#32 Array of Objects in Java
Summary
TLDRIn 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
📚 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.
👨🎓 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.
Mindmap
Keywords
💡Array
💡Length
💡Index
💡Exception
💡Object
💡Class
💡Instance Variable
💡Reference
💡Loop
💡Concatenation
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
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