What does object reference mean?

What does object reference mean?

A link to an object. Object references can be used exactly like the linked objects. Rather than holding a copy of the object, each assigned property holds object references that link to the same object, so that when the object changes all properties referring to the object reflect the change. ...

What is a null object reference?

Special object reference null explicitly refers to no object. Since there is no object there, you cannot ask the nonexistent object to do something for you. Any time you do a dot operation on a variable that holds null, the program throws exception NullPointerException. For example, doing statements.

How do you handle null reference exception?


  1. Method 1 - use if statement. Check the property before accessing instance members. ...
  2. Method 2 - use Null Conditional Operator( ? ) It will check the property before accessing instance members. ...
  3. Method 3 - use GetValueOrDefault() ...
  4. Method 4 - use Null Coalescing Operator. ...
  5. Method 5 - use ?: operator.

Why Runtime Exceptions are not checked?

Because the Java programming language does not require methods to catch or to specify runtime exceptions or errors, programmers can be tempted to write code that throws only runtime exceptions or to make all their exception subclasses inherit from RuntimeException .

How can you tell if an exception is checked or unchecked?

The main difference between checked and unchecked exception is that the checked exceptions are checked at compile-time while unchecked exceptions are checked at runtime.

What are checked exceptions give an example?

They occur usually interacting with outside resources/network resources e.g. database problems, network connection errors, missing files etc. Checked exceptions are subclasses of Exception class. Example of checked exceptions are : ClassNotFoundException, IOException, SQLException and so on.

What are restrictions on checked exceptions?

Checked exceptions are exceptions that the compiler require you handle in some way. In Java, checked exceptions are Throwable s that are not RuntimeException , Error , or one of their subclasses. The Java designers felt they were needed to ensure programs handled exceptions that were reasonably likely.

What are checked and unchecked exceptions?

1) Checked: are the exceptions that are checked at compile time. If some code within a method throws a checked exception, then the method must either handle the exception or it must specify the exception using throws keyword. ... 2) Unchecked are the exceptions that are not checked at compiled time.

What is the difference between error and exception?

Errors mostly occur at runtime that's they belong to an unchecked type. Exceptions are the problems which can occur at runtime and compile time. It mainly occurs in the code written by the developers. Exceptions are divided into two categories such as checked exceptions and unchecked exceptions.

Can we use throws for unchecked exception?

You can use "throws" to declare unchecked exceptions also. But unchecked exceptions are programmer headaches. So throws usually used to handle checked exceptions only as they are anticipated by the compiler whenever you are making use of certain classes and interfaces.

How do you create an unchecked exception?

We can create the custom unchecked exception by extending the RuntimeException in Java. Unchecked exceptions inherit from the Error class or the RuntimeException class.

How do you create an exception?

You can create your own exceptions in Java.

  1. All exceptions must be a child of Throwable.
  2. If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.
  3. If you want to write a runtime exception, you need to extend the RuntimeException class.

What is difference between throw and throws?

Throw is a keyword which is used to throw an exception explicitly in the program inside a function or inside a block of code. Throws is a keyword used in the method signature used to declare an exception which might get thrown by the function while executing the code.

What are custom exceptions?

If you are creating your own Exception that is known as custom exception or user-defined exception. Java custom exceptions are used to customize the exception according to user need. By the help of custom exception, you can have your own exception and message.

Why throw is used in Java?

The Java throws keyword is used to declare an exception. It gives an information to the programmer that there may occur an exception so it is better for the programmer to provide the exception handling code so that normal flow can be maintained. Exception Handling is mainly used to handle the checked exceptions.

Is finally a keyword in Java?

The finally keyword is used to execute code (used with exceptions - try.. catch statements) no matter if there is an exception or not.

Can we use try without catch?

Yes, It is possible to have a try block without a catch block by using a final block. As we know, a final block will always execute even there is an exception occurred in a try block, except System.

What is difference between static final and finally keywords?

The final keyword can be used with class method and variable. A final class cannot be instantiated, a final method cannot be overridden and a final variable cannot be reassigned. The finally keyword is used to create a block of code that follows a try block.

Can we override static method?

Can we override a static method? No, we cannot override static methods because method overriding is based on dynamic binding at runtime and the static methods are bonded using static binding at compile time. So, we cannot override static methods.

Are final variables static?

Static variables are stored in the static memory, mostly declared as final and used as either public or private constants. Static variables are created when the program starts and destroyed when the program stops. Visibility is similar to instance variables.

Are static methods Final?

Reports methods declared final and static. When a static method is overriden in a subclass it can still be accessed via the superclass making the final declaration not very necessary. Declaring a static method final does prevent subclasses from defining a static method with the same signature.

Can final method overridden?

You use the final keyword in a method declaration to indicate that the method cannot be overridden by subclasses.

Can we use static variables inside non static methods?

Static variables are class variable not instance or local variable . that is why we can use static variable in non static method also. and static variables are not per object . static variables have one copy that will be used in entire program.

Can abstract class contain final method?

As a final class cannot be inherited. However, an abstract class can have a final method. This final method is treated like a normal method with a body which cannot be overridden.