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| Bruce Eckel’s Thinking in Java | Contents | Prev | Next |
destroy
an object
In
most programming languages, the concept of the lifetime of a variable occupies
a significant portion of the programming effort. How long does the variable
last? If you are supposed to destroy it, when should you? Confusion over
variable lifetimes can lead to a lot of bugs, and this section shows how Java
greatly simplifies the issue by doing all the cleanup work for you.
most programming languages, the concept of the lifetime of a variable occupies
a significant portion of the programming effort. How long does the variable
last? If you are supposed to destroy it, when should you? Confusion over
variable lifetimes can lead to a lot of bugs, and this section shows how Java
greatly simplifies the issue by doing all the cleanup work for you.
Scoping
Most
procedural languages have the concept of
scope.
This determines both the visibility and lifetime of the names defined within
that scope. In C, C++ and Java, scope is determined by the placement of curly
braces
{}.
So for example:
procedural languages have the concept of
scope.
This determines both the visibility and lifetime of the names defined within
that scope. In C, C++ and Java, scope is determined by the placement of curly
braces
{}.
So for example:
{ int x = 12; /* only x available */ { int q = 96; /* both x & q available */ } /* only x available */ /* q “out of scope” */ }
A
variable defined within a scope is available only to the end of that scope.
variable defined within a scope is available only to the end of that scope.
Indentation
makes Java code easier to read. Since Java is a free form language, the extra
spaces, tabs and carriage returns do not affect the resulting program.
makes Java code easier to read. Since Java is a free form language, the extra
spaces, tabs and carriage returns do not affect the resulting program.
Note
that you
cannot
do the following, even though it is legal in C and C++:
that you
cannot
do the following, even though it is legal in C and C++:
{ int x = 12; { int x = 96; /* illegal */ } }
The
compiler will announce that the variable
x
has
already been defined. Thus the C and C++ ability to “hide” a
variable in a larger scope is not allowed because the Java designers thought
that it led to confusing programs.
compiler will announce that the variable
x
has
already been defined. Thus the C and C++ ability to “hide” a
variable in a larger scope is not allowed because the Java designers thought
that it led to confusing programs.
Scope
of objects
Java
objects do not have the same lifetimes as primitives. When you create a Java
object using
new,
it hangs around past the end of the scope. Thus if you use:
objects do not have the same lifetimes as primitives. When you create a Java
object using
new,
it hangs around past the end of the scope. Thus if you use:
{
String s = new String("a string");
}
/* end of scope */
the
handle
s
vanishes at the end of the scope. However, the
String
object that
s
was pointing to is still occupying memory. In this bit of code, there is no way
to access the object because the only handle to it is out of scope. In later
chapters you’ll see how the handle to the object can be passed around and
duplicated during the course of a program.
handle
s
vanishes at the end of the scope. However, the
String
object that
s
was pointing to is still occupying memory. In this bit of code, there is no way
to access the object because the only handle to it is out of scope. In later
chapters you’ll see how the handle to the object can be passed around and
duplicated during the course of a program.
It
turns out that because objects created with
new
stay around for as long as you want them, a whole slew of programming problems
simply vanish in C++ and Java. The hardest problems seem to occur in C++
because you don’t get any help from the language in making sure that the
objects are available when they’re needed. And more importantly, in C++
you must make sure that you destroy the objects when you’re done with them.
turns out that because objects created with
new
stay around for as long as you want them, a whole slew of programming problems
simply vanish in C++ and Java. The hardest problems seem to occur in C++
because you don’t get any help from the language in making sure that the
objects are available when they’re needed. And more importantly, in C++
you must make sure that you destroy the objects when you’re done with them.
That
brings up an interesting question. If Java leaves the objects lying around,
what keeps them from filling up memory and halting your program? This is
exactly the kind of problem that would occur in C++. This is where a bit of
magic happens. Java has a
garbage
collector
,
which looks at all the objects that were created with
new
and figures out which ones are not being referenced anymore. Then it releases
the memory for those objects, so the memory can be used for new objects. This
means that you never need to worry about reclaiming memory yourself. You simply
create objects, and when you no longer need them they will go away by
themselves. This eliminates a certain class of programming problem: the
so-called “memory leak,” in which a programmer forgets to release
memory.
brings up an interesting question. If Java leaves the objects lying around,
what keeps them from filling up memory and halting your program? This is
exactly the kind of problem that would occur in C++. This is where a bit of
magic happens. Java has a
garbage
collector
,
which looks at all the objects that were created with
new
and figures out which ones are not being referenced anymore. Then it releases
the memory for those objects, so the memory can be used for new objects. This
means that you never need to worry about reclaiming memory yourself. You simply
create objects, and when you no longer need them they will go away by
themselves. This eliminates a certain class of programming problem: the
so-called “memory leak,” in which a programmer forgets to release
memory.
