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| Bruce Eckel’s Thinking in Java | Contents | Prev | Next |
Until
now, composition has been used quite frequently. You simply place object
handles inside new classes. For example, suppose you’d like an object
that holds several
String
objects, a couple of primitives and an object of another class. For the
non-primitive objects, just put handles inside your new class, and for the
primitives just define them inside your class: (See page
97
if you have trouble executing this program.)
now, composition has been used quite frequently. You simply place object
handles inside new classes. For example, suppose you’d like an object
that holds several
String
objects, a couple of primitives and an object of another class. For the
non-primitive objects, just put handles inside your new class, and for the
primitives just define them inside your class: (See page
97
if you have trouble executing this program.)
//: SprinklerSystem.java // Composition for code reuse package c06; class WaterSource { private String s; WaterSource() { System.out.println("WaterSource()"); s = new String("Constructed"); } public String toString() { return s; } } public class SprinklerSystem { private String valve1, valve2, valve3, valve4; WaterSource source; int i; float f; void print() { System.out.println("valve1 = " + valve1); System.out.println("valve2 = " + valve2); System.out.println("valve3 = " + valve3); System.out.println("valve4 = " + valve4); System.out.println("i = " + i); System.out.println("f = " + f); System.out.println("source = " + source); } public static void main(String[] args) { SprinklerSystem x = new SprinklerSystem(); x.print(); } } ///:~
One
of the methods defined in
WaterSource
is
special:
toString( ).
You will learn later that every non-primitive object has a toString( )
method, and it’s called in special situations when the compiler wants a
String
but it’s got one of these objects. So in the expression:
of the methods defined in
WaterSource
is
special:
toString( ).
You will learn later that every non-primitive object has a toString( )
method, and it’s called in special situations when the compiler wants a
String
but it’s got one of these objects. So in the expression:
System.out.println("source
= " + source);
= " + source);
the
compiler sees you trying to add a
String
object (
“source
= “
)
to a
WaterSource.
This doesn’t make sense to it, because you can only “add” a
String
to another
String,
so it says “I’ll turn
source
into a
String
by calling
toString( )!”
After doing this it can combine the two
Strings
and pass the resulting
String
to
System.out.println( ).
Any time you want to allow this behavior with a class you create you need only
write a
toString( )
method.
compiler sees you trying to add a
String
object (
“source
= “
)
to a
WaterSource.
This doesn’t make sense to it, because you can only “add” a
String
to another
String,
so it says “I’ll turn
source
into a
String
by calling
toString( )!”
After doing this it can combine the two
Strings
and pass the resulting
String
to
System.out.println( ).
Any time you want to allow this behavior with a class you create you need only
write a
toString( )
method.
At
first glance, you might assume – Java being as safe and careful as it is
– that the compiler would automatically construct objects for each of the
handles in the above code, for example calling the default constructor for
WaterSource
to initialize
source.
The output of the print statement is in fact:
first glance, you might assume – Java being as safe and careful as it is
– that the compiler would automatically construct objects for each of the
handles in the above code, for example calling the default constructor for
WaterSource
to initialize
source.
The output of the print statement is in fact:
valve1 = null valve2 = null valve3 = null valve4 = null i = 0 f = 0.0 source = null
Primitives
that are fields in a class are automatically initialized
to zero, as noted in Chapter 2. But the object handles are initialized to
null,
and if you try to call methods for any of them you’ll get an exception.
It’s actually pretty good (and useful) that you can still print them out
without throwing an exception.
that are fields in a class are automatically initialized
to zero, as noted in Chapter 2. But the object handles are initialized to
null,
and if you try to call methods for any of them you’ll get an exception.
It’s actually pretty good (and useful) that you can still print them out
without throwing an exception.
It
makes sense that the compiler doesn’t just create a default object for
every handle because that would incur unnecessary overhead in many cases. If
you want the handles initialized, you can do it:
makes sense that the compiler doesn’t just create a default object for
every handle because that would incur unnecessary overhead in many cases. If
you want the handles initialized, you can do it:
- At
the point the objects are defined. This means that they’ll always be
initialized before the constructor is called. - In
the constructor for that class - Right
before you actually need to use the object. This can reduce overhead, if there
are situations where the object doesn’t need to be created.
All
three approaches are shown here:
three approaches are shown here:
//: Bath.java // Constructor initialization with composition class Soap { private String s; Soap() { System.out.println("Soap()"); s = new String("Constructed"); } public String toString() { return s; } } public class Bath { private String // Initializing at point of definition: s1 = new String("Happy"), s2 = "Happy", s3, s4; Soap castille; int i; float toy; Bath() { System.out.println("Inside Bath()"); s3 = new String("Joy"); i = 47; toy = 3.14f; castille = new Soap(); } void print() { // Delayed initialization: if(s4 == null) s4 = new String("Joy"); System.out.println("s1 = " + s1); System.out.println("s2 = " + s2); System.out.println("s3 = " + s3); System.out.println("s4 = " + s4); System.out.println("i = " + i); System.out.println("toy = " + toy); System.out.println("castille = " + castille); } public static void main(String[] args) { Bath b = new Bath(); b.print(); } } ///:~
Note
that in the
Bath
constructor
a statement is executed before any of the initializations take place. When you
don’t initialize at the point of definition, there’s still no
guarantee that you’ll perform any initialization before you send a
message to an object handle – except for the inevitable run-time exception.
that in the
Bath
constructor
a statement is executed before any of the initializations take place. When you
don’t initialize at the point of definition, there’s still no
guarantee that you’ll perform any initialization before you send a
message to an object handle – except for the inevitable run-time exception.
Here’s
the output for the program:
the output for the program:
Inside Bath() Soap() s1 = Happy s2 = Happy s3 = Joy s4 = Joy i = 47 toy = 3.14 castille = Constructed
