The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Dev.java for updated tutorials taking advantage of the latest releases.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
The
Object
class, in the java.lang
package, sits at the top of the class hierarchy tree. Every class is a descendant, direct or indirect, of the Object
class. Every class you use or write inherits the instance methods of Object
. You need not use any of these methods, but, if you choose to do so, you may need to override them with code that is specific to your class. The methods inherited from Object
that are discussed in this section are:
protected Object clone() throws CloneNotSupportedException
public boolean equals(Object obj)
protected void finalize() throws Throwable
public final Class getClass()
public int hashCode()
public String toString()
The notify
, notifyAll
, and wait
methods of Object
all play a part in synchronizing the activities of independently running threads in a program, which is discussed in a later
lesson
and won't be covered here. There are five of these methods:
public final void notify()
public final void notifyAll()
public final void wait()
public final void wait(long timeout)
public final void wait(long timeout, int nanos)
clone
method.
If a class, or one of its superclasses, implements the Cloneable
interface, you can use the clone()
method to create a copy from an existing object. To create a clone, you write:
aCloneableObject.clone();
Object
's implementation of this method checks to see whether the object on which clone()
was invoked implements the Cloneable
interface. If the object does not, the method throws a CloneNotSupportedException
exception. Exception handling will be covered in a later
lesson.
For the moment, you need to know that clone()
must be declared as
protected Object clone() throws CloneNotSupportedException
or:
public Object clone() throws CloneNotSupportedException
if you are going to write a clone()
method to override the one in Object
.
If the object on which clone()
was invoked does implement the Cloneable
interface, Object
's implementation of the clone()
method creates an object of the same class 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 is to add implements Cloneable
to your class's declaration. then your objects can invoke the clone()
method.
For some classes, the default behavior of Object
's clone()
method works just fine. If, however, an object contains a reference to an external object, say ObjExternal
, you may need to override clone()
to get correct behavior. Otherwise, a change in ObjExternal
made by one object will be visible in its clone also. This means that the original object and its clone are not independentto decouple them, you must override clone()
so that it clones the object and ObjExternal
. Then the original object references ObjExternal
and the clone references a clone of ObjExternal
, so that the object and its clone are truly independent.
The equals()
method compares two objects for equality and returns true
if they are equal. The equals()
method provided in the Object
class uses the identity operator (==
) to determine whether two objects are equal. For primitive data types, this gives the correct result. For objects, however, it does not. The equals()
method provided by Object
tests whether the object references are equalthat is, if the objects compared are the exact same object.
To test whether two objects are equal in the sense of equivalency (containing the same information), you must override the equals()
method. Here is an example of a Book
class that overrides equals()
:
public class Book { String ISBN; public String getISBN() { return ISBN; } public boolean equals(Object obj) { if (obj instanceof Book) return ISBN.equals((Book)obj.getISBN()); else return false; } }
Consider this code that tests two instances of the Book
class for equality:
// Swing Tutorial, 2nd edition Book firstBook = new Book("0201914670"); Book secondBook = new Book("0201914670"); if (firstBook.equals(secondBook)) { System.out.println("objects are equal"); } else { System.out.println("objects are not equal"); }
This program displays objects are equal
even though firstBook
and secondBook
reference two distinct objects. They are considered equal because the objects compared contain the same ISBN number.
You should always override the equals()
method if the identity operator is not appropriate for your class.
equals()
, you must override hashCode()
as well.
The Object
class provides a callback method, finalize()
, that may be invoked on an object when it becomes garbage. Object
's implementation of finalize()
does nothingyou can override finalize()
to do cleanup, such as freeing resources.
The finalize()
method may be called automatically by the system, but when it is called, or even if it is called, is uncertain. Therefore, don't rely on this method to do your cleanup for you. For example, if you don't close file descriptors in your code after performing I/O and you expect finalize()
to close them for you, you may run out of file descriptors. Instead, use a try
-with resources statement to automatically close your application's resources. See
The try-with-resources Statement and Finalization and Weak, Soft, and Phantom References in Java Platform, Standard Edition HotSpot Virtual Machine Garbage Collection Tuning Guide.
You cannot override getClass
.
The getClass()
method returns a Class
object, which has methods you can use to get information about the class, such as its name (getSimpleName()
), its superclass (getSuperclass()
), and the interfaces it implements (getInterfaces()
). For example, the following method gets and displays the class name of an object:
void printClassName(Object obj) { System.out.println("The object's" + " class is " + obj.getClass().getSimpleName()); }
The
Class
class, in the java.lang
package, has a large number of methods (more than 50). For example, you can test to see if the class is an annotation (isAnnotation()
), an interface (isInterface()
), or an enumeration (isEnum()
). You can see what the object's fields are (getFields()
) or what its methods are (getMethods()
), and so on.
The value returned by hashCode()
is the object's hash code, which is an integer value generated by a hashing algorithm.
By definition, if two objects are equal, their hash code must also be equal. If you override the equals()
method, you change the way two objects are equated and Object
's implementation of hashCode()
is no longer valid. Therefore, if you override the equals()
method, you must also override the hashCode()
method as well.
You should always consider overriding the toString()
method in your classes.
The Object
's toString()
method returns a String
representation of the object, which is very useful for debugging. The String
representation for an object depends entirely on the object, which is why you need to override toString()
in your classes.
You can use toString()
along with System.out.println()
to display a text representation of an object, such as an instance of Book
:
System.out.println(firstBook.toString());
which would, for a properly overridden toString()
method, print something useful, like this:
ISBN: 0201914670; The Swing Tutorial; A Guide to Constructing GUIs, 2nd Edition