Data Types and Variable

How Java Differs from C and C++

Pointers

Java does not have an explicit pointer type. Instead of pointers, all references to objects - 

including variable assignments, arguments passed into methods, and array elements — are

accomplished by using implicit references. References and pointers are essentially the same thing

except that you can’t do pointer arithmetic on references (nor do you need to).

Reference semantics also enable structures such as linked lists to be created easily in Java without

explicit pointers; merely create a linked list node with variables that point to the next and the

previous node. Then, to insert items in the list, assign those variables to other node objects.

Arrays

Arrays in Java are first class objects, and references to arrays and their contents are accomplished

through explicit references rather than via point arithmetic. Array boundaries are strictly

enforced; attempting to read past the ends of an array is a compile or run-time error. As with

other objects, passing an array to a method passes a reference to the original array, so changing

the contents of that array reference changes the original array object.

Arrays of objects are arrays of references that are not automatically initialized to contain actual

objects. Using the following Java code produces an array of type MyObject with ten elements, but

that array initially contains only nulls:

MyObject arrayofobjs[] = new MyObject[10];

You must now add actual MyObject objects to that array:

for (int i; i< arrayofobjs.length. i++) {
arrayofobjs[i] = new MyObject();
}

Java does not support multidimensional arrays as in C and C++. In Java, you must create arrays
that contain other arrays.

Strings

Strings in C and C++ are arrays of characters, terminated by a null character (\0). To operate

on and manage strings, you treat them as you would any other array, with all the inherent

difficulties of keeping track of pointer arithmetic and being careful not to stray off the end of

the array.

Strings in Java are objects, and all methods that operate on strings can treat the string as a

complete entity. Strings are not terminated by a null, nor can you accidentally overstep the end

of a string (like arrays, string boundaries are strictly enforced.

Memory Management

All memory management in Java is automatic; memory is allocated automatically when an

object is created, and a run-time garbage collector (the “GC”) frees that memory when the object

is no longer in use. C’s malloc and free functions do not exist in Java.

To “force” an object to be freed, remove all references to that object (assign variables holding

it to null, remove it from arrays, and so on). The next time the Java GC runs, that object is

reclaimed.

Data Types

As mentioned in the early part of this book, all Java primitive data types (char, int, long, and

so on) have consistent sizes and behavior across platforms and operating systems. There are no

unsigned data types as in C and C++ (except for char, which is a 16-bit unsigned integer).

The boolean primitive data type can have two values: true or false. Boolean is not an integer,

nor can it be treated as one, although you cannot cast 0 or 1 (integers) to boolean types in Java.

Composite data types are accomplished in Java exclusively through the use of class definitions.

The struct, union, and typedef keywords have all been removed in favor of classes.

Casting between data types is much more controlled in Java; automatic casting occurs only when

there will be no loss of information. All other casts must be explicit. The primitive data types

(int, float, long, char, boolean, and so on) cannot be cast to objects or vice versa; there are

methods and special “wrapper” classes to convert values between objects and primitive types.

Control Flow

Although the if, while, for, and do statements in Java are syntactically the same as they are in

C and C++, there is one significant difference. The test expression for each control flow construct

must return an actual boolean value (true or false). In C and C++, the expression can return

an integer.

Arguments

Java does not support mechanisms for optional arguments or for variable-length argument lists

to functions as in C and C++. All method definitions must have a specific number of arguments.

Command-line arguments in Java behave differently from those in C and C++. The first element

in the argument vector (argv[0]) in C and C++ is the name of the program itself; in Java, that

first argument is the first of the additional arguments. In other words, in Java, argv[0] is argv[1]

in C and C++; there is no way to get hold of the actual name of the Java program.

Other Differences

The following other minor differences from C and C++ exist in Java:

• Java does not have a preprocessor, and as such, does not have #defines or macros.
• Constants can be created by using the final modifier when declaring class and instance variables.
• Java does not have template classes as in C++.
• Java does not include C’s const keyword or the ability to pass by const reference explicitly.
• Java classes are singly inherited, with some multiple-inheritance features provided through interfaces.
• All functions are implemented as methods. There are no functions that are not tied to classes.
• The goto keyword does not exist in Java (it’s a reserved word, but currently unimplemented). You can, however, use labeled breaks and continues to break out of and continue executing complex switch or loop constructs.

Java Object Oriented Programming Concept

Object Oriented Programming is a paradigm that provides many concepts such as inheritance, data binding, polymorphism etc.
Simula is considered as the first object-oriented programming language. The programming paradigm where everything is represented as an object, is known as truly object-oriented programming language.
Smalltalk is considered as the first truly object-oriented programming language.

---

OOPs (Object Oriented Programming System)

Object means a real word entity such as pen, chair, table etc. Object-Oriented Programming is a methodology or paradigm to design a program using classes and objects. It simplifies the software development and maintenance by providing some concepts:

• Object
• Class
• Inheritance
• Polymorphism
• Abstraction
• Encapsulation

---

Object
Any entity that has state and behavior is known as an object. For example: chair, pen, table, keyboard, bike etc. It can be physical and logical.

---

Class
Collection of objects is called class. It is a logical entity.

---

Inheritance
When one object acquires all the properties and behaviours of parent object i.e. known as inheritance. It provides code reusability. It is used to achieve runtime polymorphism.

---

Polymorphism
When one task is performed by different ways i.e. known as polymorphism. For example: to convense the customer differently, to draw something e.g. shape or rectangle etc. In java, we use method overloading and method overriding to achieve polymorphism. Another example can be to speak something e.g. cat speaks meaw, dog barks woof etc.

---

Static Polymorphism (compile time polymorphism/ Method overloading):
The ability to execute different method implementations by altering the argument used with the method name is known as method overloading. For Example We have three print methods each with different arguments. When you properly overload a method, you can call it providing different argument lists, and the appropriate version of the method executes.

---

Dynamic Polymorphism (run time polymorphism/ Method Overriding)
When you create a subclass by extending an existing class, the new subclass contains data and methods that were defined in the original superclass. In other words, any child class object has all the attributes of its parent. Sometimes, however, the superclass data fields and methods are not entirely appropriate for the subclass objects; in these cases, you want to override the parent class members.

---

Abstraction
Hiding internal details and showing functionality is known as abstraction. For example: phone call, we don't know the internal processing. In java, we use abstract class and interface to achieve abstraction.

---

Encapsulation
Binding (or wrapping) code and data together into a single unit is known as encapsulation. For example: capsule, it is wrapped with different medicines. A java class is the example of encapsulation. Java bean is the fully encapsulated class because all the data members are private here. Encapsulation means putting together all the variables (instance variables) and the methods into a single unit called Class. It also means hiding data and methods within an Object. Encapsulation provides the security that keeps data and methods safe from inadvertent changes. Programmers sometimes refer to encapsulation as using a “black box,” or a device that you can use without regard to the internal mechanisms. A programmer can access and use the methods and data contained in the black box but cannot change them.

---

Advantage of OOPs over Procedure-oriented programming language

1. OOPs makes development and maintenance easier where as in Procedure-oriented programming language it is not easy to manage if code grows as project size grows.
2. OOPs provides data hiding whereas in Procedure-oriented prgramming language a global data can be accessed from anywhere.
3. OOPs provides ability to simulate real-world event much more effectively. We can provide the solution of real word problem if we are using the Object-Oriented Programming language.

Java Comments

Comments

The purpose of including comments in your code is to explain what the code is doing. Java supports both single and multi-line comments. All characters that appear within a comment are ignored by the java compiler.

---

Single-line comment

A single-line comment starts with two forward slashes and continues until it reaches the end of the line.
For example:

// this is a single-line comment
X= 5; //Assign 5 to Variable X

Adding comments as you write code is a good practice,because they provide clarification and understanding when you need to refer back to it, as well as for others who might need to read it.

---

Multi-line comments

Java also supports comments that span multiple lines.
You start this type of comment with a forward slash followed by an asterisk, and end it with an asterisk followed by a forward slash.
For example:

/* 
* This is also a Comment 
* Spanning multiple lines
*/

Note that Java does not support nested mutli-line comments.
However, you can nest Single-line comments within multi-line comments.

/*
* JavaSunderesan - MultiLine Comments 
// a Single Line Comment
*/

---

Documentation Comments

Documentation comments are special comments that have the appearance of multi-line comments, with the difference being that they generate external documentation of your source code. These begin with a forward slash followed by two asterisks, and end with an asterisk followed by an forward slash.
For Example:

/** Welcome To Java Sunderesan
* Copyright (C)2016 King  . All Rights Reserved
*/

Javadoc is a tool which comes with JDK and it is used for generating Java Code documentation in HTML format from Java source code which has required documentation in a predefined format.
When a documentation comment begins with more than two asterisks, Javadoc assumes that you want to create a "box" around the comment in the source code. It simply ignores the extra asterisks.
For Example:

/**********
* Welcome To Java Sunderesan Copyright (C)2016 King . All Rights Reserved
*/

Write a program for a String must contain three digit using String.format()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

package com.javasunderesan; public class StringThreeDigit { public static void main(String[] args) { try { int no = 0; String formattingString = null; for (int i = 0; i < 10; i++) { formattingString = String.format("Dev - %03d", no); System.out.println(formattingString); no++; } } catch(Exception e) { e.printStackTrace(); } } }

Output

Dev -  000
Dev -  001
Dev -  002
Dev -  003
Dev -  004
Dev -  005
Dev -  006
Dev -  007
Dev -  008
Dev -  009

Write a Program to Add spaces before and After a String using String.format()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

package com.javasunderesan; public class AddSpaceBeforeAndAfterString { public static void main(String[] args) { try { String msg = "java"; System.out.println(String.format("%8s", msg)); // adding spaces // before // string ; System.out.println(String.format("%-8s", msg));// adding spaces // after // string // 8 is a number of characters String and spaces } catch (Exception e) { e.printStackTrace(); } } }

Output::

    java
java    

Java - String Format

String Format

Description:

String.format() method helps us to format the given string.
It replaces each format item in a specified string with the text equivalent of a corresponding object's value.

---

Syntax:
Here is the syntax of this method:

public static String format(String format, Object... args)

---

Parameters:

format : format of the string.
args : arguments for the format string. It may be zero or more.

---

Return Value

formatted string

---

String.format() Example Program
Write a Program to Add spaces before and After a String using String.format()
Write a program for a String must contain three digit using String.format()

How to do split the String using more than one delimiter

Split the String using more than one delimiter

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

package com.javasunderesan; public class SplitStringDelimitters { public static void main(String[] args) { String str = "name Java Sunderesan field computer engineering grade 9.98"; try { String[] splits = str.split("name |field |grade "); for (int i = 1; i < splits.length; i++) { System.out.println(splits[i]); } } catch (Exception e) { e.printStackTrace(); } } }

Output::
Java Sunderesan
computer engineering
9.98

---

Java - String split() Method

Description:

This method has two variants and splits this string around matches of the given regular expression.

---

Syntax:
Here is the syntax of this method:

public String[] split(String regex, int limit)
or
public String[] split(String regex)

---

Parameter

regex : regular expression to be applied on string.

limit : limit for the number of strings in array. If it is zero, it will returns all the strings matching regex.Parameter

---

Returns

array of strings

---

Throws

PatternSyntaxException if pattern for regular expression is invalid

---

How to do Convert the date from one format to another date object in another form Program

convert-the-date-from-one-format-to-another-date-object-in-another-form Program

   package com.javasunderesan;

import java.text.SimpleDateFormat;
import java.util.Date;

public class ConvertDateObject {

 public static void main(String[] args) {
  SimpleDateFormat ddmmyyyy = new SimpleDateFormat("dd/mm/yyyy");
  SimpleDateFormat datetimeFormatyyyymmdd = new SimpleDateFormat("yyyy-mm-dd");
  try {
   ConvertDateObject cdo = new ConvertDateObject();
   String fromDate = "26/05/2016";
   System.out
     .println("Converted Date Time::" + cdo.convertDateType(fromDate, ddmmyyyy, datetimeFormatyyyymmdd));

  } catch (Exception e) {
   e.printStackTrace();
  }

 }

 private String convertDateType(String date, SimpleDateFormat orgFormat, SimpleDateFormat targetFormat) {
  String formattedDate = null;
  try {
   Date calDate = orgFormat.parse(date);
   formattedDate = targetFormat.format(calDate); // 20120821
  } catch (Exception e) {
   e.printStackTrace();
  }
  return formattedDate;
 }

}

Output:: Converted Date Time::2016-05-26

parse() and format() method used to convert One date object to another date Object

public final String format(Date date)

Formats a Date into a date/time string.
Parameters:
date - the time value to be formatted into a time string.
Returns:
the formatted time string.

---

Parsing a Date

Parse()

---

How to calculate difference between two date String in Java

Parsing Date

public Date parse(String source) throws ParseException

Parses text from the beginning of the given string to produce a date. The method may not use the entire text of the given string.
See the parse(String, ParsePosition) method for more information on date parsing.
Parameters:
source - A String whose beginning should be parsed.
Returns:
A Date parsed from the string.
Throws: ParseException - if the beginning of the specified string cannot be parsed.

   package com.javasunderesan;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class CompareDateTime {

 public static void main(String[] args) {
  SimpleDateFormat sdf = new SimpleDateFormat("dd/mm/yyyy");  
  
  Date start = null, end = null;
  String fromDate = null;
  String toDate = null;
  try {
   CompareDateTime compareDate = new CompareDateTime();
   fromDate = "24/05/2016";
   toDate = "25/05/2016";
   start = sdf.parse(fromDate);
   end = sdf.parse(toDate);   
   System.out.println("Days Between " + start + " and " + end + ":::" +compareDate.daysBetween(start, end));
  } catch (ParseException e) {
    e.printStackTrace();
  }

 }

 private Long daysBetween(Date d1, Date d2) {
  return (Long) ((d2.getTime() - d1.getTime()) / (1000 * 60 * 60 * 24));
 }
}

output: Days Between Sun Jan 24 00:05:00 IST 2016 and Mon Jan 25 00:05:00 IST 2016:::1

public static void main(string args) in java

Let's see what is the meaning of class, public, static, void, main, String[], System.out.println().

• class keyword is used to declare a class in java.
• public keyword is an access modifier which represents visibility, it means it is visible to all.
• static is a keyword, if we declare any method as static, it is known as static method. The core advantage of static method is that there is no need to create object to invoke the static method. The main method is executed by the JVM, so it doesn't require to create object to invoke the main method. So it saves memory.
• void is the return type of the method, it means it doesn't return any value.
  main represents startup of the program.
• String[] args is used for command line argument. We will learn it later.
• System.out.println() is used print statement. We will learn about the internal working of System.out.println statement later.

    package com.javasunderesan;

public class MainMethodExample {

 public static void main(String[] args) {
  System.out.println("Welcome to http://javasunderesan.blogspot.in/");
 }

}

---

public static void main(string args) interview Questions

• Why main method is static?

  To execute main method without creating object then the main method oshould be static so that JVM will call main method by using class name itself.

  ---

• Why main method return type is void?

  main method wont return anything to JVM so its return type is void.

  ---

• Why the name main?

  This is the name which is configured in the JVM.

  ---

• Why the arguments String[] args?

  Command line arguments. String array variable name args : by the naming conventions they named like that we can write any valid variable name.

  ---

• Can we write static public void main(String [] args)??

  Yes we can define main method like static public void main(String[] args){}
  Order of modifiers we can change.

  ---

• What are the possible public static void main(String [] args)method declaration with String[] args?

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

  Instead of args can place valid java identifier.

  ---

• Overloading main method

  • We can declare multiple main methods with same name as per overloading concept.
  • So we can overload main method but every time JVM looks for main method with String[] args method.
  • Lets see an example program on main method overloading.
  • main(String[] args) method will be called automatically by JVM.
  • If we want to call other methods we need to call explicitly.

      package com.javasunderesan;
  
  public class OverloadingMainMethod {
  
   public static void main(String[] args) {
  
    System.out.println("Java Sunderesan - Main method String [] args");
   }
   public static void main(int[] args) {
    System.out.println("Java Sunderesan - Main method int[] args");
   }
  
  }
  

  ---

JRE,JVM and JDK

Java Virtual Machine

The Java Virtual Machine (JVM) is an execution environment for Java applications.
It interprets compiled Java binary code ( called bytecode) to enable a computer's processor to carry out a Java Program's instructions.Java was designed to allow application programs to be built that could be run on any platform without having to be rewritten or recompiled by the programmer for each separate platform. The Java Virtual Machine makes this possible.

• The JVM is a main component of Java architecture, and is a part of the JRE(Java Runtime Environment).
• The JVM is operating system-dependent. In other words, the JVM must translate the bytecode into machine language, and the machine language depends on which operating system is being used, which makes the JVM platform-dependent.
• The JVM is responsible for allocating memory needed by the Java program.

Basically, the Java compiler reads Java language source (.java) files, translates the source into Java bytecodes, and places the bytecodes into class (.class) files.
The class files can then be executed on the Java Virtual Machine(JVM)

The JVM performs following main tasks:

• Loads code
• Verifies code
• Executes code
• Provides runtime environment

Every device that has a JVM installed is able to translate and run your .class files.
This makes it possible to run the same bytecode on different platforms.

---

JRE

JRE is an acronym for Java Runtime Environment.It is used to provide runtime environment.It is the implementation of JVM. It physically exists. It contains set of libraries + other files that JVM uses at runtime.

Implementation of JVMs are also actively released by other companies besides Sun Micro Systems.

---

Java Development Kit(JDK)

The Java Development Kit(JDK) is a software development environment for java applications and applets. It includes

• The Java Compiler(javac)
• The Java Archiving Tool(jar)
• The Java Debugging Tool(jdb)
• A complete Java Runtime Environment(JRE), for running Java programs.

After installing the JDK, you will be able to run javac,jar and jdb from the console(Command Prompt in Windows).

JDK is available for free at www.oracle.com under Java SDKs and Tools-> Java SE.

What is Keywords in Java?

Java Keywords

* Keywords are predefined identifiers available directly throughout the JVM.

* They have a special meaning inside java source code and outside of comments and strings.

* For Example : public, static, void

Q1) What are different types of access modifiers in Java?

Ans) There are four different types of modifiers:

Modifier	Accessible in the same package	Accessible in different package
Private	No	Yes, only if the class extends the main class
Protected	Yes	Yes, only if the class extends the main class
Default	Yes	No
Public	Yes	Yes

---

Q2) What is the use of final keyword?

Ans) The final keyword can be assigned to

1. Class level variable
2. method
3. class
4. Objects

If a final is assigned to a variable, the variable behaves as a constant. It means that the value of variable once set cannot be changed.

final int i=1;
i =5; // error

If a final is assigned to a method then it cannot be overridden in its child class.

1 2 3 4 5 6 7 8 9 10 11 12 13

package com.javasunderesan; class Parent { final void print() { System.out.println("Inside"); } } class Child extends Parent { public final void print() {// error cannot override final method System.out.println("Inside"); } }

If a class is made as final, then no other class can extend it and make it as parent class. E.g. String Class.

Final objects are instantiated only once. i.e
final Map map = new HashMap();
map.put("key","value");
map = new HashMap();  // error

---

Q3) What is use of throws keyword?

Ans) The Java throws keyword is used to declare an exception. It gives an information to the programmer that there may occur an exception so it is better for the programmer to provide the exception handling code so that normal flow can be maintained. Exception Handling is mainly used to handle the checked exceptions. If there occurs any unchecked exception such as NullPointerException, it is programmers fault that he is not performing check up before the code being used.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

package com.javasunderesan; import java.io.IOException; public class ThrowsKeyword { void callTestThrow_1() throws IOException { throw new IOException("device error");// checked exception } void callTestThrow() throws IOException { callTestThrow_1(); } void test() { try { callTestThrow(); } catch (Exception e) { System.out.println("exception handled"); } } public static void main(String args[]) { ThrowsKeyword obj = new ThrowsKeyword(); obj.test(); System.out.println("normal flow..."); } }

Output
exception handled
normal flow...

---

Q4) Why static methods cannot access non static variables or methods?

Ans) A static method cannot access non static variables or methods because static methods doesn't need the object to be accessed. So if a static method has non static variables or non static methods which has instantiated variables they will no be initialized since the object is not created and this could result in an error.

---

Q5) What is a static method?

Ans)A method defined as static is called static method. A static method can be accessed without creating the objects. Just by using the Class name the method can be accessed. Static method can only access static variables and not local or global non-static variables. For eg:

1 2 3 4 5 6 7 8 9 10 11 12 13 14

package com.javasunderesan; public class StaticMethodExample { public static void main(String args[]) { MainClass.printMe(); } } class MainClass { public static void printMe() { System.out.println("Welcome to JavaSunderesan.blogspot.in"); } }

OutPut:
Hello World

Also static method can call only static methods and not non static methods. But non-static methods can call static mehtods.

---

Is Possible to print message without using system.out.println?

Is possible to print message without using system.out.println?

Yes. It is possible to print message without using system.out.println.

package com.javasunderesan;

import java.io.FileDescriptor;
import java.io.FileOutputStream;
import java.io.PrintStream;
public class PrintMessage {
  public static void main(String[] args) {
    // https://javasunderesan.blogspot.in/
    // Display message without System.out.println
    try {
      String message = "javasunderesan - Learn Java\n";
      System.out.write(message.getBytes());
      System.out.format("%s", message);
      PrintStream myout = new PrintStream(new FileOutputStream(FileDescriptor.out));
      myout.print(message);
      System.err.print(message);
      System.console().writer().println(message);
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

Coding Standards and Naming conventions

Why Coding Standards?

• A program is written once , but read many times

  • During debugging
  • When adding to the program
  • When updating the program
  • When trying to understand the program

• Anything that makes a program readable and understandable saves lots of time , even in the shot run.

Naming Conventions

• Naming a class

  • Class name should be in title case , means Every word first letter should be capital letter.
  • This is also known as UpperCamelCase.
  • Choose descriptive and simple names.
  • Try to avoid acronyms and abbreviations.

  • Naming Class Example

    package com.javasunderesan
    public JavaNamingVariableDemo{
    }

• Naming a Variable

  • Variable name should be start with small letter.
  • Use lowerCamelCase.
  • Example: name, mobileNumber, age.
  • For final variable all its letters should be capital and words must be connected with "_".
  • Example: MAX_COUNT, MIN_COUNT.

  • Naming Variable Example

    package com.javasunderesan
    public JavaNamingVariableDemo{
    String firstName;
    String lastName;
    int mobileNumber;
    final static int MIN_WORDS=300;
    }

• Naming a Method

  • Method name should be verb because it represents action.
  • Use lowerCamelCase for naming methods
  • Example: setFirstName(), getFirstName(), getLastName().

  • Naming Variable Example

    package com.javasunderesan
    public JavaNamingMethodDemo{
    String firstName;
    String lastName;
    int mobileNumber;
    final static int MIN_WORDS=300;
    public void getFirstName()
    {
    return firstName;
    }
    public void setFirstName(String firstName)
    {
    this.firstName = firstName;
    }
    }

• Naming a Package

  • All letters should be small.
  • like in java io, util, applet
  • Example: com.javasunderesan

  • Naming Variable Example

    package com.javasunderesan
    public JavaNamingMethodDemo{
    }