This is the 17th part of the 500+ Core JAVA interview questions and answers. These questions can also be used as a quick study guide for preparing for an interview or for any Core JAVA concepts revision. These questions cover all the topics in core JAVA from basics to advanced concepts.
1. What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.
Pass by reference means, passing the address itself rather than passing the value. Pass by value means passing a copy of the value.
3. If you’re overriding the method equals() of an object, which other method you might also consider?
hashCode()
All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent.
5. Can there be an abstract class with no abstract methods in it?
Yes
Yes
7. What are the different identifier states of a Thread?
The different identifiers of a Thread are:
R - Running or runnable thread,
CW - Thread waiting on a condition variable,
MW - Thread waiting on a monitor lock,
MS - Thread suspended waiting on a monitor lock.
8. What if the main method is declared as private?
9. Can an Interface be final?
No
10. What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
11. Can we define private and protected modifiers for variables in interfaces?
No
12. What is Externalizable?
Externalizable is an Interface that extends Serializable Interface and sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
13. What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn’t force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to
re-use (because it is not a subclass).
14. What is a local, member and a class variable?
Variables declared within a method are “local” variables. Variables declared within the class i.e not within any methods are “member” variables (global variables). Variables declared within the class i.e not within any methods and are defined as “static” are class variables.
15. How do I convert a numeric IP address like 192.18.97.39 into a hostname like java.sun.com?
String hostname = InetAddress.getByName("192.18.97.39").getHostName();
16. What is the difference between JRE/JVM/JDK?
JDK is the Java Development Kit i.e. JDK is bundle of software that you can use to develop Java based software. JRE is the Java Runtime Environment i.e. JRE is an implementation of the Java Virtual Machine which actually executes Java programs. Typically, each JDK contains one (or more) JRE’s along with the various development tools like the Java source compilers, bundling and deployment tools, debuggers, development libraries, etc.
17. Why isn’t there operator overloading?
Because C++ has proven by example that operator overloading makes code almost impossible to maintain. In fact there very nearly wasn’t even method overloading in Java, but it was thought that this was too useful for some very basic methods like print(). Note that some of the classes like DataOutputStream have unoverloaded methods like writeInt() and writeByte().
18. What does it mean that a method or field is “static”?
Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too).
19. What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
20. Is null a keyword?
The null value is not a keyword.
21. What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object’s value. This often leads to significant errors.
22. Why do threads block on I/O?
Threads block on I/O (that is enters the waiting state) so that other threads may execute while the I/O operation is performed.
23. What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
24. What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
25. Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
26. What is a native method?
A native method is a method that is implemented in a language other than Java.
27. What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left
28. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
29. Which characters may be used as the second character of an identifier, but not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.
30. What restrictions are placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank lines and comments).
31.What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.
32. What is the range of the char type?
The range of the char type is 0 to 2^16 - 1.
33. What is the difference between sleep() and wait() methods?
Difference between sleep() and wait() is that since sleep() is an static method of class Thread so whenever it gets executed it puts the “thread of execution” i.e. currently executing thread to sleep and it will again goes to runnable state once the specified time is finished whereas wait() is a method of class Object so whenver it executes on an object it puts the thread who attains the object’s lock in waiting stage and once this thread get notified (using notify()) by other thread or its wait time is elapsed, thread again comes to runnable state. I
hope this will help you to distinguish between sleep() and wait() methods.
34.What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
35.What are the practical benefits, if any, of importing a specific class rather than an entire package (e.g. import java.net.* versus import java.net.Socket)?
It makes no difference in the generated class files since only the classes that are actually used are referenced by the generated class file. There is another practical benefit to importing single classes, and this arises when two (or more) packages have classes with the same name. Take java.util.Timer and javax.swing.Timer, for example. If I import java.util.* and javax.swing.* and then try to use "Timer", I get an error while compiling (the class name is ambiguous between both packages). Let's say what you really wanted was the javax.swing.Timer class, and the only classes you plan on using in java.util are Collection and HashMap. In this case, some people will prefer to import java.util.Collection and import java.util.HashMap instead of importing java.util.*. This will now allow them to use Timer, Collection, HashMap, and other java.swing classes without using fully qualified class names in.
36.Can a method be overloaded based on different return type but same argument type ?
No, because the methods can be called without using their return type in which case there is ambiquity for the compiler
37.What happens to a static var that is defined within a method of a class ?
Can't do it. You'll get a compilation error
38.What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.
39. What is the use of anonymous class?
Annoymous classes are basically used where you want your class to do only one kind of job eg. catching the user action (i.e. he clicked on some button or change the value of any UI component). It will help you to maintain the modularity in the code whithin a single file.
40.How many static init can you have ?
As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope.
41.What is constructor chaining and how is it achieved in Java ?
A child object constructor always first needs to construct its parent (which in turn calls its parent constructor.). In Java it is done via an implicit call to the no-args constructor as the first statement.
42. What is the difference between Vector & ArrayList?
Both Vector and arraylist are same exept of one reason. Vector methods are synchronized(only one thread can execute it) where as arraylist method are not sysnchronized. but u asked diff. between vector and array. Vector is collection of objects and the space allocated for vector is in heap. Simple array is collection of primitive values and space allocated is not heap.
43.What is the difference between the >> and >>> operators?
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
44.Is sizeof a keyword?
The sizeof operator is not a keyword.
45.Which Java operator is right associative?
The = operator is right associative.
46.What is the difference between method overriding and overloading?
Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments
47.How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
48.Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.
49.What does it mean that a class or member is final?
A final class cannot be inherited. A final method cannot be overridden in a subclass. A final field cannot be changed after it's initialized, and it must include an initializer statement where it's declared.
50.What does it mean that a method or class is abstract?
An abstract class cannot be instantiated. Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses or it also should be declared abstract.
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