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In this lecture, we’ll explore what references are [br]and how they relate to objects in Java.

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All variables hold a single and only a single value.

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You should be familiar with the 8 primitive types such as byte and boolean.

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Each type reserves a fixed amount of memory to hold each [br]time that you create that type of variable

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For example, a byte is 8 bits so each time you create [br]a byte, Java reserves 8 bits of space in memory.

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A reference, however, is a type that refers to any value that is not a primitive.

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It's a convenient way to label data that refers to a single instance of [br]a class, interface, array, enum or annotation.

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For example, when you create a String, you create [br]an object that is a String type in memory.

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Then if you create a String variable and assign it the value of the string,

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you’re actually creating a reference to that String object.

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In Java, when you create a variable that refers to a class,

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you create a bond between that variable and the class type.

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This creates a strong reference bond between [br]that object and the reference variable.

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That reference doesn’t “refer” directly to the object, but to its location in memory.

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This is an important aspect of reference variables that I’ll highlight more.

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The variable points to the memory location and not the object itself.

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The variable follows the lifecycle of the [br]object from creation to garbage collection.

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You should note that objects, which are typically class types,

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can have properties that are other class types and those [br]classes may have properties that refer to other classes.

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For each of these references, the object created must allocate [br]memory for all of the properties (primitive or reference type).

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You can imagine how complex the task would be if [br]you had to manage the memory allocation yourself.

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Thankfully, Java handles memory management for you.

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One of the effects of references that Java supports [br]is the ability to pass variables by value.

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This means that when you pass a reference type to a method, [br]that method has access to the data and can change it.

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It does not have access to the original reference [br]itself, and cannot change it to point elsewhere.

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This makes Java different than languages like C++

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where pointers are supported and variables are pass-by-reference instead of pass-by-value.

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In pass-by-reference, the location in memory of where the value [br]is stored is passed as opposed to a copy of the value itself.

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Furthermore, in Java, pass-by-value means that [br]the value is copied to a new location.

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This is why you can alter a value within a method and [br]it won’t affect the variable outside of the method.

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Let’s take a look at another example.

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The following is a simple declaration statement.

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Here we’re creating a reference variable that will point to a type specified by ClassName.

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Moving on, when you create an object from a class,

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typically by using the ‘new’ keyword,

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then Java allocates the amount of memory that [br]the object requires to store the object.

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The code snippet here creates an object of type Sample.

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It also creates a variable named ‘ob’ that refers to a Sample type

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and is associated with the object that was created by the same line of code.

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Remember, in Java, the variable is assigned a [br]reference to the object and not the object itself.

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This reference is the address of the memory location where the object is stored.

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We can also create multiple references to the same object.

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For example, the following added line creates a reference variable [br]‘r’ that is associated with the instance of Sample.

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Here, both variables, ‘ob’ and ‘r’, refer to the same Sample instance.

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Now let’s talk about objects briefly. [br]An entity that has state and behavior and exists in memory is known as an object.

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An object is considered an instance of a class, a living, breathing copy of it.

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So how do objects and references relate?

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Well, an object is an instance of a class.

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A reference describes[br]the memory location of where an object is stored.

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Thus, a reference variable may be associated with a single memory [br]address of where an object of the indicated type exists.

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One final note before we end this lesson is that a reference variable does not have to match exactly the type of object that it is associated with.

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If you have learned about Inheritance, then you know that a [br]class can extend another which creates a parent-child relationship.

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The child inherits properties and behavior of the parent.

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This creates an “IS-A” relationship.

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Child classes are not the only constructs that form IS-A relationships.

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A class that implements an interface also forms an IS-A relationship with that interface.

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So, what does this all mean?

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It means that a reference variable can be the child or parent type [br]of the memory location of the object that pairs with it.

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Let’s review again, how an object, a class, and a reference relate to one another.

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A class is a template used to instantiate objects.

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It’s also called a ‘type’ in some circumstances, [br]such as when used with a reference variable.

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An ‘object’ is an instance of a class in memory.

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It cannot be accessed directly; only through a ‘reference’.

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A ‘reference variable’ is a variable that stores the reference to an object in memory.

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In this lesson, we covered how a reference is a type of variable that stores the memory address of where an object is located.

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It isn’t the object itself, but the doorway to use to access the object.

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Reference variable types do not have to exactly [br]match the type of the referenced object.

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It can coincide with an IS-A relationship. [br]

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Thanks again for tuning in.

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I hope that you found this lesson helpful in understanding [br]reference variables and that you’ll watch more of our videos.