class at run time and compile time.

96.What is path?

It is an the location where the OS will look for finding out the executable files and commands.

97.What is the difference between Iterator and Enumeration?
Iterator differ from enumeration in two ways Iterator allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics. And , method names have been improved.

98.Why Java is not fully objective oriented ?

Due to the use of primitives in java, which are not objects.

99. What is multi-threading?

Multi-threading is the scenario where more than one threads are running.

100. What is the difference between URL and URLConnection?
A URL represents the location of a resource, and a URLConnection represents a link for accessing or communicating with the resource at the location.

101. What are the two important TCP Socket classes?
ServerSocket and Socket. ServerSocket is useful for two-way socket communication. Socket class help us to read and write through the sockets. getInputStream() and getOutputStream() are the two methods available in Socket class.

102. What is the Locale class?

A Locale object represents a specific geographical, political, or cultural region

103. Strings are immutable. But String s=”Hello”; String s1=s+”World” returns HelloWorld how ?

Here actually a new object is created with the value of HelloWorld

104. What is anonymous class ?

An anonymous class is a type of inner class that don’t have any name.

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