Java

Question 1

What is java ?

Answer 1

Java is a programming language and a platform. Java is a high level, robust, object-oriented and secure programming language.

Question 2

What is platform ?

Answer 2

Platform: Any hardware or software environment in which a program runs, is known as a platform. Since Java has a runtime environment (JRE) and API, it is called a platform.

Question 3

There are 4 platforms or editions of Java:

Answer 3

• Java SE: java programming platform
• Java EE: It is an enterprise platform that is mainly used to develop web and enterprise applications
• JAVA ME: micro platform that is dedicated to mobile applications.
• JAVA FX: used to develop rich internet applications. It uses a lightweight user interface API.

Question 4

Features of Java

Answer 4

Simple
Object-Oriented
Portable
Platform independent
Secured
Robust
Architecture neutral
Interpreted
High Performance
Multithreaded
Distributed
Dynamic

Question 5

Why java is simple ?

Answer 5

• No pointers
• Automatic garbage collector
• syntax is simple and undestandable

Question 6

Why java is platform independent ?

Answer 6

Java is a write once, run anywhere language

Question 7

why java is secured ?

Answer 7

Java is secured because:

No explicit pointer

Java Programs run inside a virtual machine sandbox

Classloader: Classloader in Java is a part of the Java Runtime Environment (JRE) which is used to load Java classes into the Java Virtual Machine dynamically. It adds security by separating the package for the classes of the local file system from those that are imported from network sources.

Bytecode Verifier: It checks the code fragments for illegal code that can violate access rights to objects.

Security Manager: It determines what resources a class can access such as reading and writing to the local disk.

Question 7

why java is secured ?

Answer 7

Java is secured because:

No explicit pointer

Java Programs run inside a virtual machine sandbox

Classloader: Classloader in Java is a part of the Java Runtime Environment (JRE) which is used to load Java classes into the Java Virtual Machine dynamically. It adds security by separating the package for the classes of the local file system from those that are imported from network sources.

Bytecode Verifier: It checks the code fragments for illegal code that can violate access rights to objects.

Security Manager: It determines what resources a class can access such as reading and writing to the local disk.

Question 8

other features for java ?

Answer 8

Portable
Java is portable because it facilitates you to carry the Java bytecode to any platform. It doesn’t require any implementation.

High-performance
Java is faster than other traditional interpreted programming languages because Java bytecode is “close” to native code. It is still a little bit slower than a compiled language (e.g., C++). Java is an interpreted language that is why it is slower than compiled languages, e.g., C, C++, etc.

Dynamic
Java is a dynamic language. It supports the dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.

Java supports dynamic compilation and automatic memory management (garbage collection).

Question 8

other features for java ?

Answer 8

Portable
Java is portable because it facilitates you to carry the Java bytecode to any platform. It doesn’t require any implementation.

High-performance
Java is faster than other traditional interpreted programming languages because Java bytecode is “close” to native code. It is still a little bit slower than a compiled language (e.g., C++). Java is an interpreted language that is why it is slower than compiled languages, e.g., C, C++, etc.

Dynamic
Java is a dynamic language. It supports the dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.

Java supports dynamic compilation and automatic memory management (garbage collection).

Question 8

other features for java ?

Answer 8

Portable
Java is portable because it facilitates you to carry the Java bytecode to any platform. It doesn’t require any implementation.

High-performance
Java is faster than other traditional interpreted programming languages because Java bytecode is “close” to native code. It is still a little bit slower than a compiled language (e.g., C++). Java is an interpreted language that is why it is slower than compiled languages, e.g., C, C++, etc.

Dynamic
Java is a dynamic language. It supports the dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.

Java supports dynamic compilation and automatic memory management (garbage collection).

Question 9

What is JVM ?

Answer 9

JVM (Java Virtual Machine) is an abstract machine. It is called a virtual machine because it doesn’t physically exist. It is a specification that provides a runtime environment in which Java bytecode can be executed. It can also run those programs which are written in other languages and compiled to Java bytecode.

The JVM performs the following main tasks:

Loads code
Verifies code
Executes code
Provides runtime environment

Question 10

What is JRE ?

Answer 10

The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide the runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.

Question 11

What is JDK ?

Answer 11

The Java Development Kit (JDK) is a software development environment which is used to develop Java applications and applets. It physically exists. It contains JRE + development tools.

Question 12

FInal attribute

Answer 12

if variable final , you can’t reassign it
i method, you can t override it
if class , you can t extend it

Question 13

Can we declare a constructor final?

Answer 13

No, because constructor is never inherited.

Question 14

What is final parameter?

Answer 14

If you declare any parameter as final, you cannot change the value of it.

Question 15

static blank final variable

Answer 15

A static final variable that is not initialized at the time of declaration is known as static blank final variable. It can be initialized only in static block.

Question 16

What is covariant ?

Answer 16

The covariant return type specifies that the return type may vary in the same direction as the subclass.

Question 17

Some benefis of covariant return type ?

Answer 17

Covariant return type helps in preventing the run-time ClassCastExceptions on returns.

Question 18

Usage of Java super Keyword

Answer 18

super can be used to refer immediate parent class instance variable.
super can be used to invoke immediate parent class method.
super() can be used to invoke immediate parent class constructor.

Note: super() is added in each class constructor automatically by compiler if there is no super() or this().

Question 19

STATIC BINDING

Answer 19

f there is any private, final or static method in a class, there is static binding.²

Question 20

Java clonning

Answer 20

Why use clone() method ?
The clone() method saves the extra processing task for creating the exact copy of an object. If we perform it by using the new keyword, it will take a lot of processing time to be performed that is why we use object cloning.

Question 21

Wrapper classes

Answer 21

Change the value in Method: Java supports only call by value. So, if we pass a primitive value, it will not change the original value. But, if we convert the primitive value in an object, it will change the original value.
Serialization: We need to convert the objects into streams to perform the serialization. If we have a primitive value, we can convert it in objects through the wrapper classes.
Synchronization: Java synchronization works with objects in Multithreading.
java.util package: The java.util package provides the utility classes to deal with objects.
Collection Framework: Java collection framework works with objects only. All classes of the collection framework (ArrayList, LinkedList, Vector, HashSet, LinkedHashSet, TreeSet, PriorityQueue, ArrayDeque, etc.) deal with objects only.

Question 22

What is encapsulation ?

Answer 22

Encapsulation in Java is a process of wrapping code and data together into a single unit, for example, a capsule which is mixed of several medicines.

Question 23

CharSequence Interface

Answer 23

The CharSequence interface is used to represent the sequence of characters. String, StringBuffer and StringBuilder classes implement it. It means, we can create strings in Java by using these three classes.

Question 24

immutability String

Answer 24

The Java String is immutable which means it cannot be changed. Whenever we change any string, a new instance is created. For mutable strings, you can use StringBuffer and StringBuilder classes.

Question 25

Java string constant pool

Answer 25

Each time you create a string literal, the JVM checks the “string constant pool” first. If the string already exists in the pool, a reference to the pooled instance is returned. If the string doesn’t exist in the pool, a new string instance is created and placed in the pool. For example:

Question 26

Why String objects are immutable in Java?

Question 27

Why String class is Final in Java?

Answer 27

The reason behind the String class being final is because no one can override the methods of the String class. So that it can provide the same features to the new String objects as well as to the old ones.

Question 28

String comparison

Answer 28

There are three ways to compare String in Java:

By Using equals() Method
By Using == Operator
By compareTo() Method

Question 29

String buffer

Answer 29

Java StringBuffer class is used to create mutable (modifiable) String objects. The StringBuffer class in Java is the same as String class except it is mutable i.e. it can be changed.

Question 30

String builder

Answer 30

Java StringBuilder class is used to create mutable (modifiable) String. The Java StringBuilder class is same as StringBuffer class except that it is non-synchronized. It is available since JDK 1.5.

StringBuffer is synchronized i.e. thread safe. It means two threads can’t call the methods of StringBuffer simultaneously. StringBuilder is non-synchronized i.e. not thread safe. It means two threads can call the methods of StringBuilder simultaneously.

StringBuilder is more efficient than StringBuffer.

Question 31

immutable class

Answer 31

There are many immutable classes like String, Boolean, Byte, Short, Integer, Long, Float, Double etc. In short, all the wrapper classes and String class is immutable. We can also create immutable class by creating final class that have final data members as the example given below:

Question 32

STRING TOKENIZER

Answer 32

The java.util.StringTokenizer class allows you to break a String into tokens. It is simple way to break a String. It is a legacy class of Java.

Question 33

Type of java exceptions

Answer 33

There are mainly two types of exceptions: checked and unchecked. An error is considered as the unchecked exception. However, according to Oracle, there are three types of exceptions namely:

Checked Exception
Unchecked Exception
Error

Question 33

Type of java exceptions

Answer 33

There are mainly two types of exceptions: checked and unchecked. An error is considered as the unchecked exception. However, according to Oracle, there are three types of exceptions namely:

Checked Exception
Unchecked Exception
Error

Question 34

Java Regex

Answer 34

Java Regex API provides 1 interface and 3 classes in java.util.regex package.
Matcher class
It implements the MatchResult interface. It is a regex engine which is used to perform match operations on a character sequence.

Question 35

Java inner class or nested class is a class that is declared inside the class or interface.

Answer 35

Nested classes represent a particular type of relationship that is it can access all the members (data members and methods) of the outer class, including private.
Nested classes are used to develop more readable and maintainable code because it logically group classes and interfaces in one place only.
Code Optimization: It requires less code to write.

Question 36

types of nested classes

Answer 36

There are two types of nested classes non-static and static nested classes. The non-static nested classes are also known as inner classes.

Non-static nested class (inner class)
Member inner class
Anonymous inner class
Local inner class
Static nested class

Question 37

Rules for Java Local Inner class

Answer 37

1) Local inner class cannot be invoked from outside the method.
Local inner class cannot access non-final local variable till JDK 1.7. Since JDK 1.8, it is possible to access the non-final local variable in the local inner class.

Question 38

Nest static class

Answer 38

A static class is a class that is created inside a class, is called a static nested class in Java. It cannot access non-static data members and methods. It can be accessed by outer class name.

It can access static data members of the outer class, including private.
The static nested class cannot access non-static (instance) data members or

Question 39

nested interfaces

Answer 39

There are given some points that should be remembered by the java programmer.

The nested interface must be public if it is declared inside the interface, but it can have any access modifier if declared within the class.
Nested interfaces are declared static

Question 40

Can we define a class inside the interface?

Answer 40

Yes, if we define a class inside the interface, the Java compiler creates a static nested class. Let’s see how can we define a class within the interface:

Question 41

Advantages of Java Multithreading

Answer 41

It doesn’t block the user because threads are independent and you can perform multiple operations at the same time.

2) You can perform many operations together, so it saves time.
3) Threads are independent, so it doesn’t affect other threads if an exception occurs in a single thread.

Question 42

Multitasking

Answer 42

Multitasking is a process of executing multiple tasks simultaneously. We use multitasking to utilize the CPU. Multitasking can be achieved in two ways:

Process-based Multitasking (Multiprocessing)
Thread-based Multitasking (Multithreading)

Question 43

1) Process-based Multitasking (Multiprocessing)

Answer 43

Each process has an address in memory. In other words, each process allocates a separate memory area.
A process is heavyweight.
Cost of communication between the process is high.
Switching from one process to another requires some time for saving and loading registers, memory maps, updating lists, etc.

Question 44

Thread-based Multitasking (Multithreading)

Answer 44

Threads share the same address space.
A thread is lightweight.
Cost of communication between the thread is low.

Question 45

Java Thread class

Answer 45

Java provides Thread class to achieve thread programming. Thread class provides constructors and methods to create and perform operations on a thread. Thread class extends Object class and implements Runnable interface.

Question 46

Life cycle of a Thread

Answer 46

New
Active
Blocked / Waiting
Timed Waiting
Terminated

Question 47

Explanation of Different Thread States

Answer 47

New: Whenever a new thread is created, it is always in the new state. For a thread in the new state, the code has not been run yet and thus has not begun its execution.
Active: When a thread invokes the start() method, it moves from the new state to the active state. The active state contains two states within it: one is runnable, and the other is running.

Blocked or Waiting: Whenever a thread is inactive for a span of time (not permanently) then, either the thread is in the blocked state or is in the waiting state.
Timed Waiting: Sometimes, waiting for leads to starvation. For example, a thread (its name is A) has entered the critical section of a code and is not willing to leave that critical section. In such a scenario, another thread (its name is B) has to wait forever, which leads to starvation. To avoid such scenario, a timed waiting state is given to thread B. Thus, thread lies in the waiting state for a specific span of time, and not forever. A real example of timed waiting is when we invoke the sleep() method on a specific thread. The sleep() method puts the thread in the timed wait state. After the time runs out, the thread wakes up and start its execution from when it has left earlier.

Terminated: A thread reaches the termination state because of the following reasons:

When a thread has finished its job, then it exists or terminates normally.
Abnormal termination: It occurs when some unusual events such as an unhandled exception or segmentation fault.

Question 48

Examplke of thread and state

Answer 48

Whenever we spawn a new thread, that thread attains the new state. When the method start() is invoked on a thread, the thread scheduler moves that thread to the runnable state. Whenever the join() method is invoked on any thread instance, the current thread executing that statement has to wait for this thread to finish its execution, i.e., move that thread to the terminated state. Therefore, before the final print statement is printed on the console, the program invokes the method join() on thread t2, making the thread t1 wait while the thread t2 finishes its execution and thus, the thread t2 get to the terminated or dead state. Thread t1 goes to the waiting state because it is waiting for thread t2 to finish it’s execution as it has invoked the method join() on thread t2.

Question 49

create a thread

Answer 49

There are two ways to create a thread:

By extending Thread class
By implementing Runnable interface.

Question 50

Starting a thread:

Answer 50

The start() method of Thread class is used to start a newly created thread. It performs the following tasks:

A new thread starts(with new callstack).
The thread moves from New state to the Runnable state.
When the thread gets a chance to execute, its target run() method will run.

Question 51

Thread Scheduler in Java

Answer 51

A component of Java that decides which thread to run or execute and which thread to wait is called a thread scheduler in Java
n Java, a thread is only chosen by a thread scheduler if it is in the runnable state. However, if there is more than one thread in the runnable state, it is up to the thread scheduler to pick one of the threads and ignore the other ones. There are some criteria that decide which thread will execute first. There are two factors for scheduling a thread i.e. Priority and Time of arrival.

Question 52

priority and time of arrival

Answer 52

Priority: Priority of each thread lies between 1 to 10. If a thread has a higher priority, it means that thread has got a better chance of getting picked up by the thread scheduler.

Time of Arrival: Suppose two threads of the same priority enter the runnable state, then priority cannot be the factor to pick a thread from these two threads. In such a case, arrival time of thread is considered by the thread scheduler. A thread that arrived first gets the preference over the other threads.

Question 53

First Come First Serve Scheduling:

Answer 53

In this scheduling algorithm, the scheduler picks the threads thar arrive first in the runnable queue.

Question 54

Time-slicing scheduling:

Answer 54

Usually, the First Come First Serve algorithm is non-preemptive, which is bad as it may lead to infinite blocking (also known as starvation). To avoid that, some time-slices are provided to the threads so that after some time, the running thread has to give up the CPU. Thus, the other waiting threads also get time to run their job.

Question 55

Preemptive-Priority Scheduling:

Answer 55

The name of the scheduling algorithm denotes that the algorithm is related to the priority of the threads.

Question 56

how tread schelder works?

Answer 56

Let’s understand the working of the Java thread scheduler. Suppose, there are five threads that have different arrival times and different priorities. Now, it is the responsibility of the thread scheduler to decide which thread will get the CPU first.

The thread scheduler selects the thread that has the highest priority, and the thread begins the execution of the job. If a thread is already in runnable state and another thread (that has higher priority) reaches in the runnable state, then the current thread is pre-empted from the processor, and the arrived thread with higher priority gets the CPU time.

When two threads (Thread 2 and Thread 3) having the same priorities and arrival time, the scheduling will be decided on the basis of FCFS algorithm. Thus, the thread that arrives first gets the opportunity to execute first.

Question 57

sleep thread

Answer 57

The Java Thread class provides the two variant of the sleep() method. First one accepts only an arguments, whereas the other variant accepts two arguments. The method sleep() is being used to halt the working of a thread for a given amount of time. The time up to which the thread remains in the sleeping state is known as the sleeping time of the thread. After the sleeping time is over, the thread starts its execution from where it has left.

Question 58

Important things to remember in sleep

Answer 58

Whenever the Thread.sleep() methods execute, it always halts the execution of the current thread.

Whenever another thread does interruption while the current thread is already in the sleep mode, then the InterruptedException is thrown.

If the system that is executing the threads is busy, then the actual sleeping time of the thread is generally more as compared to the time passed in arguments. However, if the system executing the sleep() method has less load, then the actual sleeping time of the thread is almost equal to the time passed in the argument.

Question 59

Can we start a thread twice

Answer 59

No. After starting a thread, it can never be started again. If you does so, an IllegalThreadStateException is thrown. In such case, thread will run once but for second time, it will throw exception.

Question 60

What if we call Java run() method directly instead start() method?

Answer 60

Each thread starts in a separate call stack.
Invoking the run() method from the main thread, the run() method goes onto the current call stack rather than at the beginning of a new call stack.

Question 61

the join method

Answer 61

The join() method in Java is provided by the java.lang.Thread class that permits one thread to wait until the other thread to finish its execution. Suppose th be the object the class Thread whose thread is doing its execution currently, then the th.join(); statement ensures that th is finished before the program does the execution of the next statement. When there are more than one thread invoking the join() method, then it leads to overloading on the join() method that permits the developer or programmer to mention the waiting period. However, similar to the sleep() method in Java, the join() method is also dependent on the operating system for the timing, so we should not assume that the join() method waits equal to the time we mention in the parameters. The following are the three overloaded join() methods.²

Question 62

Daemon Thread in Java

Answer 62

Daemon thread in Java is a service provider thread that provides services to the user thread. Its life depend on the mercy of user threads i.e. when all the user threads dies, JVM terminates this thread automatically.

Points to remember for Daemon Thread in Java
It provides services to user threads for background supporting tasks. It has no role in life than to serve user threads.
Its life depends on user threads.
It is a low priority thread.

If you want to make a user thread as Daemon, it must not be started otherwise it will throw IllegalThreadStateException.

Question 63

Jaba thread pool

Answer 63

Java Thread pool represents a group of worker threads that are waiting for the job and reused many times.

Advantage of Java Thread Pool
Better performance It saves time because there is no need to create a new thread.