Instance variables in Java - Step-by-Step Execution

Concept Flow - Instance variables

Create Object

↓

Allocate Memory for Instance Variables

↓

Assign Default or Given Values

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Use Instance Variables in Methods

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Object Accesses Its Own Data

When an object is created, it gets its own copy of instance variables stored in memory, which methods can use and change.

Execution Sample

Java

class Car {
  String color; // instance variable
  void paint(String newColor) {
    color = newColor;
  }
}
Car myCar = new Car();
myCar.paint("red");

This code creates a Car object, then changes its color instance variable to "red" using a method.

Execution Table

Step	Action	Instance Variable 'color'	Output/Notes
1	Create Car object myCar	null (default)	Object created, color is null by default
2	Call myCar.paint("red")	Set color to "red"	color changed to red
3	Access myCar.color	"red"	color is now red
4	End	"red"	No more actions

💡 No more code to execute, instance variable 'color' holds "red"

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
myCar.color	undefined	null	"red"	"red"

Key Moments - 2 Insights

Why is the instance variable 'color' null before calling paint()?

Because when the object is created (see step 1 in execution_table), instance variables get default values (null for String) until assigned.

Does changing 'color' inside paint() affect all Car objects?

No, it only changes the 'color' of the specific object calling paint(), as instance variables belong to each object separately.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what happens to 'color'?

A It remains null

B It is set to "red"

C It is deleted

D It becomes an integer

💡 Hint

Check the 'Instance Variable color' column at step 2 in execution_table

At which step does 'color' first get a non-null value?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Instance Variable color' column in execution_table

If we create another Car object without calling paint(), what is its 'color' value?

A null

B "default"

C "red"

D empty string ""

💡 Hint

Instance variables get default values on object creation as shown in step 1

Concept Snapshot

Instance variables belong to objects.
Each object has its own copy.
They get default values when object is created.
Methods can change them.
Changes affect only that object.

Full Transcript

Instance variables are variables that belong to each object created from a class. When you create an object, Java allocates memory for these variables and sets them to default values like null for Strings. You can change these variables by calling methods on the object. Each object keeps its own copy, so changing one object's instance variable does not affect others. For example, creating a Car object sets its color to null. Calling paint("red") on it changes its color to red. This change is stored inside that object only.