Questions and Answers
What does an interface do?
It defines common behaviors for classes.
Can abstract classes be used to create objects?
False
How do you denote an abstract method?
Using the 'abstract' modifier in the method header.
How are abstract methods and abstract classes denoted in UML?
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If a class contains any abstract methods, must it be defined as abstract?
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Abstract methods must be implemented in the subclass?
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How are abstract methods defined?
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What happens if you don't implement all the abstract methods in a subclass?
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Can you create an abstract class that contains non-abstract methods?
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Can a subclass be abstract even if the superclass is concrete?
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Since you cannot create an instance of an abstract class, can you use the class as a datatype?
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An abstract method must be non-static?
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What's the definition of static?
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What is an interface?
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What modifier is used to indicate an interface?
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Can you create an instance of an interface using the new operator?
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What is the syntax for implementing an interface?
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When a class implements an interface how are the methods used in the subclass?
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What does the Comparable interface do?
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How many interfaces can a class implement?
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Which package is the Comparable interface in?
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What is a marker interface?
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A class can extend only 1 object but can implement many interfaces.
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What class is the clone() method in?
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When can the object.clone method be accessed?
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Study Notes
Java Interfaces & Abstract Classes
- An interface defines common behaviors for classes, containing only constants and abstract methods.
- Instantiation of abstract classes is not possible using the
new
operator; they cannot be instantiated. - Abstract methods are denoted by the
abstract
modifier in their method header. - In UML, both abstract methods and abstract classes are represented in italics.
- Any class that contains abstract methods must be declared as abstract.
- Abstract methods must be implemented in the subclass; failure to do so results in the subclass becoming abstract.
- Abstract methods allow subclasses to provide specific implementations, as they are left open-ended.
- An abstract class can contain non-abstract methods alongside abstract methods.
- A subclass can be abstract even if its superclass is concrete.
- Although you can't instantiate an abstract class, it can still be used as a datatype, for example:
abstractClass[] ac = new abstractClass[];
. - Abstract methods must be non-static; static methods indicate some level of functionality.
- The term static refers to a class-level attribute or method that does not require an instance of the class.
- Syntax to define an interface includes
public interface InterfaceName { }
. - You cannot instantiate an interface using the
new
operator. - To implement an interface, the syntax used is
public class ClassName extends AnotherClass implements InterfaceName { }
. - A class that implements an interface must override all methods in the subclass with the exact same method signatures.
- The Comparable interface is utilized for defining the
compareTo
method that allows for object comparison. - A single class can implement one or more interfaces, but it can only extend one class.
- The Comparable interface is part of the
java.lang
package. - A marker interface is an empty interface that serves to signify certain properties; for instance, Cloneable is a marker interface.
- A class can extend only one superclass but can implement multiple interfaces simultaneously.
- The
clone()
method is found in the Object class. - The
object.clone
method is accessible only when the class implements the Cloneable interface.
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