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Questions and Answers
What is an anonymous object in terms of Java object creation?
What is an anonymous object in terms of Java object creation?
- An object that retains a specific reference.
- An object that cannot be accessed after creation.
- An object created and not assigned to a variable. (correct)
- An object created without a class definition.
Which statement correctly describes an instance variable?
Which statement correctly describes an instance variable?
- It refers to methods that belong to all instances equally.
- It is static and shared across all instances of a class.
- It is a data field associated with a specific instance of a class. (correct)
- It can be accessed without creating an object.
How is a method invoked on an object in Java?
How is a method invoked on an object in Java?
- Through a constructor call.
- By simply writing the method name.
- Using the object member access operator. (correct)
- Using square brackets.
What happens when a reference type data field is not referencing any object?
What happens when a reference type data field is not referencing any object?
Which of the following is true about instance methods?
Which of the following is true about instance methods?
What is the purpose of the dot operator in Java?
What is the purpose of the dot operator in Java?
What would be the output of System.out.println('Area is ' + new Circle(5).getArea()) if getArea returns the area of a Circle?
What would be the output of System.out.println('Area is ' + new Circle(5).getArea()) if getArea returns the area of a Circle?
What does the 'new' keyword indicate when used in new Circle();?
What does the 'new' keyword indicate when used in new Circle();?
What is the typical order of operations when creating and using an object in OOP?
What is the typical order of operations when creating and using an object in OOP?
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Study Notes
Object Creation and Constructors
- A constructor is a method used to initialize an object, invoked with the
newoperator. - Constructors must share the same name as their class and do not have a return type, not even void.
- Overloading is allowed, enabling multiple constructors with different parameters within the same class.
- A default constructor is provided by Java if no constructors are explicitly defined, allowing instantiation without arguments.
Accessing Objects
- Objects are accessed using reference variables, which hold the memory address of the object instead of the object itself.
- The dot (.) operator is used to access the object’s data fields and methods, denoting member access.
- A class is a reference type, meaning a variable of that class can reference an instance; e.g.,
Circle myCircle;defines a reference variable for aCircleobject.
Object vs. Reference Variable
- An object reference variable points to an object in memory, while the object is the actual instance created.
- It is common to refer to a reference variable as if it were the object for simplicity.
Arrays as Objects
- In Java, arrays are treated as objects created using the
newoperator. - An array variable holds a reference to the created array object, similar to other object types.
Accessing Data Fields and Methods
- Data fields and methods of an object are accessed using the dot operator:
- For data fields:
objectRefVar.dataField - For methods:
objectRefVar.method(arguments)
- For data fields:
- Example:
myCircle.radiusaccesses theradiusfield, whilemyCircle.getArea()invokes thegetAreamethod.
Instance Variables and Methods
- Instance variables depend on the specific object instance, while instance methods are invoked on that object.
- The object on which an instance method is called is termed the calling object.
Anonymous Objects
- Objects can be created without variable assignment, known as anonymous objects.
- Example:
System.out.println("Area is " + new Circle(5).getArea());creates and immediately uses aCircleobject without referencing it.
Reference Data Fields and Null Values
- Data fields can also be of reference types, like
String, which is a predefined Java class. - If a reference field contains no object reference, it holds the value
null, indicating the absence of an object.
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