JAVA interview questions 30

Q291. What is object lock? Explain when it is required? 
Ans. Every object in java has a unique lock whenever we are using synchronization concept then only lock concept will coming to the picture. 
If a Thread wants to execute a synchronized method first it has to get the lock of the object. Once a Thread got the lock then it is allow to execute any synchronized method on that object. After completing synchronized method execution Thread releases the lock automatically.While a 
Thread executing synchronized method on the given object the remaining Threads are not allow to execute any synchronized method on that object simultaneously. But remaining Threads are allow to execute any non-synchronized method simultaneously. (Lock concept is implemented based on object but not based on method.) 

Q292.What is the class level lock? Explain its purpose. 
Ans. Every class in java has a unique lock if a Thread wants to execute static synchronized 
method that Thread has to get class level lock once a Thread got class level lock then only it is allow to execute static synchronized method. 
While a Thread executing any static synchronized method then remaining Threads are not allow to execute any static synchronized method of the same class simultaneously. But the remaining Threads are allow to execute the following method simultaneously: 
1. Any static non-synchronized method. 
2. Synchronized instance methods 
3. Non-synchronized instance method. 
There is no relationship between object lock and class level lock, both are independent. 

Q293. While a thread executing any synchronized method on the given object is it possible toexecute remaining synchronized method of the same object simultaneously by any other thread?
Ans. No, it is no possible.

Q294. What is the difference between synchronized method and static synchronized method? 
Ans. If a Thread wants to execute a synchronized method first it has to get the lock of the 
object. Once a Thread got the lock then it is allow to execute any synchronized method on that 
object.If a Thread wants to execute static synchronized method that Thread has to get class 
level lock once a Thread got class level lock then only it is allow to execute static synchronized method. 

Q295. What is the advantage of synchronized block over synchronized method? 
Ans. If very few lines of the code required synchronization then declaring entire method as the synchronized is not recommended. We have to declare those few lines of the code inside 
synchronized block. This approach reduces waiting time of the Thread and improves performance of the system. 

Q296. What is synchronized statement? 
Ans. The Statement which is inside the synchronized area (synchronized method or synchronized block) is called synchronized statement. 

Q297. How we can declare synchronized block to get class level lock? 
Ans. To get the class level lock we can declare synchronized block as follows: 
synchronized(Display.class) 
{ 
} 

Q298. How two thread will communicate with each other? 
Ans. Two Threads will communicate with each other by using wait(), notify(), notifyAll() methods. 

Q299.wait(), notify(), notifyAll() method can available in which class? 
Ans. These methods are defined in Object class. 

Q300.Why wait(), notify(), notifyAll() method defines in object class instead of Thread class? 
Ans. These methods are defined in Object class but not in Thread because Threads are calling this method on the shared object. 

JAVA interview questions 29

Q281. If we are trying to set priority of a Thread as 100 what will happen? 
Ans. If we are trying to set priority of a Thread as 100 then we will not get any compile time error but at the runtime we will get Runtime exception IllegalArgumentException. Because the valid range of the Thread priority is (1-10) only. 

Q282. If two threads having same priority then which thread will get chance first for execution? 
Ans. If two threads having same priority then which thread will get the chance first for execution decided by Thread Scheduler. It is the part of JVM and its behavior is vendor dependent and we can’t expect exact output. 

Q283. If two threads having different priority then which thread will get chance first for execution? 
Ans. If two threads having different priority then the Thread which is having highest priority will get chance first for execution. 

Q284 .How we can prevent a thread from execution? 
Ans. We can prevent a Thread from executin by using the following methods: 
1. Yield() 
2. Join() 
3. Sleep() 

Q285. What is yield() method? Explain its purpose? 
Ans. yield() method causes the current executing thread to pause execution and give the chance for waiting thread are same priority. If there is no waiting thread or all the remaining waiting thread have low priority then the same thread will get chance once again for execution. The 
Thread which is yielded when it will get chance once again for execution depends upon mercy of Thread scheduler.Public static native void yield(); 

Q286.What is purpose of join() method? 
Ans. If a Thread wants to wait until some other Thread completion then we should go for join() method. 
Example: if a Thread t1 execute t2.join() ; then t1 will entered into waiting state until t2 Thread completion. 

Q287. Is join() method is overloaded? 
Ans. Yes join() method is overloaded method. 
Public final void join() throws InterruptedException 
By using this method thread will wait up to another thread completion . 
Public final void join(long ms) throws InterruptedException 
By using this method thread will wail upto sometime what we are passing as a argument in millisecond 
Public final void join(long ms, int ns)throws InterruptedException 
By using this method thread will wait up to sometime what we are passing as a argument in millisecond and nanosecond. 

Q288. What is the purpose of sleep() method? 
Ans. If a Thread don’t want to perform any operation for a particular amount of time then we should go for sleep() method.Whenever we are using sleep() method compulsory we should 
handle InterruptedException either by using try-catch or by using throws keyword otherwise we will get compile time error. 

Q289. What is synchronized keyword? Explain its advantages and disadvantages. 
Ans. Synchronized keyword is applicable for method and blocks only. We can’t use for
variables and classes.
If a method declared as a synchronized then at a time only one Thread is allow to
execute that method on the given object.
The main advantages of synchronized keyword are, we can prevent data inconsistency problems and we can provide Threadsafty. 
But the main limitation of synchronized keyword is it increases waiting time of 
Threads and effect performance of the system. Hence if there is no specific requirement it is not recommended to use synchronized keyword. 

Q290.Where we can use synchronized keyword? 
Ans. Synchronization concept is applicable whenever multiple Threads are operating on the same object simultaneously. But whenever multiple Threads are operating on different objects then there is no impact of synchronization. 

JAVA interview questions 28

Q271. Explain life cycle of a Thread? 
Ans. Once we create a Thread object then the Thread is said to be in New/Born state once we call t.start() method now the Thread will be entered into ready/Runnable state that is Thread is ready to execute. If Thread Scheduler allocates CPU now the Thread will entered into the Running state and start execution of run() method. After completing run() method the Thread entered into Dead State. 

Q272. What is the importance of Thread class start() method? 
Ans. Start() method present in Thread class performing all low level joining formalities for the newly created thread like registering thread with Thread Scheduler etc and then start() method 
invoking run() method.As the start() method is doing all low level mandatory activities, 
Programmer has to concentrate only on run() method to define the job. Hence, start() method is a big assistant to the programmer.Without executing Thread class start() method there is no chance of starting a new Thread. 

Q273. After starting a Thread if we trying to restart the same thread once again what will happen? 
Ans. After starting a Thread restarting of the same Thread once again is not allowed violation leads to Runtime Exception saying IllegalThreadStateException. 

Q274. Explain Thread class constructors? 
Ans. There are eight constructors are available in Thread class: 
1. Thread t=new Thread(); 
2. Thread t=new Thread(Runnable r); 
3. Thread t=new Thread(String name); 
4.Thread t=new Thread(Runnable r, String name); 
5.Thread t=new Thread(ThreadGroup g, String name); 
6.Thread t=new Thread(ThreadGroup g, Runnable r); 
7.Thread t=new Thread(ThreadGroup g, Runnable r, String name); 
8.Thread t=new Thread(ThreadGroup g, Runnable r, String name, long stacksize); 

Q275. How to get and set name of a Thread? 
Ans. For every Thread in java there is a name. To set and get the name of a Thread we can use the following methods. All methods are final. 
1.Public final void setName(String name); - To set the name of a Thread 
2.Public final String getName(); - To get the name of a Thread. 

Q276. What is the range of Thread priority in java? 
Ans. The valid range of a Thread priority is 1-10. (1 is least priority and 10 is highest priority) 

Q277. Who uses Thread priority? 
Ans. Thread Scheduler uses priorities while allocating CPU. The Thread which is having highest priority will get chance first for execution. 

Q278. What is the default priority of the Thread? 
Ans. The default priority only for the main thread is 5 but for all remaining threads default 
priority will be inheriting from parent to child. Whatever priority parent thread has the same will be inherited to the child thread. 

Q279. Once we created a new Thread what about its priority? 
Ans. Whatever priority parent thread has the same will be inherited to the new child thread. 

Q280. How to get and set priority of a Thread? 
Ans. To get and set priority of a Thread, Thread class defines the following two methods:; 
1. Public final int
getPriority(); 
2. Public final void setPriority(int priority);

JAVA interview questions 27

Q261. What is Multithreading and explain its application areas? 
Ans. Executing several thread simultaneously where each thread is a separate independent part of the same program is called multithreading. Java language provides inbuilt support for 
multithreading by defining a reach library, classes and interfaces like Thread, ThreadGroup, 
Runnable etc. The main important application area of multithreading are video games implementation, animation development, multimedia graphics etc. 

Q262.What is advantage of Multithreading? 
Ans. The main advantage of multithreading is reduces response time and improves performance of the system. 

Q263. When compared with C++ what is the advantage in java with respect to Multithreading? 
Ans.Java language provides inbuilt support for multithreading by defining a reach library, classes and interfaces like Thread, ThreadGroup, Runnable etc. But in c++ there is no inbuilt support for multithreading. 

Q264. In how many ways we can define a Thread? Among extending Thread and implementing Runnable which is recommended? 
Ans. We can define a Thread in the following two ways: 
1. by extending Thread class or 
2. by implementing Runnable interface. 

Among the two ways of defining a thread implementing Runnable mechanism is always 
recommended. In the first approach as our Thread class already extending Thread there is no chance of extending any other. Hence, we missing the key benefit of oops(inheritance properties). 

Q265. What is the difference between t.start() and t.run() method? 
Ans. In the case of t.start() method, a new thread will be created which is responsible for the
execution of run() method.
But in the case of t.run() method no new thread will be created main thread executes
run() method just like a normal method call.

Q266. Explain about Thread Scheduler? 
Ans. If multiple threads are waiting for getting the chance for executing then which thread will get chance first decided by Thread Scheduler. It is the part of JVM and its behavior is vendor dependent and we can’t expect exact output.Whenever the situation comes to multithreading the guarantee behavior is very- very low. 

Q267. If we are not overriding run() method what will happened? 
Ans.If we are not overriding run() method then Thread class run() method will executed which has empty implementation and hence we will not get any output. 

Q268.Is overloading of run() method is possible? 
Ans.Yes, we can overload run() method but Thread class start() method always invokes no-
argument run() method only. The other run() method we have to call explicitly then only will be executed. 

Q269.Is it possible to override start() method? 
Ans. Yes it is possible. But not recommended. 

Q270.If we are overriding start() method then what will happen? 
Ans. It we are overriding start() method then our own start() method will be executed just like a normal method call. In this case no new Thread will be created. 

JAVA interview questions 26

Q251. Which class act as root for entire java Exception hierarchy? 
Ans. Throwable class act as root for entire java Exception hierarchy. 

Q252. What is the difference between Error and Exception? 
Ans. Throwable class contain two child classes. 
Exception:- These are mostly caused by our program and are recoverable. 
Error:- These are not caused by our program, mostly caused by lake of system resources. These are non recoverable. 

Q253. What is difference between checked exception and unchecked exception? 
Ans. The exceptions which are checked by the compiler for smooth execution of the program at Runtime is called checked exception. Example: IOException, InterruptedException.The 
exceptions which are not checked by the compiler are called unchecked exception. Example: ArithmeticException,RuntimeException. 

Q254.What is difference between partially checked and fully checked Exception? 
Ans. A checked exception is said to be fully checked if and only if all the child classes also checked otherwise it is called partially checked exception. 
Example: 
IOException:- fully checked exception 
Exception:- partially checked exception 
Throwable:- partially checked exception 
RuntimeException:- unchecked exception 

Q255. What is a customized Exception? 
Ans. Sometimes based on our programming requirement we have to create our own exception such type of exception are called customize Exception. 
Example: 
TooYoungException 
TooOldException 
InsufficientFundException 

Q256. Explain the process of creating the customized Exception. 
Ans. Creating customized Exception: 
Class TooYoungException extends RuntimeException{ TooYoungExcetpion(String desc){ 
Super(desc); 
} 
} 
Class TooOldException extends RuntimeException { 
TooOldException(String desc){ 
super(desc); 
} 
} 
Class custExcepiton{ 
Public static void main(String[] args){ int age=Integer.parseInt(args[0]); 
if(age>60) 
{ 
Throw new TooYoungException(“Please wait some more time, definitely you will get best match”); 
} 
else if(age<18) { 
Throw new TooOldException(“Your age is already crossed of marriage, no chance to getting marriage”); 
} 
else { 
System.out.println(“Congratulation! You will get match details soon by your
email”);
}
}

Q257. Explain control flow in try, catch, finally.
Ans. try{
Statement1;
Statement2;
Statement3;
}
catch(X e){ 
Statement4; } 
Finally{ 
Statement5; } 
Statement6; 
Case1: 
If there is no Exception then output is 
Statement1 
Statement2 
Statement3 
Statement5 
Statement6 
Normal termination 
Case2: 
If an exception raised at statement2 and corresponding catch block has matched then output is Statement1 
Statement4 
Statement5 
Statement5 
Normal termination 
Case3: 
An exception raised at statement2 and corresponding catch has not matched then output is Statement1 
Statement5 
Abnormal termination 
Case4: 
An exception occurs at statement4 it always Abnormal termination but before that finally block will be executed and output is 
Statement1 
Statement2 
Statement5 
Abnormal termination 
Case5: 
If an exception raised at statement5 or statement6, it is always abnormal termination. 

Q258. Can you give the most common occurred exception in your previous project. 
Ans. NullPointerException, ArrayIndexOutofBoundException, StackOverFlowError, 
ClassCastException, NoClassDefFoundError, ExceptionInitilizerError, 
IllegalArgumentException, NumberFormatException, IllegalStateException, AssertionError. 

Q259. What is Multitasking? 
Ans. Executing several task simultaneously is called multitasking. 

Q260. What is the difference between process-based and Thread-based Multitasking? 
Ans.Process-based multitasking:- Executing several task simultaneously where each task is a 
separate independent process such type of multitasking is called process based Multitasking. 
Example:-While typing a program in the editor we can listen MP3 audio songs. At the same time we download a file from the net. all these task are executing simultaneously and each task is 
a separate independent program. hence it is process based multitasking. It is best suitable at operating system level. Thread-based multitasking:-Executing several task simultaneously where each task is a separate independent part of the same program is called Thread-based multitasking. and every independent part is called a thread. This type of multitasking is best suitable at programmatic level.