What is a green thread?

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A green thread refers to a mode of operation for the Java Virtual Machine (JVM) in which all code is executed in a single operating system thread. If the Java program has any concurrent threads, the JVM manages multi-threading internally rather than using other operating system threads. There is a significant processing overhead for the JVM to keep track of thread states and swap between them, so green thread mode has been deprecated and removed from more recent Java implementations. Current JVM implementations make more efficient use of native operating system threads.

What is a working thread?

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A working thread, more commonly known as a worker thread is the key part of a design pattern that allocates one thread to execute one task. When the task is complete, the thread may return to a thread pool for later use. In this scheme a thread may execute arbitrary tasks, which are passed in the form of a Runnable method argument, typically execute(Runnable). The runnable tasks are usually stored in a queue until a thread host is available to run them. The worker thread design pattern is usually used to handle many concurrent tasks where it is not important which finishes first and no single task needs to be coordinated with another. The task queue controls how many threads run concurrently to improve the overall performance of the system. However, a worker thread framework requires relatively complex programming to set up, so should not be used where simpler threading techniques can achieve similar results.

What are “static initializers” or “static blocks with no function names”?

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When a class is loaded, all blocks that are declared static and don’t have function name (i.e. static initializers) are executed even before the constructors are executed. As the name suggests they are typically used to initialize static fields.

public class StaticInitilaizer {

public static final int A = 5;

public static final int B;

//Static initializer block, which is executed only once when the class is loaded.

static {

if(A == 5)

B = 10;

else

B = 5;

}

public StaticInitilaizer(){} // constructor is called only after static initializer block

}

The following code gives an Output of A=5, B=10.

public class Test {

System.out.println("A =" + StaticInitilaizer.A + ", B =" + StaticInitilaizer.B);

}

Explain static vs. dynamic class loading?

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Explain Java class loaders? Explain dynamic class loading?

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Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that is already running in the JVM. So how is the very first class loaded? The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of primordial class loader. Let us look at the class loaders created by the JVM.

Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their

parent to load the class first before attempting to load it themselves. When a class loader loads a class, the  child class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true as explained in the above diagram.

Important: Two objects loaded by different class loaders are never equal even if they carry the same values, which mean a class is uniquely identified in the context of the associated class loader. This applies to singletons too, where each class loader will have its own singleton.

What is the difference between C++ and Java?

• 
  
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• Java does not support pointers. Pointers are inherently tricky to use and troublesome.
• 
  
  Java does not support multiple inheritances because it causes more problems than it solves. Instead Java supports multiple interface inheritance, which allows an object to inherit many method signatures from different interfaces with the condition that the inheriting object must implement those inherited methods. The multiple interface inheritance also allows an object to behave polymorphically on those methods.
• 
  
  Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method. This means you do not know when the objects are going to be finalized. Avoid using finalize() method to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to release these resources through the finalize() method.
• 
  
  Java does not include structures or unions because the traditional data structures are implemented as an object oriented framework (Java collection framework).
• 
  
  All the code in Java program is encapsulated within classes therefore Java does not have global variables or functions.
• 
  
  C++ requires explicit memory management, while Java includes automatic garbage collection.

What is the main difference between the Java platform and the other software platforms?

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Java platform is a software-only platform, which runs on top of other hardware-based platforms like UNIX, NT etc.
The Java platform has 2 components:

•  Java Virtual Machine (JVM) – ‘JVM’ is a software that can be ported onto various hardware platforms. Byte codes are the machine language of the JVM.
• Java Application Programming Interface (Java API)