Java Basics

Question 1

Java Object

Answer 1

A runtime instance of a class in memory

Question 2

Class’s elements

Answer 2

Fields & methods

Question 3

How many public classes a file can have?

Answer 3

1. Each file can contain only one public class

2. The file name must match the class name, including case, and have a .java extension.

Question 4

What is the main method structure?

Answer 4

public static void main(String [] args)
public static void main(String args [])
public static void main(String… args)
public static final void main(String [] args)

Question 5

package

Answer 5

A grouping of classes, interfaces, enumerations, and annotated types.

Question 6

What is the package that is automatically imported?

Answer 6

java.lang

Question 7

What are imports needed for?

Answer 7

Imports are needed to be able to reference an external class.

Question 8

default package

Answer 8

An unnamed package used when no package statement is present in the code. This is a special unnamed package that you should use only for throwaway code

Question 9

What are wildcards used for?

Answer 9

Wildcards (*) are used to import all the classes from the a package. Importing all the classes in a package vs just the class that is going to be used does not slow down the program. Listing each of the classes makes the code easy to read. Using the wildcard can shorten the import list.
Only one (at the end) is permited in an import.

Question 10

Imports precedence explicit vs wildcard

Answer 10

Explicit import takes precedence over any wildcards.

Question 11

package compilation errors

Answer 11

package error on runtime only;

import at compile time.

Question 12

class structure

Answer 12

package.
imports.
class declaration.

Question 13

bytecode

Answer 13

Compiled Java code. Bytecode appears in files with the .class extension.

Question 14

What is the purpose of a constructor?

Answer 14

The purpose of the constructor is to initialize fields, although you can put any code in there.

Question 15

Constructor return type

Answer 15

It has no return type.

Question 16

Instance initializer

Answer 16

code block outside methods.

Question 17

Order of initialization

Answer 17

Fieds and instance initilizer blocks are run in the order in wich they appear in the file.
The constructor runs after above have run.

Question 18

What is a default constructor?

Answer 18

default constructor is the one that is not provided by the user but by the jvm.