OCJA(1Z0 - 808) || String Objects Creation Heap and String Constant Pool (SCP) Part - 1

Summary

This video explores key differences between creating String objects in Java using `new String()` and string literals. It explains that using `new String()` creates two objects—one in the heap and another in the String Constant Pool (SCP)—while a string literal creates only one in the SCP. The video also delves into JVM memory management, garbage collection, and how objects in the SCP are reused for efficiency. The session clarifies how the heap and SCP handle string objects differently, ensuring memory optimization and object reuse.

Takeaways

• 📜 String object creation depends on the use of the 'new' keyword, and it leads to different memory allocation behaviors.
• 🏗️ When 'new String()' is used, two objects are created: one in the heap and one in the String Constant Pool (SCP).
• 💡 In contrast, without 'new', only one object is created in the SCP, as JVM reuses the object if it already exists.
• 🗂️ The purpose of the SCP is to allow the JVM to reuse objects, improving memory efficiency.
• 🗑️ Objects created in the SCP are not eligible for garbage collection since JVM keeps an implicit reference to them.
• ⚠️ Until Java version 1.6, the SCP was part of the method area or permgen space, but from Java 1.7 onwards, it moved to the heap for better memory management.
• 🔄 In cases where string literals are already present in the SCP, JVM reuses them instead of creating new objects, enhancing efficiency.
• ⚙️ When runtime operations modify strings, new objects are always created in the heap, not the SCP.
• ❌ Objects without explicit reference in the heap are eligible for garbage collection, unlike those in the SCP.
• 💾 Using 'new' ensures a new object is created in the heap even if an identical object exists in the SCP, avoiding reuse in the heap.

Q & A

What is the difference between using 'string s = new string("druga")' and 'string s = "druga"'?

Using 'string s = new string("druga")' creates a new object in the heap memory each time, while 'string s = "druga"' utilizes the string literal pool in the string constant pool (SCP), reusing the same object for the same literal, thus saving memory.

Why is it said that using the 'new' operator is dangerous in the context of strings?

Using the 'new' operator for strings can lead to unnecessary object creation in the heap, which might not be garbage collected immediately due to implicit references maintained by the JVM, leading to increased memory usage.

What is the role of the string constant pool (SCP) in Java?

The string constant pool (SCP), also known as the string intern pool, stores literals to ensure that each string literal is unique in memory, thus saving space and improving memory utilization.

How does the JVM handle string literals?

The JVM checks if a string literal already exists in the SCP before creating a new object. If it exists, it reuses the existing object; if not, it creates a new object and adds it to the SCP for future use.

What happens to the object created with 'new string("druga")' in the SCP?

When a new object is created using 'new string("druga")', a copy is also maintained in the SCP for future references, but the original object in the heap is not eligible for garbage collection due to implicit references by the JVM.

Why are objects created with string literals not immediately eligible for garbage collection?

Objects created with string literals are not immediately eligible for garbage collection because the JVM maintains an implicit reference to them in the SCP, ensuring they are available for future use.

What is the impact of Java version on how the string constant pool is managed?

Prior to Java 7, the SCP was part of the permanent generation in the heap. From Java 7 onwards, the SCP is part of the heap itself, allowing it to expand and contract as needed for efficient memory utilization.

Can you provide an example of how many objects are created with 'string s1 = new string("druga"); string s2 = "druga";'?

In this example, three objects are created: one in the heap for 's1', one in the SCP for the literal 'druga', and another in the heap for 's2' if it's a new object not found in the SCP.

What is the significance of the string pool in memory management?

The string pool is significant in memory management as it prevents the creation of duplicate string objects for identical literals, thus conserving memory and enhancing performance.

How does the JVM decide whether to reuse an object from the SCP or create a new one?

The JVM checks if the content of the string literal already exists in the SCP. If it does, it reuses the existing object; if not, it creates a new object and adds it to the SCP for potential future reuse.

What is the difference between the heap area and the string constant pool (SCP) in terms of object creation?

The heap area always creates a new object when using the 'new' operator, whereas the SCP may reuse existing objects for identical string literals, thus optimizing memory usage.

Outlines

00:00

🔍 Understanding String Object Creation and the 'New' Operator

This paragraph delves into the differences between creating string objects using the 'new' keyword and direct string assignment in Java. It explains that when using 'new String()', two objects are created—one in the heap memory and one in the String Constant Pool (SCP). In contrast, when directly assigning a string, only one object is created in the SCP. The paragraph emphasizes the JVM’s role in reusing existing SCP objects for optimization and avoiding duplicate object creation.

05:01

🧠 JVM's String Reuse Mechanism in SCP

This section continues the discussion on string object creation but focuses on how the JVM checks if an identical object already exists in the SCP before creating a new one. If an object with the same content is found, it is reused instead of creating a new one. Additionally, it highlights that string literals always create objects in the SCP and that objects created via 'new' will exist in the heap memory, with SCP used for future reuse.

10:03

🔄 Memory Allocation in Heap vs SCP with Multiple Strings

The paragraph uses multiple string creation examples to explain how the heap and SCP store string objects. It illustrates that creating strings with 'new' leads to new heap objects, while direct string assignment results in objects being checked for reuse in the SCP. The JVM optimizes memory by ensuring no duplicate objects exist in the SCP, but duplicate heap objects can exist when 'new' is used repeatedly with the same string content.

15:04

💡 Detailed Example of String Object Creation and Reuse

The final paragraph further explains string object creation through an example where multiple strings are created with the same content. It shows that each 'new String()' creates separate objects in the heap, but SCP objects are reused. It also touches on garbage collection, stating that objects without references in the heap are eligible for garbage collection, while SCP objects are maintained for future reuse by the JVM. The paragraph concludes by encouraging readers to continue learning through examples to fully grasp the heap and SCP concepts.

Keywords

💡String Literal

A string literal is a sequence of characters enclosed in double quotes, like 'Durga' in the script. In the context of the video, string literals are created in a special area of memory called the String Constant Pool (SCP), allowing them to be reused for efficiency. The concept of string literals ties into memory management in Java, where the system optimizes the storage of strings to avoid duplication.

💡Heap Memory

Heap memory is the area where new objects in Java are stored when the 'new' keyword is used. In the script, it's explained that when 'String s = new String('Durga')' is executed, a new object is created in heap memory. Heap memory is important for understanding how memory allocation occurs dynamically at runtime and how garbage collection works in Java.

💡String Constant Pool (SCP)

The String Constant Pool (SCP) is a special area in memory where string literals are stored for reuse. When a string is created using a literal, it is stored in the SCP to avoid duplicating the same string. In the video, it's mentioned that when a string literal like 'Durga' is used, it is stored in SCP for future use, improving memory efficiency.

💡Garbage Collection

Garbage collection is the process by which Java automatically reclaims memory that is no longer referenced by the program. In the script, it's explained that objects created in heap memory but not referenced anymore are eligible for garbage collection. However, objects in the SCP are not eligible because they are implicitly referenced by the JVM for reuse.

💡New Operator

The 'new' operator in Java is used to create a new object in heap memory. The script repeatedly mentions the 'new' operator when explaining how new strings are created in heap memory, as opposed to reusing strings from the SCP. Understanding the distinction between creating new objects and using literals is key to understanding Java's memory model.

💡Immutability

Immutability refers to the property of an object whose state cannot be changed after it is created. Strings in Java are immutable, which means that once a string is created, its value cannot be modified. In the video, immutability is critical to understanding why new objects must be created when modifications to strings occur, as demonstrated by operations like concatenation.

💡Memory Management

Memory management in Java involves the allocation, usage, and deallocation of memory during program execution. The script provides a detailed explanation of how memory is managed for strings in both the heap and SCP. Efficient memory management ensures that the same string is not stored multiple times unnecessarily, thus optimizing memory usage.

💡Runtime Operation

Runtime operations refer to actions performed by a program during its execution, such as object creation or string manipulation. In the video, runtime operations like string concatenation lead to the creation of new objects in heap memory. The script explains that even if strings are immutable, new objects are created during runtime operations, emphasizing the importance of understanding the difference between compile-time and runtime behaviors.

💡Reference Variable

A reference variable is used to store the address of an object in memory. In Java, when a string is created, a reference variable points to its memory location. In the video, the script discusses how strings created in heap memory have explicit reference variables, while those in the SCP have implicit references maintained by the JVM, ensuring they are not collected by the garbage collector.

💡Efficiency

Efficiency in this context refers to how Java optimizes memory usage, particularly with strings. The script explains that using the SCP for string literals enhances memory efficiency by allowing the reuse of the same object across multiple parts of a program. This concept is important for understanding why certain design decisions in Java are made, particularly when it comes to managing strings and reducing memory overhead.

Highlights

Understanding the difference between 'new String()' and string literals is crucial for Java memory management.

In the case of 'String s = new String("Durga")', two objects are created: one in the heap and one in the String Constant Pool (SCP).

When using the 'new' operator, a new object is always created in the heap, regardless of existing objects in the SCP.

For string literals like 'String s = "Durga"', only one object is created in the SCP if it doesn't already exist.

The SCP is used for memory optimization, ensuring that identical string literals are reused rather than recreated.

From Java 1.7 onwards, the SCP is moved to the heap for more efficient memory utilization.

Objects in the SCP are not eligible for garbage collection because they are implicitly referenced by the JVM.

Using 'new String()' forces the creation of a new object in the heap, even if the string already exists in the SCP.

Without the 'new' keyword, the JVM first checks the SCP for existing objects with the same content and reuses them if available.

When a string is created using 'new', two objects may exist with the same content—one in the heap and one in the SCP.

Strings are immutable in Java, so any modification results in the creation of a new object in the heap.

Runtime operations that modify a string do not affect objects in the SCP, only creating new objects in the heap.

The JVM manages memory efficiently by maintaining implicit references to objects in the SCP, ensuring they aren't collected by the garbage collector.

For every string literal, the JVM creates only one object in the SCP for reuse across the program.

Understanding the interplay between the heap and SCP is key to managing memory effectively and avoiding unnecessary object creation in Java.

Transcripts

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you

in the last two videos we covered about

immutability mutability equals method

what is the difference between sting and

sing before okay up to that clear now

the next case is very important case

right sir if I can't a string yes is

equal to new string of juga string yes

he's equal to new string of druga next

string yes is equal to du --ga what EDA

difference between these two things can

you please tell the most dangerous point

string s is equal to new sting of druga

string s is equal to draaga what is the

difference between these two things

friends the most dangerous concept next

da almost up another discussion based on

this example only you require two lesion

very very carefully sir can you please

tell what the difference between these

two things

swinging a silicon using of druga

stingiest is equal to druga remember

that in the posture case okay in the

posture case two objects will be created

but in the second case only one object

will be created remember that in the

posture case two objects will be created

but in the second case only one object

will be created how we read are very the

difference it's not like do you know

whenever we are using new operator

whenever we are using new operator

compulsory universe it will be created

in the hip area okay Veta contain two

Duga and ax you see the reference

variable for this forgetting

inter-country

Duga you see the reference variable for

this next our new string of druga new

string of droga sorry it's a little

bright for you first string the literal

for the future purpose okay one object

will be created in SCP area forgetting

what is that yes

P string a constant a pool remember this

one what it is yes it is a string a

constant the pool okay you new object

will be created in the ACP area survey D

the reference variable no explicit

reference variable but internally

implicit reference variable will be

maintained by the JVM can immediately

Mosca serve I did the deed of this one

the need of this one is reusing same

object for future purpose are getting so

two objects will be created one is in

the hip area

yes he's pointing to that heap object

another copy will be created in ACP area

for the future purpose sir water future

purpose simple point you should have a

reuse operating in the next level

examples you can able to get okay well

then immediately Mosca sir this object

this object is available for garbage

collection not not no no because

implicitly implicitly reference variable

will be maintained by JVM if any object

created because of because I'm string

the constant Litoral Litoral Litoral

implicit reference variable will be

maintained by JVM this object is not

eligible for garbage collection remember

that but immediately you may ask her

suck this year CP v r TC area is there

until 1.6 version until 1.6 version yes

CP it apart

ah sir me that a year yeah method again

yeah yes if it apart of method area are

permit Jen are getting like permanent

permanent generation okay hit the bottom

method area are permit Jen okay like

buttah from one point seven version

onwards for efficient memory utilization

sir ACP area is moving to keep only okay

in the hip area some water reservoir for

SCP okay in the in the permian do you

know meter area yes CP zombies fixer

sizes but once it is more to the paper

no my ACP can

expandable regarding so just because of

memory utilization you can just stuff

from 1.7 version and word this SCP is

move order to okay apparently remember

but whoever Chan it is a part of me that

area but not inside hip area

okay well sub this is just for internal

sake but anyway you are not required to

worry so here how many objects you got

can you attend two objects got created

one is in the hip area second one in the

ACP yes he is always pointing to that

hip area right so quietly subject to

play Sarah this object up for future

purposeful what is that future purpose I

will explain in the future don't worry

about that okay well but in this case

string EEOC's equal to du gard easy

collateral or not yes it's a literal

it's an apron

something you know in this case in this

case only one object will be created in

the SCP year yeah in this case only one

object will be created in SCP year yeah

remember this concern in the ACP only

one object will be created you see the

reference point will do that but anyway

mistake JVM will check is there any

object the content with this content

already available in there CPR now if it

is already there if it is already there

then only then only so then it is going

to reuse the same object if the object

is not already there then only the new

object will be created and yes he's

pointing to that but anyway object at

creation in the hip area is always

mandatory whenever we are using new

operator compulsory a new object will be

created but uh if you are going to

create like this so object creation is

not mandatory pasta JVM wheelchair in

the SCP area do you have any object

within this content or not if the object

is already there ryuuta same object so

if the object is not already there then

only create a new object that's all so

what is the difference between this one

under this one UD scare total hominy

object you are created to object will be

created under in these case how many

objects will be created sir only one

object that

in the SCP area that's not no sir why it

is a second object placed in SCP because

the reason is for future purpose flour

we use reusable deep purpose look at

this object not having any reference

easy to people forgot my collection

aetna sorry it is not visible for

garbage collection because internal

difference will be maintained by JVM

itself clear sir in the last example

just we had a discussion about the keep

and ACP object creation let me continue

some more example so that you people can

get much cloud directive here observe

I'm taking string s1 is equal to new

string off Duga string s1 is you got you

still got broken string yes 2 is equal

to new string ah-oogah string s2 is you

continue single Turki next string yes

sir 3 is equal to-- daruka string s 3 is

equal to Trueba string ha yessuh 4 is

equal to 2 string s4 is equalto troopa

Tata

can you please tell just because of this

line how many objects would got created

yeah respond just because of this line

how many objects have got could get it

how many are there in the general heap

how many are there in the SCP yet done

please confirm total ammonium Jake's got

created is it only one object to object

R 3 R 4 R 5 are getting R 5 r 6 how many

how many will be there in the hip area

how many will be there in SCP area yes

if we want to be have some guess in your

mind so that you can able to understand

dancer very clearly if I start

explaining

can you please guess how many up there

in the hip area - how many are there in

their City area huh remember okay just

let me start explanation so that you

people can feel

more comfortable right here do you know

after first align deep area SCP area no

legend legend there are some conclusions

also I want to talk string s1 is equal

to new singer Drogo

whenever we are using new operator sorry

any website will be created in the hip

area

remember compulsory you new object will

be created in the hip area

that's why Durga Durga yes someone is

pointing to him next

it is a literal literal right for the

future purpose one copy will be

maintained in the ACP area

yes CP area right okay been sub just

took because a post line how many

objects got to get it to objects got

created one is in the hip area second

one in the ACP area no string s2 is

equal to neo stink after ooga singing

STDs you called me on string of Durga so

now whenever we are using new operator

come on sir can you object will be

created sir in the hip area can you

watch it will be created

yes - next ah this a copy has to place

in the ACP but in ACP Durga contain all

of the object is available or not yes if

it is already there we are not required

to place remember this one already care

we are not required to place okay second

line also completed trinka yes sir three

is equal to do god where I require to

create the subject

I told already in the last example where

I record a cricketer subject in the SCP

area but in ACP area object creation is

always optional posture JVM wheelchair

is there any object already already

pointing already not having the same

content or not if any object already

having the same content then existing

object will be reused it won't create a

new object that's why string yes a three

yes a three is also pointing to druga

string yes four is equal to druga yes up

for is also pointing to the graph

yes four is also fine filter Durga one

day now can you please confirm

total total sir how many objects got

created only three objects got created

one is in the ACP area two objects in

the heap area right so the conclusion is

whenever we are using new operator

compulsory you watch it will be created

in the hip area that's why there may be

a chance of existing two objects are

with the same content in the hip area

yes Isidro god raga sim kundan de ba

there is no chance of existing two

objects with the same content in the

ASAP area remember these ones are in the

ICP area with CM contain only one object

will be there same object will be reused

multiple times so internally memory will

be C over memory utilization will be

improve all right clear now you for

knowledge about ACP on the heap from

zero to twenty percent price I'm sure if

I will go for two more two or three more

example then you will get much clarity

don't require to worry to this clear so

the next example related to heap and ACP

I will upon sir string s is equal to new

string of druga string is equal to

string of draaga yes sir doctor can get

up software yes sir

is equal to your start can't get up

solutions okay well can you please tell

total how many objects will be created

in the hip area how many there sa P area

how many there

can you please yes friends can you

please tell

total how many objects will be created

how many present in the hip area how

many present in SCP area yes okay well

so compulsory you people should know

some answer in your mind okay maybe 3 4

2 4 or 6 for something like you should

have some clarity okay

now have a look once

period general heap and the SCP area

poster line string a city called new

string of Duga what this one not

required to keep any explanation even in

the last two examples also we covered

yes is equal to using of druga one copy

will be created in the hip area for the

future purpose another copy will be

created in SCP area

okay well Duga like this yes sir da can

get all software yes a dot can get our

software software like this I'm taking

some do you know yes sir that can get up

software here abjure software easy a

string literal software is a string a

little later but if restring a literal

one copy will be created in the string

concerned full because the name a we

observed string constant a pole

it is doing a constant string electro

means a string constant for a first

finger constant one copy will be created

in the SCP area that's why a faster

software will be created in the SCP area

ok software got created in ACP area okay

Ben next yes

do you know so yes sir doc can get up

software already what is yes yes it

druga container software

draaga software do you know so string is

immutable

you can't change the content if you are

trying to perform the change compulsory

new object will be created

do you know because of runtime operation

may be made that a call because of

runtime operation if an object is

required to create that option is always

going to create only in the hip area but

not in ACP remember this one sir

sir I call can get because of that

univox it will be created

runtime operation if an object God

created that the new object will be

created only in the hip area but not in

ESCP yes see Pimenta far

constants string literals ax not because

of from running operation created

objects so now yes Eric can get up

software

yes he's already dooga doo Ruger

software

yaniv object got created forgetting

durga software any website God created

right okay

for this a new object do you have any

reference variable no no no we are not

assigning to any reference variable

that's fine

this object eligible for got pay

collection okay anyway it is eligible

for GC next ah yes is equal to yes it

are can get our solutions is here

yes rot can get our solution solution

solutions are is a constant string

constant compulsory one copy will be

there in the SCP year yeah okay

solutions are easier contain easy easy

one copy will be there in the SCP area

because it's a string electron why for

every single a troll one copy will be

created in a city for the reusable D

purpose next time yes sir the arc can

get absolution because he is always

pointing to druga durga that target of

solutions

now Duga solutions will be created a new

object because of runtime operation if

an object is required to create that

object is always going to be placed only

in the area next uh DISA resulta

we are assigning to yes here I 70s now

onward this yes sir is pointing to this

object

regarding a 1 watt TCS is pointing to

this object if you print yes happily

your content through girl solutions

that's all can you please tell total how

many objects were created how many there

in the hip area how many there in the

SCP six up chicks got created because of

the score three present in the hip area

three present in the ASAP under these

are two objects eligible for garbage

collection only these objects not

eligible for garbage collection in the

heap anyway these are not eligible for

DC because implicit reference variables

will be maintained by J

YUM now are you getting clarity okay I

hope but just to take a bit special care

so for every example you are going to

learn some new thing maybe after one not

pre-exam 'pl yes all the things are

going to be completed then you will get

much clarity about this concept clear

for love read but keep on listening very

important

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