Core Java - Interview Questions and Answers for 'Core Java' | Search Interview Question

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Q511. Is it a valid identifier in Java ?

int 123Abc = 5;

Core Java

Ans. No, identifier in java cannot start with a numeral.

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Q512. What is the difference between Collections.emptyList() and creating new instance of List using new ?

Core Java

Ans. But Collections.emptyList() returns an Immutable list whereas new arraylist() creates a mutable list.

Advantage of getting an empty list using Collections.emptyList is that it returns a singleton list which can be shared among many references and hence made immutable. This is good fit for situations where we would like to initialize a list to basic minimum empty to avoid null pointer exception.

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Very frequently asked.

Q513. What is the use of synchronized keyword ?

Core Java

Ans. Synchronize is used to achieve mutual exclusion i.e at one time, the segment of the code, method that has been declared synchronized should be executed by single thread only and hence the lock needs to be retrieved before executing the segment and then released.

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Usually asked with Questions related to Generics.

Q514. What are Type Erasures in Java ?

Core Java

Ans. Type erasure applies to the use of generics. When generics are used, they're pre compiled into compile time checks and execution-time casts.

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Q515. Have you ever tried compressing data using Java ?

Core Java

Ans. Yes, we can use ZipInputStream class for compressing the FileInputstream object.

Sample Code for ZipInputStream

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Q516. Watching a directory for changes. Monitoring a directory.

Core Java

Ans. http://www.logicbig.com/tutorials/core-java-tutorial/java-nio/java-watch-service/

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Q517. How can one determine if JVM is 32-bit or 64-bit from Java Program ?

Core Java

Ans. There is a Java system property "sun.arch.data.model" that can tell if JVM is 32 bit of 64 bit

System.getProperty("sun.arch.data.model") can be used to get that property.

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Q518. Is ++ or -- operation thread safe in Java ?

Core Java

Ans. No

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Q519. How can we find the memory usage of JVM from Java code?

Core Java

Ans. Using Runtime object.

Runtime runtime = Runtime.getRuntime();

runtime.totalMemory();
runtime.freeMemory();

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Q520. How can we catch an exception thrown by another thread ?

Core Java

Ans. http://stackoverflow.com/a/6548203/2789764

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Q521. How can you check if a String is a 10 digit number by using regular expression ?

Core Java

Ans. We can use \d{10} as the expression to check that

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Q522. How to check the version of Java through Java program itself ?

Core Java

Ans. We can use System.getProperty("java.version");

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Q523. how can we get the System Environment variables from within Java program ?

Core Java

Ans. We can use System.getenv for getting the environment variables. For ex - System.getenv("JAVA_HOME")

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Q524. Why calculations in float or Double sometimes gives unpredictable results ?

Core Java

Ans. Because the way float values are stored its not precise.

For example - 0.1 is actually stored as
0.1000000000000000055511151231257827021181583404541015625

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Q525. If we parse any string to get another data type and the String doesn't abide by the type of data it should have hold, what would happen ?

Core Java

Ans. It will throw Parse Exception.

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Q526. Can we modify the variables defined in an interface ?

Core Java

Ans. No, member variables of a interface are final by default. The final ensures the value assigned to the interface variable is a true constant that cannot be re assigned by program code.

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Q527. Can we declare a member element as abstract ?

Core Java

Ans. No, only methods can be declared abstract.

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Q528. What are the primitive data types in Java ?

Core Java

Ans. boolean byte char double float int long short void

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Q529. Have you ever heard of InstanceAlreadyExistsException ?

Core Java

Ans. This exception usually occur if we try to register the bean with name already present in the registry or repository. Usually this happens when multiple tests are executed at once in single container.

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Q530. What is Java NumberFormatException ?

Core Java

Ans. NumberFormatException is the exception that Java throws when we try to convert one data type to the Number data type but the value is not parsable to be a Number.

For example , the following code will result in the NumberFormatException

String string = "Buggy";
int strtoInt = Integer.parseInt(string);

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Q531. Write a Java Program to check Armstrong Number ?

Core Java

Ans. http://www.programmingsimplified.com/java/source-code/java-program-armstrong-number

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Q532. How Spliterator in Java 8 different than iterator ?

Core Java

Ans. Though there are many differences the way internally they both iterates the collections and streams respectively, but the main difference in performance that is achieved by spliterator as it can iterate Streams in Parallel whereas iterator only iterates collections sequentially.

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Q533. Which is better in terms of performance - Iterator or Spliterator ?

Core Java

Ans. Spliterator has better performance potential than iterators but only if the potential is used. Spliterator can iterate streams in parallel as well as in sequence whereas iterator can only iterate in sequence.

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Q534. Difference between System.out.printf and System.out.println ?

Core Java

Ans. println prints an additional end of line whereas printf doesnt.

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Q535. What is the use of return statement without any argument ?

Core Java

Ans. Such return is only expected in methods that returns void. the purpose of having return in such a method is to return the flow out of method and return to the place where it was called. Its actually used to prematurely return the control from the method.

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Q536. Does it make sense to have return statement at the end of method ?

Core Java

Ans. It make sense if its returning something. It doesnt make sense for void methods where return statement is not preceded by anything. In case of void methods , return as last statement has no impact and is actually a overhead.

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Q537. What is meant by following error ?

Source folder is not on the Java build class path

Core Java

Ans. It means that the respective folder is not looked upon by Java to look for java or resource files and hence not being included as part of build.It needs to be added to Java Build Path.

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Q538. How Java is different than C/C++ in terms of memory allocation and de-allocation ?

Core Java

Ans. Java provides a faster allocation mechanism but at the cost of expensive deallocation. Deallocation in java is done by mechanism called garbage collection that runs periodically to free up unclaimed memory space , which actually slows down the application. Hence Java is usually faster if memory is not of concern and sufficiently large memory can be allocated to an application.

C/C++ on other hand have equal cost of allocation and de-allocation.

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Q539. What is a shutdown hook ?

Core Java

Ans. Shutdown hook is a thread that is invoked implicitly by JVM just before the shut down. It is used to clean up unused resources.

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Q540. Can we start the same thread twice ?

Core Java

Ans. No we cannot start the same thread twice. Each thread has a lifecycle.

But Yes, we can run the same code in parallel using different threads.

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