Object

Question 1

The Object Class

Answer 1

 The mother of all classes (quite literally!)
⚫ Not just a placeholder… Provides core functionality
⚫ Any class defined without an explicit superclass implicitly extends Object. Every single Java class therefore has Object as a superclass.
⚫ Any methods and instance variables defined in Object are therefore present in all classes…
⚫ Override them to provide useful tailored behaviour in your classes.
⚫ Examples include:
 toString()
 equals()
 clone()
 finalize()
 hashCode()
 Concurrency primitives (wait/notify) – See SCC211

Question 2

toString()

Answer 2

 Returns a humanly readable serialization of the object instance.

 The format and content of the returned String is totally unconstrained by Java, is application specific, and left to the programmer.
 Normally, a textual representation of the key instance variables held within the instance is provided.

 Default behaviour: A textual representation of the unique ID used within the Java Virtual Machine to identify this object instance…
 …not very useful for application programmers or end users!

Question 3

equals()

Answer 3

 Tests if the instance the method is called on and that provided as aparameter are, by the programmers definition, equivalent.

 Remember all instances are referred to by Object References in Java.
 So, if we have two different instances of a class, but their instance variables
have the same value, are they the same? The answer is subjective…

 Default behaviour: reference equality. As tested by the “==“ operation. Two object instances are considered equal if and only if they reside at the same place in memory.

Question 4

Equality and Equivalence 1

Answer 4

 Reference equality has positive benefits, in that this guarantees uniqueness. However, there are many different types of equivalence
relationship…

 Equivalence relations are implemented in Java using standard methods.
 The equals method provides this functionality. Programmers can use
method overriding to create an equals method for their classes that implements their ‘preferred’ equivalence definition for that class:

Question 5

Equality and Equivalence 2

Answer 5

Question 6

finalize()

Answer 6

e that timing is non-deterministic –GC does not necessarily happen at
the same time as an object becomes eligible for garbage collection…
 Default behaviour: No operation
 Finalize() is deprecated in the Java API since 9.0
⚫ Notoriously difficult to implement safely, and no real need for it….
⚫ So don’t use it! ☺

Question 7

hashcode()

Answer 7

 Provides a hash value for the object
 As used by the Collections API (HashMap and HashSet)
 Objects that are equal() must return the same hashcode.
 Hashcodes should be as distinct as possible, but not all objects are required
to have a unique hashcode.

 Default behaviour: returns the memory address for the object as used by the underlying implementation of the Java Virtual Machine.

 Always override the hashcode() method if you override equals()

Question 8

clone() 1

Answer 8

 Creates and returns a duplicate copy of the current object
instance.
 A new instance is created, and a reference to that new instance is returned
 The new instance is of the same type as the original, and has the same value

Question 9

clone() 2

Answer 9

 Clone and Equality… By contract, the new instance is created such that:
p == p.clone() false
p.equals(p.clone()) true or false !!
▪ However, cloning is not necessarily transient…
▪ What if an object being cloned refers to another object?

Question 10

The Blue Pill or the Red Pill

Answer 10

 We have a choice…
 Do we clone just the first object instance, and simply take a duplicate of any
object references contained therein?
The story ends – you believe what you want to believe…
 Or do we also clone any object instances the first object is referencing?
You see how deep the rabbit hole goes…

Question 11

The Blue Pill: Shallow Copy

Answer 11

 Duplicate only the first object
⚫ Primitive values are duplicated as values.
⚫ Object references are duplicated as values.
⚫ Relatively lightweight process
⚫ Preserves identity and semantics of any referenced instances
⚫ Default Behaviour: The clone() method in Object implements a shallow copy

Question 12

The Red Pill: Deep Copy

Answer 12

 Duplicate all references to objects
⚫ Primitive values are duplicated as values.
⚫ Object references are cloned.
⚫ Potentially expensive process, as number of object being cloned is unbounded!
⚫ Totally decouples cloned instance from the original, but can affect semantics
⚫ Risk of infinite cloning, if references are circular! (e.g. if bestMate is symmetric!)

Question 13

implementing clone()

Answer 13

 The clone() is by default disabled on new classes
⚫ Implement the Cloneable interface to allow your objects to be cloned:
⚫ n.b. This is purely permissive. Cloneable has no methods!
⚫ Shallow Copy: No other action is necessary
⚫ Deep Copy: Override the clone() method, and clone any (or all) of the object
references we are holding as instance variables.
Class Person implements Cloneable
Class Person implements Cloneable
{
public Object clone()
{
Person that = super.clone();
that.bestMate = that.bestMate.clone();
return that;
}
}
n.b. It is the programmer’s decision which (if any) related objects should
be cloned. This should be informed by your OO design… (c.f. SCC204!)