What is user defined Exception?

These are separate exception classes which users define so that they may assist them in certain particular purposes, user-defined exception can be created just by doing a subclass of the exception class, there by allowing custom exceptions to be obtained, using throw, and caught in the way as which resembles the way the normal exceptions are done, for instance,

Class myCustomException extends Exception

{

//It is a necessity that a class exists so that an exception may exist

}

What new attributes are found in the Java Database Connectivity (JDBC) 2.0 Application/Abstract Programming Interface (API) or the JDBC 2.0 core API?

Java Database Connectivity 2.0 Application/Abstract Programming Interface (JDBC 2.0 API) has the following attributes: A full set of JDBC API that has both optional packages API as well as the core API and avails to the programmer a strong industrial level database for computing.

These are:

Result tests which can be scrolled, by the help of the methods which are new in the interface ResultSet, which facilitates one by one move to a desired row or to the relevant position regarding the present position.
It also gives a functionality based on Batch Update to the applications in java.
ResultSet.updateXXX methods can then be used by java applications
Data types of type SQL3 for interface mapping which are generally new data types
UDT (User defined types) for mapping in a costumed way.
Other features like performance hints, ability to use the character streams, java.math.BigDecimal values that are generally of full precision in nature, security is enhanced and support regarding time zone with respect to timestamp values as well as time and dates.

What is Garbage Collection?

This is an important feature of Java Garbage Collection and is called automatic memory management since Java Virtual Machine removes unused objects or variables when the value is null automatically from the memory.

The programs done by users usually are not in a position to remove the objects from the memory and as a result the garbage collector must be responsible for the removal of objects which the program no longer references to, each class inherits the finalize() method usually from the java.lang.object, this method is invoked by the garbage collector on determination that an object no longer experiences references.

When using Java, one should ensure to assign null into a variable explicitly when not in use any more, the Java Virtual Machine tries to recycle unused objects by calling the Runtime.gc and Systems.gc() but this doesn’t guarantee time for garbage collection of all the objects

How can garbage collection are forced.

No way, this happens automatically

Briefly describe OOPs.

It constitutes the initials of Object Oriented Programming

Describe the principles of OOPs.

The main principles of OOP are possibility of Inheritance, Encapsulation and polymorphism.

What is encapsulation?

It is wrapping or binding of data as well as the code that operates on the data into a single thing. Its purpose is to ensure data safety from misuse as well as outside interface; it is like a protective wrapper, which hinders data and code from arbitral access by code, which have been described outside the wrapper.

What is inheritance?

It is the occurrence whereby an object acquires the characteristics of another object

What is the principle of polymorphism?

It implies a situation whereby many forms are attributed to one name.

It is important in allowing a particular entity to take place of a general category involving different types of events. The particular event is determined by nature of the prevailing situation, this concept can be resembled to the one of multiple methods attributed to one interface in elaboration.

What are the forms of polymorphism?

These can be three as from the view of a typical programmer in Java namely Method Overloading, method overriding using inheritance as well as overriding the given methods through Java interfaces

Name and explain the Java Access Specifiers

These are the keywords responsible in determining the level of access to a given member of a class and they include private, public, and protected as well as defaults

What are the wrapper classes in java?

This is a wrapper, which occurs around the primitive data, its instance may wrap a value, which is primitive, and have a corresponding type.

The following table provides an insight into the primitive types and its corresponding wrapper class

Primitive	Wrapper
short	java.lang.Short
void	java.lang.Void
long	java.lang.Long
int	java.lang.Integer
float	java.lang.Float
double	java.lang.Double
char	java.lang.Character
boolean	java.lang.Boolean
byte	java.lang.Byte

What is the output of the following program?

public class test {
public static void main(String [] args) {
    int x = 3;
    int y = 1;
   if (x = y)
     System.out.println("Not equal");
else
    System.out.println("Equal");
}
}

A: Equal

B: Not Equal

C: compilation fall is caused at if(x=y), thus result in an error

D: no output is realized on console even though the program executes

Explain class variables.

A common copy of variables is shared every time one creates several objects of a one class, implying that only one copy per class exist disregard the number of objects created from it. Static words in a class are used to declare static variables or the class variables though it is important to note that declaration should be done outside the class. Static memory provides the storage for these variables. The most common use of the class variables include the constants, which are the variable that once declared, can never change again, i.e. the initial value is not changed at any instance and are called by the class name always.

These are the variables, which are created immediately the program begins, that is, before creation of the instance of the class with the help of operators and destroyed immediately the program stops. A class variable has a similar scope to that of instance variable.

Definition of a class variable can take place anywhere at the level of the class with a keyword static, they share a common initial value with the instance variable. When definition is of an int in nature, then zero becomes its default value, when a type bool is used, then the default value is false and null regarding the object reference. These class variables are never associated with the any object but with the class.

Is instanceof and getClass similar?

No, they are not the in the sense that the instanceof refers to an operator whereas the getClass refers to a method, usually of the class java.lang.Object class. Take into consideration a scenario where if(o.getClass().getName().equals("java.lang.Math")){ }

is used, the only check by this method is whether we have passed a classname equal to java.lang,Math, the class java.lang.Math is loaded by the bootsrap classLoader of which is an abstract class. The responsibility of a classLoader is definitely to load classes and all objects of the given classes have references to this classLoader. Returning of the runtime class of object is performed by the getClass method by fetching the instance of java for a type name, which has been qualified fully. The code which we wrote is of no significance as we aren’t supposed to compare getClass.getName(). This is because if two separate class loaders load the same class, Java Virtual Machine will consider them as separate classes. The only thing it can provide is the implementing class and no interface comparison though the instanceof operator is in a position to do that. Comparison between an object to a specified type can be preformed by the instance of operator, it can be used for testing whether an object is a class instance, a subclass instance or an instance to a class implementing an interface of a certain order thus it is incumbent that we make efforts to use the instanceof operator rather than getClass() method. Note that there is no similarity between instanceof operator and the getClass method; this can be clearly depicted in the following example:

Interface oone {
}

Class Two implements oone {
}
Class Three implements oone {
}

public class Test {
public static void main(String args[]) {
one test1 = new Two();
one test2 = new Three();
System.out.println(test1 instanceof one); //true
System.out.println(test2 instanceof one); //true
System.out.println(Test.getClass().equals(test2.getClass())); //false
}
}

Describe the use of observables and observer.

The objects usually maintain list of observers, subclass to the observable class. update() method for all pertinent observers are invoked upon updating of the observable object in order to bring note to observers that a change in state has occurred. Observer interface is usually implemented by the by the objects which observe the observable objects

Explain the difference between preemptive scheduling and time-sharing.

When it comes to preemptive scheduling, there is no stoppage in a process until a task is accomplished or a highly prioritized task is availed whereas in time slicing, there exists time division such that all a task is processed for a specific period of time upon completion of time, it reenters a pool of ready tasks as the scheduler is left to select the next task to execute depending on the priorities as well as predefined factors.

Describe order of precedence and the associativity and also explain their usages.

Order in which operators are evaluated is determined by the order of precedence whereas determination as to whether to evaluate expression right to left or left to right is performed by the associativity.

Explain the meaning of transient variable.

This is a variable in which serialization can not be performed, an example is a situation whereby one declares a variable transient in a class which can be serialized and then writes the class to an ObjectSream, here, there is no way to write the value of the variable to the stream instead when the class is retrieved out of the ObjectStream rendering the value of the variable null.

What is the name of the container known to use Border Layouts as their default layouts?

Dialog classes, Frames and Windows

Describe your understanding of the term Synchronization.

This is the responsiveness/control of access to shared resources using many threads to attain a situation whereby only one thread is in a position to access a particular resource at any one given time. In a situation where there exists no synchronization, there exists a possibility for modification of a shared object by a thread as the other thread is using the object’s value thus such data should protected by synchronization. The following is an example of synchronization:

Public synchronized void Method1 () {
// Appropriate method-related code.
}
Example on how to synchronize a block of code (inside a function)

Public myFunction (){
    synchronized (this) {
            // Synchronized code here.
         }
}

What is API?

API is the Abbreviation for Application/Abstract Programming Interface, it consists of classes and interfaces responsible for providing support in the operations on the collections of given objects. These are classes and interfaces, which are more powerful, flexible and regular as compared to others like arrays, hashtables, vectors, if used carefully.

Example of classes includes: ArrayList, LinkedList, TreeMap, HashMap, HashSet, TreeSet

Examples of Interfaces Include: Set, Maps, Lists, Collection

State whether an Iterator is a Class or an Interface.

It is an Interface and applicable while performing step through the elements of a collection

Compare and contrast an interface with an abstract class.

Contrasts:

Interface is responsible while carrying out multiple inheritances while a class is only responsible for extending another class.
Interfaces have got limitations to public methods and constants without limitations while the classes can as well bear protected parts, static methods, protected parts, partial implementation, etc.
Unlike an ordinary class, which can implement several interfaces, a class may extend only one abstract class when it comes to abstract classes.
Abstract classes are relatively faster than interfaces as interfaces require further indirections so as to locate a corresponding method in the expected actual class.

Comparison

The only comparison is that instantiation cannot be performed in either of them.

Define an abstract class.

When abstract methods are contained in a class, then it becomes an abstract class and no instantiation can be performed in such a class.

Example includes:

abstract class AbstractClass {
    protected String herString;
    public String getherString() {
        return herString;
        }
    public abstract string theAbstractFunction();
}

Define an interface.

They are the paraphernalia used to define but not implement methods in java, constants can be included in interfaces. For a class to implement an interface, it is necessary that that class implement all methods, which are defined in the interface.