Interview Questions On Java

Interview Questions on Java-part 1

What if the main method is declared as private? The program compiles properly but at runtime it will give “Main method not public.” message.
What is meant by pass by reference and pass by value in Java? Pass by reference means, passing the address itself rather than passing the value. Pass by value means passing a copy of the value. If you’re overriding the method equals() of an object, which other method you might also consider? hashCode()
What is Byte Code? Or What gives java it’s “write once and run anywhere” nature? All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any platform and hence java is said to be platform independent.
Expain the reason for each keyword of public static void main(String args[])? public- main(..) is the first method called by java environment when a program is executed so it has to accessible from java environment. Hence the access specifier has to be public. static: Java environment should be able to call this method without creating an instance of the class , so this method must be declared as static. void: main does not return anything so the return type must be void The argument String indicates the argument type which is given at the command line and arg is an array for string given during command line.
What are the differences between == and .equals() ? Or what is difference between == and equals Or Difference between == and equals method Or What would you use to compare two String variables - the operator == or the method equals()? Or How is it possible for two String objects with identical values not to be equal under the == operator? The == operator compares two objects to determine if they are the same object in memory i.e. present in the same memory location. It is possible for two String objects to have the same value, but located in different areas of memory. == compares references while .equals compares contents. The method public boolean equals(Object obj) is provided by the Object class and can be overridden. The default implementation returns true only if the object is compared with itself, which is equivalent to the equality operator == being used to compare aliases to the object. String, BitSet, Date, and File override the equals() method. For two String objects, value equality means that they contain the same character sequence. For the Wrapper classes, value equality means that the primitive values are equal. public class EqualsTest { public static void main(String[] args) { String s1 = “abc”; String s2 = s1; String s5 = “abc”; String s3 = new String(”abc”); String s4 = new String(”abc”); System.out.println(”== comparison : ” + (s1 == s5)); System.out.println(”== comparison : ” + (s1 == s2)); System.out.println(”Using equals method : ” + s1.equals(s2)); System.out.println(”== comparison : ” + s3 == s4); System.out.println(”Using equals method : ” + s3.equals(s4)); } } Output == comparison : true == comparison : true Using equals method : true false Using equals method : true
What if the static modifier is removed from the signature of the main method? Or What if I do not provide the String array as the argument to the method? Program compiles. But at runtime throws an error “NoSuchMethodError”. Why oracle Type 4 driver is named as oracle thin driver? Oracle provides a Type 4 JDBC driver, referred to as the Oracle “thin” driver. This driver includes its own implementation of a TCP/IP version of Oracle’s Net8 written entirely in Java, so it is platform independent, can be downloaded to a browser at runtime, and does not require any Oracle software on the client side. This driver requires a TCP/IP listener on the server side, and the client connection string uses the TCP/IP port address, not the TNSNAMES entry for the database name.
What is the difference between final, finally and finalize? What do you understand by the java final keyword? Or What is final, finalize() and finally? Or What is finalize() method? Or What is the difference between final, finally and finalize? Or What does it mean that a class or member is final? o final - declare constant o finally - handles exception o finalize - helps in garbage collection Variables defined in an interface are implicitly final. A final class can’t be extended i.e., final class may not be subclassed. This is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. A final method can’t be overridden when its class is inherited. You can’t change value of a final variable (is a constant). finalize() method is used just before an object is destroyed and garbage collected. finally, a key word used in exception handling and will be executed whether or not an exception is thrown. For example, closing of open connections is done in the finally method.
What is the Java API? The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.
What is the GregorianCalendar class? The GregorianCalendar provides support for traditional Western calendars.
What is the ResourceBundle class? The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program’s appearance to the particular locale in which it is being run.
Why there are no global variables in Java? Global variables are globally accessible. Java does not support globally accessible variables due to following reasons: • The global variables breaks the referential transparency • Global variables creates collisions in namespace.
How to convert String to Number in java program? The valueOf() function of Integer class is is used to convert string to Number. Here is the code example: String numString = “1000″; int id=Integer.valueOf(numString).intValue(); What is the SimpleTimeZone class? The SimpleTimeZone class provides support for a Gregorian calendar. What is the difference between a while statement and a do statement? A while statement (pre test) checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement (post test) checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the loop body at least once.