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Q1411. What will be result of following code and why

Integer int1 = 1;
Integer int2 = 1;
String str1 = new String("str");
String str2 = new String("str");
String str3 = "str";
String str4 = "str";

System.out.println(int1 == int2);
System.out.println(str1 == str2);
System.out.println(str3 == str4);

Ans. true
false
true

Just like Strings, java maintains an integer constant pool too. So 1 will be maintained in integer constant pool and hence reference int2 will point to same integer in pool.

String pool is only for string literals ( defined by "" ) and not for newly created objects and hence str1 == str2 will return false as they are separate objects in memory.

String pool is used for storing string literals so that they can be reused and str3 and str4 will point to same string literal in string pool.

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Q1412. When does an object cleaned or destroyed by garbage collection ?

Ans. When it is no longer referenced or it looses all references which were earlier pointing to it.

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Q1413. How can we destroy an object ?

Ans. We cannot explicitly initiate destruction of an object as no destroy method is available in java. Only Garbage collection can destroy an object.

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Q1414. Given a string and index for the opening bracket, find the index of matching closing bracket.

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Q1415. Given a string with multiple opening and closing brackets, determine if the string is valid or not ( to see if the string has all closing brackets for each opening bracket ) ?

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Q1416. What is the trade off between using Collections.synchronizedList() on an ArrayList and a CopyOnWriteArrayList?

Ans. Synchronized List locks the whole list to provide synchronization and thread safety during the read or write operation, while, CopyOnWriteArrayList doesn’t lock the whole list during these operations.
The CopyOnWriteArrayList class works according to its name i.e. copy-on-write which performs different actions for reading and write operations. For every write operation (add, set, remove, etc), it makes a new copy of the elements in the list. and for the read operations (get, iterator, listIterator, etc), it works on a different copy. So there is no additional overhead during a read operation and its read operation is faster than Collections.SynchronizedList(). Thus, COWAL is better for reading operation than Synchronized List.

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Q1417. What is the difference between CountingSemaphore and a CountDownLatch?

Operating System

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Q1418. What are the advantages and disadvantages of using lock classes from java.util.concurrent.locks vs. synchronized methods ?

Ans. http://www.devinline.com/2015/10/Lock-Vs-synchronized-in-java.html

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Q1419. What will be the output of following code

Integer x = 1;
Integer y = 2;
System.out.println(x == y);

What if you change 1 to "1" and Integer to String?

Ans. It will print "true" with integers as well as strings. The reason is "Integer constant pool" and "String pool"

String pool maintains pool of string literals. When a string literal is used for the first time, a new string object is created and is added to the pool. Upon it's subsequent usage , the reference for the same object is returned. Similarly java uses integer constant pool.

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Q1420. What are the trade offs between public constructor and static final method ?

Ans. Public constructor is simple and easy as it's the default way of object creation. So there are no additional coding overheads as compiler provides the default constructor if none is provided by coder.

With static final methods, it facilitates loose coupling by segregating the responsibility of object creation to a separate method. Validation can be done on the constructor arguments before calling it. Moreover if any adaption on the arguments is required that can achieved easily with factory method.On the flip side, there is coding overhead and additional method call.

Sample Code for constructor

Sample Code for factory

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Q1421. How can we get an array out of ArrayList ?

Ans. We can get an array out of ArrayList by using toArray() method of an array list.

String a[] = arrayList.toArray();

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Q1422. What is Livelock in relation to deadlock ?

Operating System

Ans. Livelock is when two threads or processes get deadlocked because they try to perform the same action to come out of a potential deadlock.

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Q1423. What are the four situations that are required for deadlock to happen ?

Operating System

Ans. mutual exclusion i.e the resource is not shareable and hence a process should have exclusive access to it at a particular time

Hold and Wait i.e the process doesn't release all existing resources held by it before requesting for another one.

No preemption i.e noone can forcefully release a resource held by another process and the process should voluntarily release the resource held by it.

Circular Wait - For ex, P1 is waiting on resource to be released by P2, P2 is waiting on resource to be released by P3 and P3 is waiting on the resource to be released by P1

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Q1424. Can we override the overloaded methods too ?

Ans. Yes. Both concepts are independent of each other. Overriding depends on the methods with same name and signature whereas Overloading on same name but with different signatures. All definitions of an overloaded method can be individually overridden in the derived class.

For example -

Class A has overloaded methods "public void myMethod(String str)" and "public void myMethod(int x)"

We can have derived class ClassB extends ClassA and hence can override both methods myMethod(String str) and myMethod(int x) in ClassB

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Q1425. What is "dynamic method dispatch" in java ?

Ans. dynamic method dispatch is a process in which a call to an
overridden method is resolved at runtime rather than at compile-time. This is used to achieve runtime polymorphism in java.

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Q1426. What is the difference between Enumeration and Iterator ?

Ans. Enumeration can iterate only legacy collections like Vector , HashTable and Stack whereas Iterator can iterate both legacy and non legacy collections.

Enumeration is less safer than Iterator

Enumeration is fail safe whereas Iterator is fail fast

Iterator allows for removal of element while traversal whereas Enumeration doesn't have remove method.

Enumerations were introduced in Java 1 whereas Iterators were introduced with Java 2

Enumerations have methods like hasMoreElements and nextElement whereas Iterators have methods like hasNext, next and remove

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Q1427. What is meant Java is "Write Once Run Anywhere" ?

Ans. It means that Java is compiled to byte code that is platform independent and hence can be run anywhere or any machine.

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Q1428. What does the modularity in project jigsaw means ?

Ans. It means that now individual modules will be built independently instead of complete jdk built in a single jar file. So we will have multiple jars instead of single jar. Moreover this organization would result in lighter applications as they can just built themselves with the required jars and hence ignoring the not required and legacy modules.

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Q1429. What is project Jigsaw in Java 9 ?

Ans. The projects aims to organize the Java source code into independent modules and hence facilitate building them independently.So now we will have multiple core java jars instead of single one. This way of organizing the code would offer following advantages

1. Will make applications lighter as only required modules will be built with the application. Moreover there are some legacy packages currently which anyone hardly use.

2. Reuse same class whole identifiers across different modules.

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Q1430. What are the changes in Java 9 ?

Ans. JVM and JDK are getting smaller.

New command line tool called jshell that will add native support and popularize a Java way to REPL (Read-Eval-Print-Loop)

private methods in interfaces

Introduction of the module system or project Jigsaw

JavaDoc Search

Light version of JSON API

HTTP2 client

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Q1431. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?

Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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Q1432. What are some uses of interceptors ?

Ans. Authentication / Authorization
URL forwarding
Auditing / Logging
Localization

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Q1433. Explain Interceptor Design Pattern ?

Ans. It is used for intercepting the request. It's primary use is to implement security policy. All or specific request types can be intercepted and hence forwarded to authentication / authorization module so as to facilitate authorized requests to application.

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Q1434. Can you give an example of "Convention over configuration" Design principle ?

Ans. Hibernate mapping configurations are used for mapping hibernate entities and corresponding DB Tables. Conventionally Entities and Table can share the same name and hence framework can provide implicit mapping instead of explicit mapping through configurations. Though it may result in little loss of flexibility in extreme cases but will work with all applications following the convention.

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Q1435. Are there any disadvantages of "Convention over configuration" Design paradigm ?

Ans. Yes, It may result in some loss of flexibility as the application has to follow all conventions. Moreover it can contradict with other design paradigms or principles used in the application or framework.

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Q1436. What is "convention over configuration" ? Which framework uses this design paradigm ?

Ans. Convention over configuration is a way of software design used by frameworks that attempt to decrease the number of decisions that a developer is required to make without necessarily losing flexibility. The objective is to reduce the unnecessary configurations and follow conventions to accomplish that. For example - DB Entity and DB Table name conventionally should be same and hence shouldn't require configuring mapping between the two.

"Spring Boot" and "Ruby on Rails" have adopted this design principle.

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Q1437. Why is goto not used in Java?

Ans. Goto statement results in unstructured jumps in control from one code location to another and hence makes the code very difficult
to read , maintain and debug. Moreover Goto can always be replaced using other statements like break / continue and hence results in structured movement of programming control.

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Q1438. Are Annotations better than using XML configs ?

Ans. Yes , as both code and its meta information reside in the same file, it promotes better readability and maintenability.

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Q1439. What is the use of Annotations in Java ?

Ans. Annotations are used to specify meta information about the code.

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Q1440. What are protocol buffers ?

Markup Language

Ans. Protocol buffers are language-neutral, platform-neutral, mechanism for serializing structured data. It is developed by google.

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