Posted by: lrrp | December 3, 2014

Common JDK , JVM, JRE related questions

  • What do you mean by platform independence of Java?

    Platform independence means that you can run the same Java Program in any Operating System. For example, you can write java program in Windows and run it in Mac OS.

  • What is JVM and is it platform independent?

    Java Virtual Machine (JVM) is the heart of java programming language. JVM is responsible for converting byte code into machine readable code. JVM is not platform independent, thats why you have different JVM for different operating systems. We can customize JVM with Java Options, such as allocating minimum and maximum memory to JVM. It’s called virtual because it provides an interface that doesn’t depend on the underlying OS.

  • What is the difference between JDK and JVM?

    Java Development Kit (JDK) is for development purpose and JVM is a part of it to execute the java programs.

    JDK provides all the tools, executables and binaries required to compile, debug and execute a Java Program. The execution part is handled by JVM to provide machine independence.

  • What is the difference between JVM and JRE?

    Java Runtime Environment (JRE) is the implementation of JVM. JRE consists of JVM and java binaries and other classes to execute any program successfully. JRE doesn’t contain any development tools like java compiler, debugger etc. If you want to execute any java program, you should have JRE installed.

  • Why Java is not pure Object Oriented language?

    Java is not said to be pure object oriented because it support primitive types such as int, byte, short, long etc. I believe it brings simplicity to the language while writing our code. Obviously java could have wrapper objects for the primitive types but just for the representation, they would not have provided any benefit.

    As we know, for all the primitive types we have wrapper classes such as Integer, Long etc that provides some additional methods.

  • What is difference between path and classpath variables?

    PATH is an environment variable used by operating system to locate the executables. That’s why when we install Java or want any executable to be found by OS, we need to add the directory location in the PATH variable.

    Classpath is specific to java and used by java executables to locate class files. We can provide the classpath location while running java application and it can be a directory, ZIP files, JAR files etc.

  • What is the importance of main method in Java?

    main() method is the entry point of any standalone java application. The ideal syntax of main method is public static void main(String args[]).

    main method is public and static so that java can access it without initializing the class. The input parameter is an array of String through which we can pass runtime arguments to the java program.

  • What is Classloader in Java?

    Java Classloader is the program that loads byte code program into memory when we want to access any class. We can create our own classloader by extending ClassLoader class and overriding loadClass(String name) method.

  • What are different types of classloaders?

    There are three types of built-in Class Loaders in Java:

    1. Bootstrap Class Loader – It loads JDK internal classes, typically loads rt.jar and other core classes.
    2. Extensions Class Loader – It loads classes from the JDK extensions directory, usually $JAVA_HOME/lib/ext directory.
    3. System Class Loader – It loads classes from the current classpath that can be set while invoking a program using -cp or -classpath command line options.
  • What is difference between Heap and Stack Memory?

    Major difference between Heap and Stack memory are as follows:

    • Heap memory is used by all the parts of the application whereas stack memory is used only by one thread of execution.
    • Whenever an object is created, it’s always stored in the Heap space and stack memory contains the reference to it. Stack memory only contains local primitive variables and reference variables to objects in heap space.
    • Memory management in stack is done in LIFO manner whereas it’s more complex in Heap memory because it’s used globally.

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