Questions and Answers
Study Notes
Objects
- An object has a unique identity, state, and behaviors.
- The state of an object consists of a set of data fields (also known as properties) with their current values.
- The behavior of an object is defined by a set of methods.
Classes and Objects
- A class is a template that defines the properties and behavior of an object.
- Multiple objects can be created from a single class.
- Each object has its own state (data fields with values) but shares the same behavior (methods) as other objects of the same class.
Example: Person Class
- A
Person
class has a data field calledname
and a method calledgetName
. - Multiple objects can be created from the
Person
class, each with its ownname
value (e.g.,O1
withname
"Ahmad",O2
withname
"Zaki", etc.).
Characteristics of Objects
- An object represents a real-world entity that can be distinctly identified (e.g., a student, a desk, a circle, a button, a loan).
- An object has both a state (data fields with values) and behavior (methods).
Garbage Collection
- When an object is no longer referenced, it is considered garbage.
- The JVM automatically collects garbage.
Assigning null to a Reference Variable
- If an object is no longer needed, you can explicitly assign null to a reference variable for the object.
- This helps in garbage collection.
Variables of Primitive Data Types and Object Types
- Primitive type variables hold values (e.g., int i = 1).
- Object type variables hold references to objects (e.g., Circle c = new Circle()).
- Reference variables hold the memory address of an object.
Copying Variables of Primitive Data Types and Object Types
- Primitive type assignment (e.g., i = j) copies the value.
- Object type assignment (e.g., c1 = c2) copies the reference, not the object.
- After object type assignment, both variables point to the same object.
Garbage Collection Example
- After the assignment statement c1 = c2, the object previously referenced by c1 is no longer referenced and becomes garbage.
Garbage Collection
- When an object is no longer referenced, it is considered garbage.
- The JVM automatically collects garbage.
Assigning null to a Reference Variable
- If an object is no longer needed, you can explicitly assign null to a reference variable for the object.
- This helps in garbage collection.
Variables of Primitive Data Types and Object Types
- Primitive type variables hold values (e.g., int i = 1).
- Object type variables hold references to objects (e.g., Circle c = new Circle()).
- Reference variables hold the memory address of an object.
Copying Variables of Primitive Data Types and Object Types
- Primitive type assignment (e.g., i = j) copies the value.
- Object type assignment (e.g., c1 = c2) copies the reference, not the object.
- After object type assignment, both variables point to the same object.
Garbage Collection Example
- After the assignment statement c1 = c2, the object previously referenced by c1 is no longer referenced and becomes garbage.
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Description
Learn about the fundamentals of object-oriented programming, including objects, classes, state, and behavior. Understand how classes define properties and behavior, and how multiple objects can be created from a single class.