Postfix operators i.e ()  . is at the highest precedence.
Variables used in a switch statement can only be a string, enum, byte, short, int, or char.
This method is used to get the primitive data type of a certain String.
The String class is immutable, so that once it is created a String object cannot be changed. Since String is immutable it can safely be shared between many threads ,which is considered very important for multithreaded programming.
The String class is considered as immutable, so that once it is created a String object cannot be changed. If there is a necessity to make alot of modifications to Strings of characters then StringBuffer should be used.
Use StringBuilder whenever possible because it is faster than StringBuffer. But, if thread safety is necessary then use StringBuffer objects.
java.util.regex package is used for this purpose.
java.util.regex consists of three classes − Pattern class, Matcher class and PatternSyntaxException class.
It is possible to define a method that will be called just before an object’s final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.
An exception is a problem that arises during the execution of a program. Exceptions are caught by handlers positioned along the thread’s method invocation stack.
It is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
It is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation.
The Exception class has two main subclasses : IOException class and RuntimeException Class.
If a method does not handle a checked exception, the method must declare it using the throwskeyword. The throws keyword appears at the end of a method’s signature.
An exception can be thrown, either a newly instantiated one or an exception that you just caught, by using throw keyword.
The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.
While creating your own exception −
- All exceptions must be a child of Throwable.
- 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.
- You want to write a runtime exception, you need to extend the RuntimeException class.
It is the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.
If the method overrides one of its superclass’s methods, overridden method can be invoked through the use of the keyword super. It can be also used to refer to a hidden field.
Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.
It refers to the ability to make a class abstract in OOP. It helps to reduce the complexity and also improves the maintainability of the system.
These classes cannot be instantiated and are either partially implemented or not at all implemented. This class contains one or more abstract methods which are simply method declarations without a body.
If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.
It is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. Therefore encapsulation is also referred to as data hiding.
The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this Encapsulation gives maintainability, flexibility and extensibility to our code.
An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.
It includes −
- Interface cannot be instantiated
- An interface does not contain any constructors.
- All of the methods in an interface are abstract.
A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.
Packages are used in Java in-order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations, etc., easier.
A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.
Thread can be created by: implementing Runnable interface, extending the Thread class.
An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.
An applet extends java.applet.Applet class.
It uses garbage collection to free the memory. By cleaning those objects that is no longer reference by any of the program.
An immutable object can’t be changed once it is created.
It is used with variables or methods and used to call constructer of same class.
It is a collection of element which cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.
It is a Set implemented when we want elements in a sorted order.
It is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered.
- Throw is used to trigger an exception where as throws is used in declaration of exception.
- Without throws, Checked exception cannot be handled where as checked exception can be propagated with throws.
public static void main (String args[ ])
The following shows the explanation individually −
- public − it is the access specifier.
- static − it allows main() to be called without instantiating a particular instance of a class.
- void − it affirns the compiler that no value is returned by main().
- main() − this method is called at the beginning of a Java program.
- String args[ ] − args parameter is an instance array of class String
Java Runtime Environment is an implementation of the Java Virtual Machine which executes Java programs. It provides the minimum requirements for executing a Java application;
JAR files is Java Archive fles and it aggregates many files into one. It holds Java classes in a library. JAR files are built on ZIP file format and have .jar file extension.
This is Web Archive File and used to store XML, java classes, and JavaServer pages. which is used to distribute a collection of JavaServer Pages, Java Servlets, Java classes, XML files, static Web pages etc.
The java compiler creates a default constructor only if there is no constructor in the class.
No, this is not possible.
It is used to initialize the static data member, It is excuted before main method at the time of classloading.
Holding the reference of the other class within some other class is known as composition.
If a class has multiple functions by same name but different parameters, it is known as Method Overloading.
If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding.