chap03.htm

Thinking in Java, 2nd edition

    <font color=#0000ff>int</font> j = rand.nextInt() % 100;
    prt(<font color=#004488>"i = "</font> + i);
    prt(<font color=#004488>"j = "</font> + j);
    prt(<font color=#004488>"i &gt; j is "</font> + (i &gt; j));
    prt(<font color=#004488>"i &lt; j is "</font> + (i &lt; j));
    prt(<font color=#004488>"i &gt;= j is "</font> + (i &gt;= j));
    prt(<font color=#004488>"i &lt;= j is "</font> + (i &lt;= j));
    prt(<font color=#004488>"i == j is "</font> + (i == j));
    prt(<font color=#004488>"i != j is "</font> + (i != j));

    <font color=#009900>// Treating an int as a boolean is </font>
    <font color=#009900>// not legal Java</font>
<font color=#009900>//! prt("i &amp;&amp; j is " + (i &amp;&amp; j));</font>
<font color=#009900>//! prt("i || j is " + (i || j));</font>
<font color=#009900>//! prt("!i is " + !i);</font>

    prt(<font color=#004488>"(i &lt; 10) &amp;&amp; (j &lt; 10) is "</font>
       + ((i &lt; 10) &amp;&amp; (j &lt; 10)) );
    prt(<font color=#004488>"(i &lt; 10) || (j &lt; 10) is "</font>
       + ((i &lt; 10) || (j &lt; 10)) );
  }
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> prt(String s) {
    System.out.println(s);
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can apply AND, OR, or NOT to
<B>boolean</B> values only. You can&#8217;t use a non-<B>boolean</B> as if it
were a <A NAME="Index263"></A><B>boolean</B> in a logical expression as you can
in C and C++. You can see the failed attempts at doing this commented out with a
<B>//!</B> comment marker. The subsequent expressions, however, produce
<B>boolean</B> values using relational comparisons, then use logical operations
on the results. 
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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">One output listing looked like
this:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>i = 85
j = 4
i &gt; j is <font color=#0000ff>true</font>
i &lt; j is <font color=#0000ff>false</font>
i &gt;= j is <font color=#0000ff>true</font>
i &lt;= j is <font color=#0000ff>false</font>
i == j is <font color=#0000ff>false</font>
i != j is <font color=#0000ff>true</font>
(i &lt; 10) &amp;&amp; (j &lt; 10) is <font color=#0000ff>false</font>
(i &lt; 10) || (j &lt; 10) is <font color=#0000ff>true</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Note that a <B>boolean</B> value is
automatically converted to an appropriate text form if it&#8217;s used where a
<B>String</B> is expected. 
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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can replace the definition for
<B>int</B> in the above program with any other primitive data type except
<B>boolean</B>. Be aware, however, that the comparison of floating-point numbers
is very strict. A number that is the tiniest fraction different from another
number is still &#8220;not equal.&#8221; A number that is the tiniest bit above
zero is still nonzero.<A NAME="Index264"></A>

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<A NAME="Heading136"></A><FONT FACE = "Verdana"><H4 ALIGN="LEFT">
Short-circuiting</H4></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">When dealing with
<A NAME="Index265"></A><A NAME="Index266"></A><A NAME="Index267"></A>logical
operators you run into a phenomenon called &#8220;short circuiting.&#8221; This
means that the expression will be evaluated only <I>until</I> the truth or
falsehood of the entire expression can be unambiguously determined. As a result,
all the parts of a logical expression might not be evaluated. Here&#8217;s an
example that demonstrates short-circuiting:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c03:ShortCircuit.java</font>
<font color=#009900>// Demonstrates short-circuiting behavior.</font>
<font color=#009900>// with logical operators.</font>

<font color=#0000ff>public</font> <font color=#0000ff>class</font> ShortCircuit {
  <font color=#0000ff>static</font> <font color=#0000ff>boolean</font> test1(<font color=#0000ff>int</font> val) {
    System.out.println(<font color=#004488>"test1("</font> + val + <font color=#004488>")"</font>);
    System.out.println(<font color=#004488>"result: "</font> + (val &lt; 1));
    <font color=#0000ff>return</font> val &lt; 1;
  }
  <font color=#0000ff>static</font> <font color=#0000ff>boolean</font> test2(<font color=#0000ff>int</font> val) {
    System.out.println(<font color=#004488>"test2("</font> + val + <font color=#004488>")"</font>);
    System.out.println(<font color=#004488>"result: "</font> + (val &lt; 2));
    <font color=#0000ff>return</font> val &lt; 2;
  }
  <font color=#0000ff>static</font> <font color=#0000ff>boolean</font> test3(<font color=#0000ff>int</font> val) {
    System.out.println(<font color=#004488>"test3("</font> + val + <font color=#004488>")"</font>);
    System.out.println(<font color=#004488>"result: "</font> + (val &lt; 3));
    <font color=#0000ff>return</font> val &lt; 3;
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    <font color=#0000ff>if</font>(test1(0) &amp;&amp; test2(2) &amp;&amp; test3(2))
      System.out.println(<font color=#004488>"expression is true"</font>);
    <font color=#0000ff>else</font>
      System.out.println(<font color=#004488>"expression is false"</font>);
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Each test performs a comparison against
the argument and returns true or false. It also prints information to show you
that it&#8217;s being called. The tests are used in the expression:

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<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#0000ff>if</font>(test1(0) &amp;&amp; test2(2) &amp;&amp; test3(2))</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You might naturally think that all three
tests would be executed, but the output shows otherwise:

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<BLOCKQUOTE><FONT SIZE = "+1"><PRE>test1(0)
result: <font color=#0000ff>true</font>
test2(2)
result: <font color=#0000ff>false</font>
expression is <font color=#0000ff>false</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The first test produced a <B>true</B>
result, so the expression evaluation continues. However, the second test
produced a <B>false</B> result. Since this means that the whole expression must
be <B>false</B>, why continue evaluating the rest of the expression? It could be
expensive. The reason for short-circuiting, in fact, is precisely that; you can
get a potential performance increase if all the parts of a logical expression do
not need to be evaluated.

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</FONT><A NAME="_Toc375545254"></A><A NAME="_Toc481064548"></A><BR></P></DIV>
<A NAME="Heading137"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
Bitwise operators<A NAME="Index268"></A><A NAME="Index269"></A></H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The bitwise operators allow you to
manipulate individual bits in an integral primitive data type. Bitwise operators
perform boolean algebra<A NAME="Index270"></A> on the corresponding bits in the
two arguments to produce the result.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The bitwise operators come from C&#8217;s
low-level orientation; you were often manipulating hardware directly and had to
set the bits in hardware registers. Java was originally designed to be embedded
in TV <A NAME="Index271"></A>set-top boxes, so this low-level orientation still
made sense. However, you probably won&#8217;t use the bitwise operators much.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The bitwise AND operator
(<B>&amp;</B>)<A NAME="Index272"></A><A NAME="Index273"></A> produces a one in
the output bit if both input bits are one; otherwise it produces a zero. The
bitwise OR operator (<B>|</B>)<A NAME="Index274"></A><A NAME="Index275"></A>
produces a one in the output bit if either input bit is a one and produces a
zero only if both input bits are zero. The bitwise EXCLUSIVE OR, or XOR
(<B>^</B>),<A NAME="Index276"></A><A NAME="Index277"></A><A NAME="Index278"></A>
produces a one in the output bit if one or the other input bit is a one, but not
both. The bitwise NOT<A NAME="Index279"></A> (<B>~</B>, also called the <I>ones
complement </I>operator<A NAME="Index280"></A><A NAME="Index281"></A>) is a
unary operator;<A NAME="Index282"></A><A NAME="Index283"></A> it takes only one
argument. (All other bitwise operators are binary
operators.<A NAME="Index284"></A><A NAME="Index285"></A>) Bitwise NOT produces
the opposite of the input bit&#8212;a one if the input bit is zero, a zero if
the input bit is one. 
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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The bitwise operators and logical
operators use the same characters, so it is helpful to have a mnemonic device to
help you remember the meanings: since bits are &#8220;small,&#8221; there is
only one character in the bitwise operators.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Bitwise operators can be combined with
the <B>=</B> sign to unite the operation and assignment:
<B>&amp;=<A NAME="Index286"></A></B>, <B>|=<A NAME="Index287"></A></B> and
<B>^=<A NAME="Index288"></A></B> are all legitimate. (Since <B>~</B> is a unary
operator it cannot be combined with the <B>=</B> sign.)

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</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The <B>boolean</B> type is treated as a
one-bit value so it is somewhat different. You can perform a bitwise AND, OR and
XOR, but you can&#8217;t perform a bitwise NOT (presumably to prevent confusion
with the logical NOT). For <B>boolean</B>s the bitwise operators have the same
effect as the logical operators except that they do not short circuit. Also,
bitwise operations on <B>boolean</B>s include an XOR logical operator that is
not included under the list of &#8220;logical&#8221; operators. You&#8217;re
prevented from using <B>boolean</B>s in shift expressions, which is described
next.

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</FONT><A NAME="_Toc375545255"></A><A NAME="_Toc481064549"></A><BR></P></DIV>
<A NAME="Heading138"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
Shift operators<A NAME="Index289"></A><A NAME="Index290"></A></H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The shift operators also manipulate bits.
They can be used solely with primitive, integral types. The left-shift operator
(<B>&lt;&lt;</B>)<A NAME="Index291"></A><A NAME="Index292"></A> produces the
operand to the left of the operator shifted to the left by the number of bits
specified after the operator (inserting zeroes at the lower-order bits). The
signed right-shift operator
(<B>&gt;&gt;</B>)<A NAME="Index293"></A><A NAME="Index294"></A> produces the
operand to the left of the operator shifted to the right by the number of bits
specified after the operator. The signed right shift <B>&gt;&gt; </B>uses
<I>sign extension<A NAME="Index295"></A><A NAME="Index296"></A></I>: if the
value is positive, zeroes are inserted at the higher-order bits; if the value is
negative, ones are inserted at the higher-order bits. Java has also added the
unsigned right shift <B>&gt;&gt;&gt;, </B>which<B> </B>uses <I>zero
extension<A NAME="Index297"></A><A NAME="Index298"></A></I>: regardless of the
sign, zeroes are inserted at the higher-order bits.<I> </I>This operator does
not exist in C or C++.<I>

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