#32 Array of Objects in Java

Summary

In this video, the focus is on enhancing understanding of arrays in programming. It begins with a discussion on the necessity of arrays and their creation, then delves into more complex operations such as dynamically printing array values without hardcoding the size. The instructor demonstrates the default initialization of integer arrays and the potential for runtime errors, like index out of bounds exceptions. A solution is presented using the 'length' property to avoid such errors. The tutorial progresses to creating an array of objects, specifically 'Student' objects, and iterating over them to print details. The video concludes with a preview of upcoming topics, promising to address the challenge of printing object values directly.

Takeaways

• 😀 Arrays are essential for storing multiple values in a single variable.
• 🔍 The length of an array can be dynamic, and it's crucial to know the array's length to avoid errors.
• 👀 Default values for integer arrays are zero, which is important to remember when elements are not explicitly initialized.
• ⚠️ Accessing an array index that is out of bounds will result in a runtime exception.
• 🛠️ The 'length' property of an array can be used to safely iterate over its elements without causing errors.
• 🎓 Demonstrated the creation of a 'Student' class with instance variables like roll number, name, and marks.
• 📚 Showed how to create multiple 'Student' objects and assign them to an array to manage a collection of students.
• 🔑 Explained that arrays do not create objects themselves; objects must be instantiated separately and then added to the array.
• 🖨️ Discussed how to print the details of objects stored in an array, emphasizing the need to access object properties individually.
• 🔄 Illustrated the process of iterating over an array of 'Student' objects and printing their details using a loop.

Q & A

What happens if you define an array with a size larger than the number of assigned values?

If you define an array with a size larger than the assigned values, the unassigned indexes will be filled with default values. For an integer array, the default value is zero.

What is an exception in the context of arrays?

An exception is a runtime error that occurs when you try to access an array index that is out of bounds. For example, if the array has six elements and you try to access the seventh, an exception will be thrown.

How can the length property of an array help avoid errors?

The length property can help by ensuring you only access valid indexes within the array. By using the array's length property, you can iterate over the array without exceeding its bounds and causing exceptions.

How do you create an array of objects, such as students, in this example?

To create an array of objects, you define an array of the object type (e.g., `Student[] students = new Student[3];`). Then, you manually assign objects to the array's indexes, such as `students[0] = S1;`.

What is the difference between creating an array of objects and creating the objects themselves?

Creating an array of objects only allocates space for the object references, not the objects themselves. You need to manually create the objects and assign them to the array.

Why does printing an object directly result in an address-like value?

When printing an object directly without overriding the `toString()` method, Java prints the memory reference (address) of the object rather than its values. This is why you need to explicitly access object fields like `S1.name` to print meaningful information.

How can you dynamically print the values of multiple student objects in an array?

You can loop through the array using a `for` loop, accessing each object's fields dynamically. For example, `students[i].name` and `students[i].marks` will print the name and marks of each student as the loop iterates.

Why is it necessary to use object fields like `.name` and `.marks` when printing student data?

Without explicitly accessing these fields, Java will print the object's memory reference instead of its actual data. Using `object.field` ensures that you retrieve and print the desired values stored in the object.

Can arrays hold different data types, such as integers, strings, and objects?

Yes, arrays can hold different data types, but they must be homogeneous. For example, you can have an array of integers, an array of strings, or an array of objects like `Student`. However, you cannot mix data types in a single array.

What is the purpose of using arrays in Java programming?

Arrays provide a way to store multiple values of the same type in a single variable, making it easier to manage and process large amounts of data. They allow for more efficient storage and retrieval compared to individual variables.

Outlines

00:00

📚 Introduction to Arrays and Handling Exceptions

This paragraph introduces the concept of arrays, explaining their necessity and how to create them. It discusses the importance of knowing the array's length to avoid errors such as 'index out of bounds' exceptions. The speaker demonstrates the default value assignment in arrays (e.g., zero for integers) and suggests using the 'length' property to safely iterate through arrays without causing runtime errors. The paragraph also touches on the idea of creating an array of objects, specifically 'Student' objects, and initializes them with properties like roll number, name, and marks.

05:01

👨‍🎓 Creating and Manipulating Arrays of Student Objects

The second paragraph delves into the creation of an array to hold 'Student' objects. It explains the process of manually creating 'Student' objects and then assigning them to an array of 'Student' references. The paragraph clarifies that the array itself does not create the objects but is used to store references to them. It also demonstrates how to iterate through an array of 'Student' objects to print their details, such as name and marks, using a loop. The speaker highlights the importance of using the array's 'length' property to control the loop and avoid indexing errors. The paragraph concludes with a practical example of printing student details from the array, emphasizing the use of object properties within the loop.

Keywords

💡Array

An array is a data structure that can hold a fixed-size sequential collection of elements of the same type. In the video, arrays are used to store multiple values such as integers or objects. The script discusses creating arrays and the importance of knowing their length to avoid errors like 'out of bounds' exceptions. For example, the video mentions creating an array of integers with a certain length and then assigning values to it.

💡Length

The 'length' property of an array refers to the number of elements it contains. In the video, the instructor explains how using the 'length' property can help in avoiding errors when accessing array elements. It is used to ensure that the loop iterating over the array does not exceed the number of elements present, thus preventing runtime exceptions.

💡Index

An index is used to access the elements of an array. The video script mentions that array indexes start at zero and end at the length of the array minus one. It warns about the potential for errors when accessing indexes that do not exist, such as trying to access an index beyond the last element of the array.

💡Exception

An exception in programming refers to an event that occurs during the execution of a program that disrupts the normal flow of the program's instructions. The video script uses the term 'exception' to describe runtime errors that occur when trying to access an array index that is out of bounds, which is not caught at compile time.

💡Object

An object in programming is an instance of a class that contains data in the form of fields (often known as attributes or properties) and code, in the form of procedures (often known as methods). In the video, objects of a 'Student' class are created and stored in an array, demonstrating how arrays can be used to manage collections of objects.

💡Class

A class is a blueprint for creating objects. It defines the properties and methods that the created objects will have. The video script introduces a 'Student' class with properties like roll number, name, and marks, and then creates objects of this class to be stored in an array.

💡Instance Variable

Instance variables in a class are variables whose values are unique to each instance of the class. The video script mentions that roll number, name, and marks are instance variables of the 'Student' class because they belong to each individual student object and not to a method.

💡Reference

A reference in the context of objects in programming is a way to access the object in memory. The video script explains that when objects are stored in an array, the array does not contain the actual objects but rather references to them. This is important for understanding how arrays of objects work and how memory is managed.

💡Loop

A loop is a programming construct that allows code to be executed repeatedly based on a given boolean condition. The video script discusses using a loop to iterate over the elements of an array, using the array's length to control the loop and ensure that it does not go out of bounds.

💡Concatenation

Concatenation is the process of joining two or more strings end-to-end. In the video, the script mentions concatenating strings to print out the details of a student object, such as name and marks, instead of just printing the object reference, which would not be meaningful to the user.

Highlights

Introduction to the necessity and creation of arrays.

Demonstration of how to print array values.

Discussion on the default values of an integer array.

Exploration of potential errors when the array size is not correctly managed.

Explanation of the 'length' property of arrays to avoid index errors.

Introduction to the concept of exceptions as runtime errors.

Practical example of creating an array with a variable size.

Transition from integer arrays to creating arrays of objects, specifically 'Student' objects.

Step-by-step guide to creating multiple 'Student' objects with instance variables.

Creation of an array to hold references to 'Student' objects.

Clarification on the difference between creating an array and creating objects.

Advantages of using arrays to manage and print student details.

Technique to print student details using array indexing and object properties.

Discussion on the limitations of printing objects directly and the need for property access.

Conclusion and预告 of the next video's content.

Transcripts

so once we have talked about array I

mean basically we have talked about why

do we need array and how to create an

array let's do something more with that

in now in fact in the previous video we

have seen how to create the array and

then how to print the values now in this

video let's try to see if we can find a

better way of printing this value see

what is happening now is let's say I

mean of course we know the length of

this array here right which is four uh

it can also be five or any numbers I

mentioned before right it can be 100 200

or whatever size you want to mention and

you can insert the value now before I do

anything else I just want to show you

one more thing what if the value of the

size of array is 6 and you are assigning

only four values here now in that case

the other indexes will also have some

values right let's see I just want to

see what are those values before we

continue

so compile and run you can see we got

the values but then the last two values

are zero remember we have talked about

this by default the values for the array

or integer array will be zero

and that's what is happening Here and

Now what I want to do is I want to uh

see if I can get this value see the

thing is why I should know the length of

the array it might also lead to errors

example if I say 7 here by mistake and

if I try to fetch the values see this is

where it will give you

an exception now till this point we have

not talked about exception uh we have a

separate topic what are exceptions at

this point just remember exceptions are

runtime errors okay so the errors which

comes in runtime at compile time there

is no problem when you compile this code

it will not uh blame you it will not

shout but the moment you've done this

code it will say hey something is wrong

here so that's exception now what is the

problem the problem is we have six

values index number star starts with

zero it ends at five not at six and here

it says index six is out of bounds so

basically we are trying to go beyond the

limit okay so how do we solve this

problem so instead of specifying the

number hardware values we can do one

thing you know this arrays they have a

property called length so we can simply

use this length as a variable or a

property which will show which will tell

us what's the actual length of the so

that you will not make any mistake the

only thing you have to remember is you

have to put that you have to make sure

that it's less than not greater than

okay and I will I will clear this I will

say compile and run you can see we got

the values and there's no problem the

only thing is the size is 6 and then we

are printing only we are assigning only

four values let me go back to four

values and you can see we got four

values here now this is one of the good

addition in terms of the property which

you can use so that's about length here

now what we can do is uh let's say if I

want to have a string array or maybe I

let me create a either of student can we

do that uh so remember one thing so if I

create a student here and of course I

can create one object I can get multiple

objects here and let's say I have a

integer I have a student which has some

values so let's say integer roll number

okay and the string name and int marks

of course not a good way to uh maintain

the records we don't want to maintain

marks but just for the example okay so

we have student and then we have these

three variables okay now mind you these

are the instance variable because they

belong to a class not to a method I just

want to comment the entire section here

and what I will do is I will create

student object so let me create the

first object I will say student S1 is

equal to new student and I want to

assign all the values to the student so

I will say S1 dot roll number which is 1

and S1 dot name which is let's say

Naveen and S1 dot marks which is let's

say 88 okay so we got the first object

now maybe I want to create more objects

what I can do is I can just copy this

code and paste and let's say three

objects so we want three objects here

and I will say this is two this is three

and let's say well number two let's say

this is hush and marks let's say 67 and

if I say this is 3 let's say this is

kitten and let's say the marks is

97. okay so we got these Marks here

right and then basically they are the

objects now I I made one mistake here if

you can I don't know if you have

observed but if you can see I'm still

assigning the values to S1 don't you

think it should be S2 S3 uh so it should

be S2 is 2 and S2 and there here we have

to say S3 S3 and S3 so basically we got

three objects here now what I will do is

I want to create an array of students

right we have created the array of

integers but then how do we do that

array of students it's very simple you

say student and then you have to say

students because we are creating

multiple students here that's a variable

name equal to now this students here is

not one variable it's an array so I have

to say array and then we have to say new

now in this case when you say we have

INT in the same way here it should be

student

right student array of let's say three

students will be having so all these

objects you know we got three objects

here S1 S2 and S3 they will become part

of this array example I can say student

students of 0 is equal to S1 so the

first object the first reference is

getting stored here in the first index

and likewise we have this is S1 S2 and

this is S2 S3 so you can see we got

three objects in the array so of course

you need these objects otherwise this

will not work now if you are thinking at

this point we are creating three strand

object no we are not getting three

student object most of the people have

this confusion that on this line we are

creating three student object now we are

creating an array which can hold student

references it will not create those

object by itself

you have to manually create the objects

and assign to an array now this thing

will make much more sense when we start

working with databases when you fetch

data about a student it will give you a

lot of data you can basically fetch

those data in a student array now what's

the advantage the advantages let's say

if you want to print all the details of

a student so what you can simply do is

you can use an array which will start

with 0 and I is less than equal to okay

I is less than students dot length which

we have seen and then I plus plus and

then here we can simply print but how do

you print the values so what we'll do is

you have to print one value at a time

you have to say S1 because if you try to

fetch the value of a student it will

give you a address format okay example

let me show you something at this point

uh let me just comment this section and

let me print the student object itself

and let's see what happens so if I try

to print the S1 value

you tell me what will happen what the

what will be the output

I mean think about that I will just

compile this and run you can see it is

not printing the value of a student it

is printing something else see in the

upcoming videos we'll solve this problem

at this point it's printing the address

and we don't know what this is so what

we can do is explicitly you have to

mention S1 dot name and you have to

maybe you can do a concatenation here

and you can say S1 dot marks I just I

don't want to print the roll number at

this point so I'm printing on you this

too and now if you compile and run you

can see we got the student details okay

so can we do it here so let me just

uncomment this section the only thing is

you have to print this particular

statement cut

and paste now the only problem is it

will always refer to S1 we don't want to

refer to S1 right the student value will

keep changing so I can say students

square bracket I right so this value

will keep changing right this will be S1

S2 S3 in every iteration the same can be

done here

so I can say students of I that should

make sense and let's see if this works

let me go back here compile

and run you can see we got the values

right so basically we can work with

normal objects as well uh before this we

have worked with integer of course you

can work with string as well but we just

went uh one step ahead by creating an

array of students okay so I hope uh you

got some idea regarding how do we get an

array of students and yes in the next

video we'll try to do something else