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TheObject
class sits at the top of the class hierarchy tree in the Java platform. Every class in the Java system is a descendent, direct or indirect, of the
Object
class. This class defines the basic state and behavior that all objects must have, such as the ability to compare oneself to another object, to convert to a string, to wait on a condition variable, to notify other objects that a condition variable has changed, and to return the class of the object.Your classes may want to override the following
Object
methods. Theequals
/hashCode
are listed together as they must be overridden together.Your class cannot override these
clone
equals
/hashCode
finalize
toString
Object
methods (they are final):
getClass
notify
notifyAll
wait
clone
MethodYou use theclone
method to create an object from an existing object. To create a clone, you write:aCloneableObject.clone();Object
's implementation of this method checks to see if the object on whichclone
was invoked implements theCloneable
interface, and throws aCloneNotSupportedException
if it does not. Note thatObject
itself does not implementCloneable
, so subclasses ofObject
that don't explicitly implement the interface are not cloneable.If the object on which
clone
was invoked does implement theCloneable
interface,Object
's implementation of theclone
method creates an object of the same type as the original object and initializes the new object's member variables to have the same values as the original object's corresponding member variables.The simplest way to make your class cloneable then, is to add implements Cloneable to your class's declaration. For some classes the default behavior of
Object
'sclone
method works just fine. Other classes need to overrideclone
to get correct behavior.Consider the
Stack
class, which contains a member variable that refers to aVector
. IfStack
relies onObject
's implementation ofclone
, then the original stack and its clone will refer to the same vector. Changing one stack will change the other, which is undesirable behavior.Here then is an appropriate implementation of
clone
for ourStack
class, which clones the vector to ensure that the original stack and its clone do not refer to the same vector (changes are indicated with a change in font):The implementation forpublic class Stack implements Cloneable { private Vector items; // code for Stack's methods and constructor not shown protected Object clone() { try { Stack s = (Stack)super.clone(); // clone the stack s.items = (Vector)items.clone(); // clone the vector return s; // return the clone } catch (CloneNotSupportedException e) { // this shouldn't happen because Stack is Cloneable throw new InternalError(); } } }Stack
'sclone
method is relatively simple: It callssuper.clone
, whichStack
inherits fromObject
and which creates and initializes an instance of the correct type. At this point, the original stack and its clone refer to the same vector. Next the method clones the vector.Be careful:
clone
should never usenew
to create the clone and should not call constructors. Instead, the method should callsuper.clone
, which creates an object of the correct type and allows the hierarchy of superclasses to perform the copying necessary to get a proper clone.
equals
and hashCode
MethodsYou must override theequals
andhashCode
methods together.The
equals
method compares two objects for equality and returnstrue
if they are equal. Theequals
method provided in theObject
class uses the identity function to determine if objects are equal (if the objects compared are the exact same object the method returnstrue
).However, for some classes, two distinct objects of that type might be considered equal if they contain the same information. Consider this code that tests two
Integer
s,one
andanotherOne
, for equality:This program displaysInteger one = new Integer(1), anotherOne = new Integer(1); if (one.equals(anotherOne)) System.out.println("objects are equal");objects are equal
even thoughone
andanotherOne
reference two distinct objects. They are considered equal because the objects compared contain the same integer value.Your classes should only override the
equals
method if the identity function is not appropriate for your class. If you overrideequals
, then overridehashCode
as well.The value returned by
hashCode
is anint
that maps an object into a bucket in a hash table. An object must always produce the same hash code. However, objects can share hash codes (they aren't necessarily unique). Writing a "correct" hashing function is easy--always return the same hash code for the same object. Writing an "efficient" hashing function, one that provides a sufficient distribution of objects over the buckets, is difficult and is out of the scope of the tutorial.Even so, the hashing function for some classes is relatively obvious. For example, an obvious hash code for an
Integer
object is its integer value. For an example of a class that overrides theequals
andhashCode
methods, see theBingoBall
class in BINGO!
.
finalize
MethodTheObject
class provides a method,finalize
, that cleans up an object before it is garbage collected. This method's role during garbage collection was discussed previously in Cleaning Up Unused Objects. Thefinalize
method is called automatically by the system and most classes you write do not need to override it. So you can generally ignore this method.
toString
MethodObject
'stoString
method returns aString
representation of the object. You can usetoString
along withSystem.out.println
to display a text representation of an object, such as the current thread:TheSystem.out.println(Thread.currentThread().toString());String
representation for an object depends entirely on the object. TheString
representation of anInteger
object is the integer value displayed as text. TheString
representation of aThread
object contains various attributes about the thread, such as its name and priority. For example, the previous line of code displays the following output:TheThread[main,5,main]toString
method is very useful for debugging. It behooves you to override this method in all your classes.
getClass
MethodThegetClass
method is a final method that returns a runtime representation of the class of an object. This method returns aClass
object.Once you have a
Class
object you can query it for various information about the class, such as its name, its superclass, and the names of the interfaces that it implements. The following method gets and displays the class name of an object:One handy use of avoid PrintClassName(Object obj) { System.out.println("The Object's class is " + obj.getClass().getName()); }Class
object is to create a new instance of a class without knowing what the class is at compile time. The following sample method creates a new instance of the same class asobj
, which can be any class that inherits fromObject
(which means that it could be any class):Object createNewInstanceOf(Object obj) { return obj.getClass().newInstance(); }
Note: . Take particular note of the section titled Synchronizing Threads.
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