Object-Oriented Programming: Inheritance - Java: How to Program Early Objects - page 388

Method Description

toString This method (introduced in Section 9.4.1) returns a String representation of an

object. The default implementation of this method returns the package name and
class name of the object's class typically followed by a hexadecimal representation of
the value returned by the object's hashCode method.

wait , notify ,
notifyAll

Methods notify , notifyAll and the three overloaded versions of wait are related to
multithreading, which is discussed in Chapter 23.

getClass

Every object in Java knows its own type at execution time. Method getClass (used
in Sections 10.5 and 12.5) returns an object of class Class (package java.lang )
that contains information about the object's type, such as its class name (returned
by Class method getName ).

finalize

This protected method is called by the garbage collector to perform termination
housekeeping on an object just before the garbage collector reclaims the object's
memory. Recall from Section 8.10 that it's unclear whether, or when, finalize will
be called. For this reason, most programmers should avoid method finalize .

clone

This protected method, which takes no arguments and returns an Object refer-
ence, makes a copy of the object on which it's called. The default implementation
performs a so-called shallow copy —instance-variable values in one object are cop-
ied into another object of the same type. For reference types, only the references are
copied. A typical overridden clone method's implementation would perform a
deep copy that creates a new object for each reference-type instance variable. Imple-
menting clone correctly is difficult. For this reason, its use is discouraged. Some indus-
try experts suggest that object serialization should be used instead. We discuss
object serialization in Chapter 15. Recall from Chapter 7 that arrays are objects. As
a result, like all other objects, arrays inherit the members of class Object . Every
array has an overridden clone method that copies the array. However, if the array
stores references to objects, the objects are not copied—a shallow copy is per-
formed.

Fig. 9.12 | Object methods. (Part 2 of 2.)

9.7 (Optional) GUI and Graphics Case Study:
Displaying Text and Images Using Labels
Programs often use labels when they need to display information or instructions to the
user in a graphical user interface. Labels are a convenient way of identifying GUI compo-
nents on the screen and keeping the user informed about the current state of the program.
In Java, an object of class JLabel (from package javax.swing ) can display text, an image
or both. The example in Fig. 9.13 demonstrates several JLabel features, including a plain
text label, an image label and a label with both text and an image.

1 // Fig 9.13: LabelDemo.java

2 // Demonstrates the use of labels.

3 import java.awt.BorderLayout;

4 import javax.swing.ImageIcon;

Fig. 9.13 | JLabel with text and with images. (Part 1 of 2.)