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Posted
on March 1st, 2001
| Bruce Eckel's Thinking in Java | Contents | Prev | Next |
all the objects
When
you create a handle, you want to connect it with a new object. You do so, in
general, with the
new
keyword.
new
says, “Make me a new one of these objects.” So in the above
example, you can say:
String
s = new String("asdf");
Not
only does this mean “Make me a new
String,”
but it also gives information about
how
to make the
String
by supplying an initial character string.
Of
course,
String
is not the only type that exists. Java comes with a plethora of ready-made
types. What’s more important is that you can create your own types. In
fact, that’s the fundamental activity in Java programming, and it’s
what you’ll be learning about in the rest of this book.
Where storage lives
It’s
useful to visualize some aspects of how things are laid out while the program
is running, in particular how memory is arranged. There are six different
places to store data:
- Registers. This is the fastest storage because it exists in a place different than that of other storage: inside the processor. However, the number of registers is severely limited, so registers are allocated by the compiler according to its needs. You don’t have direct control, nor do you see any evidence in your programs that registers even exist.
- The stack . This lives in the general RAM (random-access memory) area, but has direct support from the processor via its stack pointer . The stack pointer is moved down to create new memory and moved up to release that memory. This is an extremely fast and efficient way to allocate storage, second only to registers. The Java compiler must know, while it is creating the program, the exact size and lifetime of all the data that is stored on the stack, because it must generate the code to move the stack pointer up and down. This constraint places limits on the flexibility of your programs, so while some Java storage exists on the stack – in particular, object handles – Java objects are not placed on the stack.
- The heap . This is a general-purpose pool of memory (also in the RAM area) where all Java objects live. The nice thing about the heap is that, unlike the stack, the compiler doesn’t need to know how much storage it needs to allocate from the heap or how long that storage must stay on the heap. Thus, there’s a great deal of flexibility in using storage on the heap. Whenever you need to create an object, you simply write the code to create it using new and the storage is allocated on the heap when that code is executed. And of course there’s a price you pay for this flexibility: it takes more time to allocate heap storage.
- Static storage . “Static” is used here in the sense of “in a fixed location” (although it’s also in RAM). Static storage contains data that is available for the entire time a program is running. You can use the static keyword to specify that a particular element of an object is static, but Java objects themselves are never placed in static storage.
- Constant storage . Constant values are often placed directly in the program code, which is safe since they can never change. Sometimes constants are cordoned off by themselves so that they can be optionally placed in read-only memory (ROM).
- Non-RAM storage . If data lives completely outside a program it can exist while the program is not running, outside the control of the program. The two primary examples of this are streamed objects, in which objects are turned into streams of bytes, generally to be sent to another machine, and persistent objects, in which the objects are placed on disk so they will hold their state even when the program is terminated. The trick with these types of storage is turning the objects into something that can exist on the other medium, and yet can be resurrected into a regular RAM-based object when necessary. Java 1.1 provides support for lightweight persistence , and future versions of Java might provide more complete solutions for persistence.
Special case: primitive types
There
is a group of types that gets special treatment; you can think of these as
“primitive” types that you use quite often in your programming. The
reason for the special treatment is that to create an object with
new,
especially a small, simple variable, isn’t very efficient because
new
places objects on the heap. For these types Java falls back on the approach
taken by C and C++. That is, instead of creating the variable using
new,
an “automatic” variable is created that
is
not a handle
.
The variable holds the value, and it’s placed on the stack so it’s
much more efficient.
Java
determines the size of each primitive type. These sizes don’t change from
one machine architecture to another as they do in most languages. This size
invariance is one reason Java programs are so portable.
|
boolean
|
1-bit
|
–
|
–
|
Boolean
|
|
char
|
16-bit
|
Unicode
0
|
Unicode
2
16-
1
|
Character
|
|
byte
|
8-bit
|
-128
|
+127
|
Byte[11]
|
|
short
|
16-bit
|
-215
|
+215
– 1
|
Short1
|
|
int
|
32-bit
|
-231
|
+231
– 1
|
Integer
|
|
long
|
64-bit
|
-263
|
+263
– 1
|
Long
|
|
float
|
32-bit
|
IEEE754
|
IEEE754
|
Float
|
|
double
|
64-bit
|
IEEE754
|
IEEE754
|
Double
|
|
void
|
–
|
–
|
–
|
Void1
|
All
numeric types are signed, so don’t go looking for unsigned types.
The
primitive data types also have “wrapper”
classes
for them. That means that if you want to make a non-primitive object on the
heap to represent that primitive type, you use the associated wrapper. For
example:
char
c = 'x';
Character C = new Character(c);
or
you could also use:
Character
C = new Character('x');
The
reasons for doing this will be shown in a later chapter.
High-precision
numbers
Java
1.1
has added two classes for performing high-precision arithmetic:
BigInteger
and
BigDecimal.
Although these approximately fit into the same category as the
“wrapper” classes, neither one has a primitive analogue.
Both
classes have methods that provide analogues for the operations that you perform
on primitive types. That is, you can do anything with a
BigInteger
or
BigDecimal
that
you can with an
int
or
float,
it’s just that you must use method calls instead of operators. Also,
since there’s more involved, the operations will be slower. You’re
exchanging speed for accuracy.
BigInteger
supports arbitrary-precision integers. This means that you can accurately
represent integral values of any size without losing any information during
operations.
BigDecimal
is for arbitrary-precision fixed-point numbers; you can use these for accurate
monetary calculations, for example.
Consult
your online documentation for details about the constructors and methods you
can call for these two classes.
Arrays in Java
Virtually
all programming languages support arrays. Using arrays in C and C++ is perilous
because those arrays are only blocks of memory. If a program accesses the array
outside of its memory block or uses the memory before initialization (common
programming errors) there will be unpredictable results.
[12]
One
of the primary goals of Java is safety, so many of the problems that plague
programmers in C and C++ are not repeated in Java. A Java array is guaranteed
to be initialized and cannot be accessed outside of its range. The range
checking comes at the price of having a small amount of memory overhead on each
array as well as verifying the index at run time, but the assumption is that
the safety and increased productivity is worth the expense.
When
you create an array of objects, you are really creating an array of handles,
and each of those handles is automatically initialized to a special value with
its own keyword: null.
When Java sees
null,
it recognizes that the handle in question isn’t pointing to an object.
You
must assign an object to each handle before you use it, and if you try to use a
handle that’s still
null,
the problem will be reported at run-time. Thus, typical array errors are
prevented in Java.
You
can also create an array of primitives. Again, the compiler guarantees
initialization because it zeroes the memory for that array.
[11]
In Java version 1.1 only, not in 1.0.
[12]
In C++ you should often use the safer containers in the Standard Template
Library as an alternative to arrays.
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