Thinking in Java - 4th Edition
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Thinking in Java - 4th Edition

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“object references.” And (he goes on) everything is actually pass by value. So you’re not passing
by reference, you’re “passing an object reference by value.” One could argue for the precision of such convoluted
explanations, but I think my approach simplifies the understanding of the concept without hurting anything (well, the
language lawyers may claim that I’m lying to you, but I’ll say that I’m providing an appropriate abstraction).

 

the room and still control the television, you take the remote/reference with you, not the
television.

Also, the remote control can stand on its own, with no television. That is, just because you
have a reference doesn’t mean there’s necessarily an object connected to it. So if you want to
hold a word or sentence, you create a String reference:

String s;

But here you’ve created only the reference, not an object. If you decided to send a message to
s at this point, you’ll get an error because s isn’t actually attached to anything (there’s no
television). A safer practice, then, is always to initialize a reference when you create it:

String s = "asdf";

However, this uses a special Java feature: Strings can be initialized with quoted text.
Normally, you must use a more general type of initialization for objects.

You must create
 all the objects

When you create a reference, you want to connect it with a new object. You do so, in general,
with the new operator. The keyword new says, “Make me a new one of these objects.” So in
the preceding 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, Java comes with a plethora of ready-made types in addition to String. What’s
more important is that you can create your own types. In fact, creating new types is 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 five different places to store data:

1. Registers. This is the fastest storage because it exists in a place different from that of
other storage: inside the processor. However, the number of registers is severely
limited, so registers are allocated as they are needed. You don’t have direct control,
nor do you see any evidence in your programs that registers even exist (C & C++, on
the other hand, allow you to suggest register allocation to the compiler).

2. The stack. This lives in the general random-access memory (RAM) 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
system must know, while it is creating the program, the exact lifetime of all the items
that are stored on the stack. This constraint places limits on the flexibility of your
programs, so while some Java storage exists on the stack—in particular, object
references—Java objects themselves are not placed on the stack.

42 Thinking in Java Bruce Eckel

Everything Is an Object 43 

3. 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 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 an object, you
simply write the code to create it by using new, and the storage is allocated on the
heap when that code is executed. Of course there’s a price you pay for this flexibility: It
may take more time to allocate and clean up heap storage than stack storage (if you
even could create objects on the stack in Java, as you can in C++).

4. 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), in
embedded systems.2

5. 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 provides support for lightweight persistence, and
mechanisms such as JDBC and Hibernate provide more sophisticated support for
storing and retrieving object information in databases.

Special case: primitive types

One group of types, which you’ll use quite often in your programming, gets special treatment.
You can think of these as “primitive” types. 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 by using new, an “automatic” variable is
created that is not a reference. The variable holds the value directly, 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 more portable than programs in most other languages.

Primitive
type

Size Minimum Maximum Wrapper type

boolean — — — Boolean

char 16 bits Unicode 0 Unicode 216- 1 Character

byte 8 bits -128 +127 Byte

short 16 bits -215 +215-1 Short

int 32 bits -231 +231-1 Integer

long 64 bits -263 +263-1 Long

float 32 bits IEEE754 IEEE754 Float

double 64 bits IEEE754 IEEE754 Double

void — — — Void

                                                            
2 An example of this is the string pool. All literal strings and string-valued constant expressions are interned automatically
and put into special static storage.

All numeric types are signed, so don’t look for unsigned types.

The size of the boolean type is not explicitly specified; it is only defined to be able to take
the literal values true or false.

The “wrapper” classes for the primitive data types allow you to make a non-primitive object
on the heap to represent that primitive type. For example:

char c = ‘x’;
Character ch = new Character(c);

Or you could also use:

Character ch = new Character(‘x’);

Java SE5 autoboxing will automatically convert from a primitive to a wrapper type:

Character ch = ‘x’;

and back:

char c = ch;

The reasons for wrapping primitives will be shown in a later chapter.

High-precision numbers

Java includes 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.