Java Important Questions and Answers

[Kalyan]

Java Important Questions and Answers

What Output is displayed as the result of executing the following statement?

System.out.println("// Looks like a comment.");
// Looks like a comment
a)The statement results in a compilation error.
b)Looks like a comment.
c)No output is displayed.

Ans.a

In order for a source code file, containing the public class Test, to successfully compile, which of the following must be true?
a)It must have a package statement
b)It must be named Test.java
c)It must import java.lang
d)It must declare a public class named Test

Ans.b

What are identifiers and what is naming convention?

Ans:Identifiers are used for class names, method names and variable names. An identifier may be any descriptive sequence of upper case & lower case letters,numbers or underscore or dollar sign and must not begin with numbers.

What is the return type of program's main( ) method?

Ans :void

What is the argument type of program's main( ) method?

Ans :string array.

Which characters are as first characters of an identifier?

Ans :A – Z, a – z, _ ,$

What are different comments?

Ans :
1) // -- single line comment
2) /* --*/ multiple line comment
3) /** --*/ documentation

What is the difference between constructor method and method?

Ans :Constructor will be automatically invoked when an object is created. Whereas method has to be call explicitly.

What is the use of bin and lib in JDK?

Ans : Bin contains all tools such as javac, applet viewer, awt tool etc., whereas Lib contains all packages and variables.
Data types,Variables and Arrays

What is meant by variable?

Ans: Variables are locations in memory that can hold values. Before assigning any value to a variable, it must be declared.

What are the kinds of variables in Java? What are their uses?

Ans: Java has three kinds of variables namely, the instance variable, the local variable and the class variable.

Local variables are used inside blocks as counters or in methods as temporary variables and are used to store information needed by a single method.

Instance variables are used to define attributes or the state of a particular object and are used to store information needed by multiple methods in the objects.

Class variables are global to a class and to all the instances of the class and are useful for communicating between different objects of all the same class or keeping track of global states.