Java Basics

Question 1

Is the following syntax valid?

int x=10, int y;

Answer 1

No.

It must be written either:

int x=10; int y;

or

int x=10, y;

Question 2

Must a java file have a public class?

Answer 2

No.

Question 3

If a class (or enum) is public then where must it be defined?

Answer 3

In a file by the same name.

Question 4

Can there be more than one public class (or enum) per file?

Answer 4

No.

Question 5

Is final a valid modifier for a main method?

public final static void main (String[ ] args) { }

Answer 5

Yes.

Final means this cannot be overridden. Making this static method final prevents subclasses from implementing the same (hidden) static method.

Question 6

What does Object class’s toString method do?

Answer 6

It prints the name of the class + the @ sign + the object’s hash code.

Question 7

If you pass the name of an array to a print/println method, what happens?

Answer 7

It will print the symbol ` [ ` + the Element Type encoding + the ` @ ` sign + the hash code.

E.g.: [LmyArray;@

Question 8

If you do not import a given package, how can you still access its public methods?

Answer 8

Use the fully qualified class name.

Question 9

T/F: Every class belongs to a package.

Answer 9

True.

Even if you omit the package statement, the class belongs to the unnamed package.

Question 10

The following are the complete contents of TestClass.java file. Which packages are automatically imported?

class TestClass{
   public static void main(String[] args){
     System.out.println("hello");
  }
}

Answer 10

java.lang

and

“The package with no name”

Question 11

Does a main method necessarily need to take an array of strings as an argument if it is to run from the command line?

Answer 11

Yes.

Question 12

A class or interface can be referred to by using a fully qualified name or a simple name. What must happen to be able to use the simple name?

Answer 12

The class must be in the same package or imported.

Question 13

Does Java support multiple inheritance of types?

Answer 13

Yes.

Interfaces, classes and enums are all “types”. Since a class can implement multiple interfaces, Java does support multiple inheritance of types.

Question 14

Does Java support multiple inheritance of state?

Answer 14

No.

State is represented by instance fields, which only a class can have.

Question 15

What is encapsulation?

Answer 15

The technique used to package the information in such a way as to hide what should be hidden, and make visible what is intended to be visible.

It helps make sure that clients have no accidental dependence on the choice of representation

Question 16

When a program is called from the command line with no arguments, what is the length of the args array?

Answer 16

Zero.

Question 17

Name two benefits of polymorphism.

Answer 17

It makes code more reusable.

It makes code more dynamic.

Question 18

Is the following syntax valid?

import static com.foo.*

(foo is a package)

Answer 18

No. You cannot import a package statically.

Question 19

Can you import a package statically?

Answer 19

No.

You can only import static members of a class statically.

Question 20

What is the syntax for importing static fields?

Answer 20

import static…*

or

import static ..fieldname>;

Question 21

DID YOU READ THE QUESTION CAREFULLY?

Answer 21

Yes.

Question 22

Is the this reference available with a static method?

Answer 22

No.

Question 23

Does each object of a class have its own copy of each non-static member variable?

Answer 23

Yes.

Question 24

In an instance method, is calling this.foo() the same as calling foo( ) ?

Answer 24

Yes, the this keyword is a assigned to the current object automatically by the JVM.