Creating Immutable Custom Class in Java

 A frequent Java interview question is : 

How to create a Class which is immutable? 

Like any other question, the answer begins with the understandig what do we really mean by the underlying concept - in this case : immutablity !!! 

Making a class immutable entails ensuring two solemn contracts : 

1. Class is closed for extension i.e it cannot be inherited. (why is that necessary? ans at bottom)
2. Once the object is created, the state of fields cannot chage till its existence in JVM.

In other words, A variable/class of an immutable type may only be changed by re-assigning to that variable. When we wish to only modify some portion of an immutable, we are compelled to reassign the whole object.

Let's take Student class as an example. 

This class can be made Immutable in following way (observe the Student class carefully) : 

KEY TAKEAWAYS : 

1. finalize class 
2. set class mebers as "private" and "final" 
3. Assign new object to instance variable by copying all values 
4. remove all setters() 
5. return duplicate(new object) in getters() instead of actual instance variable

Finally answer to the question : Why do we need to declare an Immutable class final? 

Well the question is quite debatable. 

• Is a Java Immutable Class Always final?
• Why would one declare an immutable class final in Java
• java.math.BigInteger class is example, it's values are immutable but it is not final.

There are several reasons why we need to declare a class as final to make it immutable.

1. To prevent subclasses from overriding methods and changing the state of the object.
• If a class is not final, a subclass could override a method and change the state of the object. This would violate the principle of immutability, which states that the state of an object cannot be changed once it is created.
2. To ensure that the object is thread-safe.
• Immutable objects are thread-safe because they cannot be changed by multiple threads at the same time. This is important for objects that are shared between multiple threads, such as objects in a cache.
3. To improve performance.
• The JVM can make some optimizations for immutable objects, such as caching the hash code of the object. This can improve the performance of applications that use immutable objects.
4. To make the code more readable and easier to understand.
• Immutable objects are easier to reason about because their state cannot change. This makes the code more readable and easier to understand.

~ Happy Coding ~

Working with Multimedia(Video/Audio) files in Java : Case Study JAVE library

              

Pictorial Guide to understand Access Modifier in Java

 

Access Modifiers in Java has been explained beautifully in blogs like this. 

• Controlling Access to Members of a Class | The Java™ Tutorials
• Access Modifiers in Java | Baeldung

But this blog aims to leave a pictorial memory in reader's mind. 

Usually, it is represented in tabular form like this ... 

But I wanted to present even more practical schematic diagram. 

Private

Default (aka package-private)

Protected

Public

 

Demistifying the enigma of negative numbers in programming

I came across this linkedin blog [How are negative numbers stored in binary?] [How the negative numbers are stored in memory? ] . These blogs provoked a related question : Why are 2's complement necessary at first place and why such systems are also not used for the quotidian Decimal system ? 

Negative numbers has alwyas been difficult to undertand reason being its vicarious nature - when your friend lends you green dollars you can touch it, feel it, grasp it, but when he asks back you cannot really "see" any money, unless you press hard on your memory. The negative number are particularly recondite when it comes to how computer deals with this "negativity". 

Let's revisit DECIMAL SYSTEM :

* only symbols, we call digits, permitted are : 0 to 9 (i.e. Ten symbols) 

* we can represent all WHOLE Numbers (0,1,2,3...∞) using only these 10 symbols and their infinite jumbling combinations. (read Permutaions & Combinations). 

At this point, Primates had no idea about what "-54" meant ? They logged informations like "The bald Primate owes 50 rice grains to the hirsute Primate ". But as much water has flowed under the bridge some primate suggested why not replace "owed" with yet another symbol with, say, "-".

* Therefore, inorder to represent INTEGERS (-∞,...-3,-2,-1,0,1,2,3 ...) we agreed to use an additional symbol "-"

So, the Decimal system(0-9) is easy because we have made it easier by borrowing extra symbol of (-)ve.  

Before we strike the pith of  the blog title, bear me to answer this qustion : 

How could we have written/represented negative numbers if you were not permitted to use an extra symbol like "-" ? 

Let me rephrase question : "How will you write -7 without using (-) symbol in decimal system?" [Constraint: you can use any digit (0-9) for maximum 4 times as in 8764]

(why I have limited to 4 digits in this excercise? Because unlike when wrting on paper, in computer the size is limited based on data type - for example, an integer is stored in 32 bits)

Propsed Solution: 

We have to find B such that A + B = 0 

A = 7 = 0007 (represented as 4 digit) 

Step I : now take 9's complement* = 9999-0007 = 9992 [DEFINITION: 9's complement of A = highest number possible - A]

Step II: Add 1 : 9993 (This is B i.e. -7) [10's complement of A]

Lets verify : A + B = 0007 + 9993 = 1 0000 = 0000 (discard the leading 1 because we can only use 4 digits)

Agreeing to Agree on somethings is at the cornerstone of any system we devise to ease our transactions. What we have implictly agreed above is that if the left most symbol will be of highest value out of 10 symbols(i.e. 9) then it will represent negative number. And if the left most symbol is of the lowest value out of 10 symbols (i.e 0) then it will represent positive number. 

Then obvious counter question will be : how can we represent a number greater than 8999 , say 9000 using above agreement? Won't this 9000 be actually representing : positive 1000 

Let A = 1000

 I. 9999 - 1000 = 8999 (9's complement of 1000)

II. 8999 + 1    = 9000 (10's complement of 1000)

The answer is no, 9000 = -1000. Because for 4 spaces we cannot represent (positive) number greater than 8999. Why? Because of our prior tacit agreement about TRADEOFF between RANGE & SIGN. 

either, you can represent 10 thousand WHOLE Numbers from 0000 to 9999

or, you can represent 999 negative integers(9001to999) & 999 positive integers(0001to0999) and one zero (9000 or 0000). Total numbers = 1999 The leading number will act as sign indicator. Such drastic fall in range because one space, the leftmost space, which could have hold 10 information is used to present only two infromation (+/-). 

Now, Enter the world of BINARY SYSTEM : 

* only two symbols: 0 and 1 are permitted.

In the Binary system, at the machine level we dont have luxury of adding (-ve) symbol. Everything is black(0) & white(1) - either 0 or 1. (unless you are using quantum computer). 

So early programmers have devised an igenious way(already explained above) to overcome this scarcity. 

A = 7 is represented in binary (8-bit) as  0000 1101

Step I : 1's complement of A : (highest no possible - A) = 1111 1111 - 0000 1101 = 1111 0010

Step II: 2's complement of A :   1's complement + 1 = 1111 0010 +1 = 1111 0011

Verify : 0000 1101 + 1111 0011 =  1 0000 0000 = 0000 0000 (leading 1 will be discarded for 8-bit)

PS: the range loss in binary system is much lesser compared to decimal system . Hence it made sense to use above representation in binary systems. 

Capacity reducation for 4 spaces if we store negative numbers : 

1. Decimal system : 10,000 --> 2000 (1000 postive , 1000 negative) : 5x loss

2. Binary system :  16 (0000 to 1111) --> 15(1111/-7 to 0 to 0111/+7) - hardly any loss

Thanks. 

Google Sheet + Apps Script : Day03 (Removes blank Cells in a column consisting of hyperlinked strings)

 

Turns a spaced column into a compressed one :

 ==>

function removeEmptyCellsInColumn() {

  var ss = SpreadsheetApp.getActiveSpreadsheet();

  var sheet = ss.getSheets()[0];

    var start=130;

    var last=300;

    var i=start;

    var j=i+1;

    var rangeCut,rangePaste;

    var a_i;

    var a_j =sheet.getRange(j,2).getValue()

    while(j<last){

        if(a_j !== "") {// if a[j] isNOTempty //a[i+1] =a[j];

          rangePaste = sheet.getRange(i+1,2); //pulls up next val in  next contiguous empty col

          rangeCut =sheet.getRange(j,2);

          a_i =sheet.getRange(i,2).getValue();

          

          var richValue = SpreadsheetApp.newRichTextValue()

                        .setText(a_j)

                        .setLinkUrl(sheet.getRange(j,2).getRichTextValue().getLinkUrl())

                        .build();

          rangePaste.setRichTextValue(richValue); //paste at a[i]

          i++;

          rangeCut.clear();//clears or cuts the a[j]

        }

        j++; 

        a_j =sheet.getRange(j,2).getValue();

      }

}

Google Sheet + Apps Script : Day02(Removes/Blanks Cell containing Digit/Number)

 

This code helps remove all numbers in 3rd column of third sheet in a SpreadSheet.

function numberRemoverFromColumn() {

  var ss = SpreadsheetApp.getActiveSpreadsheet();

  var sheet = ss.getSheets()[2];

  //  var sheet = ss.getSheetByName("SheetName"); 

 

 var row=1;

  var startRowAt=2; //starting row

  var endRowAt=2192;//last row

  for ( row=startRowAt; row<endRowAt; row++ ) {

    var range = sheet.getRange(row,3);

    var presentCellValue =range.getValues(); //=values[0][0];

      if(presentCellValue%2===0 || presentCellValue%2===1){

        Logger.log(presentCellValue+" will be blanked now");

        var richValue = SpreadsheetApp.newRichTextValue()

                        .setText("")

                        .build();

        range.setRichTextValue(richValue)

      }

  }

}

Google Sheet + Apps Script : Day1

Created a program to change each strings containing cells of first column into hyperlinks redirecting to itself : 

function selfRefrencingHyperlinkMaker() {

  var ss = SpreadsheetApp.getActiveSpreadsheet();

  var sheet = ss.getSheets()[1];

  //  var sheet = ss.getSheetByName("SheetName"); 

  var row;

  var startRowAt=2;

  var endRowAt=150;

  for ( row =startRowAt; row<endRowAt; row++ ) {

    var range = sheet.getRange(row,1);

    var values = range.getValues();

      if(values[0][0].length >0){

        var richValue = SpreadsheetApp.newRichTextValue()

                        .setText(values)

                        .setLinkUrl("#gid="+sheet.getSheetId()+"&range=A"+row)

                        .build();

        range.setRichTextValue(richValue)

      }

  }

}