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<a href="http://www.bruceeckel.com/javabook.html">Bruce Eckel's Thinking in Java</a>
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<H2 ALIGN=LEFT>
Array
initialization
</H2>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Initializing
arrays in C is error-prone and tedious. C++ uses
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>aggregate
initialization
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to make it much safer.
</FONT><A NAME="fnB22" HREF="#fn22">[22]</A><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
Java has no “aggregates” like C++, since everything is an object in
Java. It does have arrays, and these are supported with <A NAME="Index329"></A><A NAME="Index330"></A>array
initialization.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">An
array is simply a sequence of either objects or primitives, all the same type
and packaged together under one identifier name. Arrays are defined and used
with the square-brackets <A NAME="Index331"></A><A NAME="Index332"></A><A NAME="Index333"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>indexing
operator
</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>[ ]</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
To define an array you simply follow your type name with empty square brackets:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">int[]
a1;
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can also put the square brackets after the identifier to produce exactly the
same meaning:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">int
a1[];
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
conforms to expectations from C and C++ programmers. The former style, however,
is probably a more sensible syntax, since it says that the type is “an
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
array.” That style will be used in this book.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
compiler doesn’t allow you to tell it how big the array is. This brings
us back to that issue of “handles.” All that you have at this point
is a handle to an array, and there’s been no space allocated for the
array. To create storage for the array you must write an initialization
expression. For arrays, initialization can appear anywhere in your code, but
you can also use a special kind of initialization expression that must occur at
the point where the array is created. This special initialization is a set of
values surrounded by curly braces. The storage allocation (the equivalent of
using
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
is taken care of by the compiler in this case. For example:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">int[]
a1 = { 1, 2, 3, 4, 5 };
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">So
why would you ever define an array handle without an array?
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">int[]
a2;
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Well,
it’s possible to assign one array to another in Java, so you can say:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">a2
= a1;
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">What
you’re really doing is copying a handle, as demonstrated here:
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: Arrays.java</font>
<font color="#009900">// Arrays of primitives.</font>
<font color="#0000ff">public</font> <font color="#0000ff">class</font> Arrays {
<font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
<font color="#0000ff">int</font>[] a1 = { 1, 2, 3, 4, 5 };
<font color="#0000ff">int</font>[] a2;
a2 = a1;
<font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i < a2.length; i++)
a2[i]++;
<font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i < a1.length; i++)
prt("a1[" + i + "] = " + a1[i]);
}
<font color="#0000ff">static</font> <font color="#0000ff">void</font> prt(String s) {
System.out.println(s);
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>a1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is given an initialization value while
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>a2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is not;
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>a2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is assigned later – in this case, to another array.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There’s
something new here: all arrays have an intrinsic member (whether they’re
arrays of objects or arrays of primitives) that you can query – but not
change – to tell you how many elements there are in the array. This
member is
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>length</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Since arrays in Java, like C and C++, start counting from element zero, the
largest element you can index is
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>length
- 1
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If you go out of <A NAME="Index334"></A><A NAME="Index335"></A>bounds,
C and C++ quietly accept this and allow you to stomp all over your memory,
which is the source of many infamous bugs. However, Java protects you against
such problems by causing a run-time error (an
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>exception</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
the subject of Chapter 9) if you step out of bounds. Of course, checking every
array access costs time and code and there’s no way to turn it off, which
means that array accesses might be a source of inefficiency in your program if
they occur at a critical juncture. For Internet security and programmer
productivity, the Java designers thought that this was a worthwhile tradeoff.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">What
if you don’t know how many elements you’re going to need in your
array while you’re writing the program? You simply use
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to create the elements in the array. Here, <A NAME="Index336"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
works even though it’s creating an array of primitives (
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
won’t create a non-array primitive):
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: ArrayNew.java</font>
<font color="#009900">// Creating arrays with new.</font>
<font color="#0000ff">import</font> java.util.*;
<font color="#0000ff">public</font> <font color="#0000ff">class</font> ArrayNew {
<font color="#0000ff">static</font> Random rand = <font color="#0000ff">new</font> Random();
<font color="#0000ff">static</font> <font color="#0000ff">int</font> pRand(<font color="#0000ff">int</font> mod) {
<font color="#0000ff">return</font> Math.abs(rand.nextInt()) % mod;
}
<font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
<font color="#0000ff">int</font>[] a;
a = <font color="#0000ff">new</font> <font color="#0000ff">int</font>[pRand(20)];
prt("length of a = " + a.length);
<font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i < a.length; i++)
prt("a[" + i + "] = " + a[i]);
}
<font color="#0000ff">static</font> <font color="#0000ff">void</font> prt(String s) {
System.out.println(s);
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Since
the size of the array is chosen at random (using the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>pRand( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method defined earlier), it’s clear that array creation is actually
happening at run-time. In addition, you’ll see from the output of this
program that array elements of primitive types are automatically initialized to
”empty” values. (For numerics, this is zero, for
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>char</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it’s
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>null</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and for
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it's
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>false</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.)</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Of
course, the array could also have been defined and initialized in the same
statement:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">int[]
a = new int[pRand(20)];
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you’re dealing with an array of non-primitive objects, you must always use
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Here, the handle issue comes up again because what you create is an array of
handles. Consider the wrapper type
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Integer,</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
which is a class and not a primitive:
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: ArrayClassObj.java</font>
<font color="#009900">// Creating an array of non-primitive objects.</font>
<font color="#0000ff">import</font> java.util.*;
<font color="#0000ff">public</font> <font color="#0000ff">class</font> ArrayClassObj {
<font color="#0000ff">static</font> Random rand = <font color="#0000ff">new</font> Random();
<font color="#0000ff">static</font> <font color="#0000ff">int</font> pRand(<font color="#0000ff">int</font> mod) {
<font color="#0000ff">return</font> Math.abs(rand.nextInt()) % mod;
}
<font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
Integer[] a = <font color="#0000ff">new</font> Integer[pRand(20)];
prt("length of a = " + a.length);
<font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i < a.length; i++) {
a[i] = <font color="#0000ff">new</font> Integer(pRand(500));
prt("a[" + i + "] = " + a[i]);
}
}
<font color="#0000ff">static</font> <font color="#0000ff">void</font> prt(String s) {
System.out.println(s);
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here,
even after
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is called to create the array:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">Integer[]
a = new Integer[pRand(20)];
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">it’s
only an array of handles, and not until the handle itself is initialized by
creating a new
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Integer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is the initialization complete:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">a[i]
= new Integer(pRand(500));
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you forget to create the object, however, you’ll get an exception at
run-time when you try to read the empty array location.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Take
a look at the formation of the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">object
inside the print statements. You can see that the handle to the
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Integer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object is automatically converted to produce a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">representing
the value inside the object.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">It’s
also possible to initialize arrays of objects using the curly-brace-enclosed
list. There are two forms, the first of which is the only one allowed in Java 1.0<A NAME="Index337"></A>.
The second (equivalent) form is allowed starting with Java 1.1<A NAME="Index338"></A>:</FONT><P></DIV>
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