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}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
result will be
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>true</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
as you would expect. Ah, but it’s not as simple as that. If you create
your own class, like this:
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: EqualsMethod2.java</font>
<font color="#0000ff">class</font> Value {
<font color="#0000ff">int</font> i;
}
<font color="#0000ff">public</font> <font color="#0000ff">class</font> EqualsMethod2 {
<font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
Value v1 = <font color="#0000ff">new</font> Value();
Value v2 = <font color="#0000ff">new</font> Value();
v1.i = v2.i = 100;
System.out.println(v1.equals(v2));
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">you’re
back to square one: the result is
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>false</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
This is because the default behavior of
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>equals( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is to compare handles. So unless you
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>override</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>equals( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in your new class you won’t get the desired behavior. Unfortunately, you
won’t learn about overriding until Chapter 7, but being aware of the way
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>equals( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
behaves might save you some grief in the meantime.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Most
of the Java library classes implement
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>equals( )</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
so that it compares the contents of objects instead of their handles.
</FONT><a name="_Toc375545253"></a><a name="_Toc408018454"></a><P></DIV>
<A NAME="Heading109"></A><H3 ALIGN=LEFT>
Logical
operators<A NAME="Index142"></A><A NAME="Index143"></A></H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
logical operators AND (&&)<A NAME="Index144"></A><A NAME="Index145"></A>,
OR (||)<A NAME="Index146"></A><A NAME="Index147"></A>
and NOT (!) produce a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
value of
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>true</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>false</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
<A NAME="Index148"></A><A NAME="Index149"></A>based
on the logical relationship of its arguments. This example uses the relational
and logical operators:
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: Bool.java</font>
<font color="#009900">// Relational and logical operators</font>
<font color="#0000ff">import</font> java.util.*;
<font color="#0000ff">public</font> <font color="#0000ff">class</font> Bool {
<font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
Random rand = <font color="#0000ff">new</font> Random();
<font color="#0000ff">int</font> i = rand.nextInt() % 100;
<font color="#0000ff">int</font> j = rand.nextInt() % 100;
prt("i = " + i);
prt("j = " + j);
prt("i > j is " + (i > j));
prt("i < j is " + (i < j));
prt("i >= j is " + (i >= j));
prt("i <= j is " + (i <= j));
prt("i == j is " + (i == j));
prt("i != j is " + (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 && j is " + (i && j));</font>
<font color="#009900">//! prt("i || j is " + (i || j));</font>
<font color="#009900">//! prt("!i is " + !i);</font>
prt("(i < 10) && (j < 10) is "
+ ((i < 10) && (j < 10)) );
prt("(i < 10) || (j < 10) is "
+ ((i < 10) || (j < 10)) );
}
<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 apply AND, OR, or NOT to
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
values only. You can’t use a non-
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
as if it were a <A NAME="Index150"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in a logical expression as you can in C and C++. You can see the failed
attempts at doing this commented out with a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>//!</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
comment marker. The subsequent expressions, however, produce
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
values using relational comparisons, then use logical operations on the results.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">One
output listing looked like this:
</FONT><P></DIV>
<font color="#990000"><PRE>i = 85
j = 4
i > j is <font color="#0000ff">true</font>
i < j is <font color="#0000ff">false</font>
i >= j is <font color="#0000ff">true</font>
i <= j is <font color="#0000ff">false</font>
i == j is <font color="#0000ff">false</font>
i != j is <font color="#0000ff">true</font>
(i < 10) && (j < 10) is <font color="#0000ff">false</font>
(i < 10) || (j < 10) is <font color="#0000ff">true</font> </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
value is automatically converted to an appropriate text form if it’s used
where a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is expected.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can replace the definition for
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in the above program with any other primitive data type except
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
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 “not equal.” A number that is the tiniest bit above zero is
still nonzero.<A NAME="Index151"></A></FONT><P></DIV>
<A NAME="Heading110"></A><H4 ALIGN=LEFT>
Short-circuiting</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
dealing with <A NAME="Index152"></A><A NAME="Index153"></A>logical
operators you run into a phenomenon called “short circuiting.” This
means that the expression will be evaluated only until 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’s an example
that demonstrates short-circuiting:
</FONT><P></DIV>
<font color="#990000"><PRE><font color="#009900">//: 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("test1(" + val + ")");
System.out.println("result: " + (val < 1));
<font color="#0000ff">return</font> val < 1;
}
<font color="#0000ff">static</font> <font color="#0000ff">boolean</font> test2(<font color="#0000ff">int</font> val) {
System.out.println("test2(" + val + ")");
System.out.println("result: " + (val < 2));
<font color="#0000ff">return</font> val < 2;
}
<font color="#0000ff">static</font> <font color="#0000ff">boolean</font> test3(<font color="#0000ff">int</font> val) {
System.out.println("test3(" + val + ")");
System.out.println("result: " + (val < 3));
<font color="#0000ff">return</font> val < 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) && test2(2) && test3(2))
System.out.println("expression is <font color="#0000ff">true</font>");
<font color="#0000ff">else</font>
System.out.println("expression is <font color="#0000ff">false</font>");
}
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Each
test performs a comparison against the argument and returns true or false. It
also prints information to show you that it’s being called. The tests are
used in the expression:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">if(test1(0)
&& test2(2) && test3(2))
</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
might naturally think that all three tests would be executed, but the output
shows otherwise:
</FONT><P></DIV>
<font color="#990000"><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><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
first test produced a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>true</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
result, so the expression evaluation continues. However, the second test
produced a
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>false</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
result. Since this means that the whole expression must be
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>false</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
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.
</FONT><a name="_Toc375545254"></a><a name="_Toc408018455"></a><P></DIV>
<A NAME="Heading111"></A><H3 ALIGN=LEFT>
Bitwise
operators<A NAME="Index154"></A><A NAME="Index155"></A></H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
bitwise operators allow you to manipulate individual bits in an integral
primitive data type. Bitwise operators perform boolean algebra<A NAME="Index156"></A>
on the corresponding bits in the two arguments to produce the result.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
bitwise operators come from C’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="Index157"></A>set-top
boxes, so this low-level orientation still made sense. However, you probably
won’t use the bitwise operators much.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
bitwise AND operator (&)<A NAME="Index158"></A><A NAME="Index159"></A>
produces a one in the output bit if both input bits are one; otherwise it
produces a zero. The bitwise OR operator (|)<A NAME="Index160"></A><A NAME="Index161"></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 (^),<A NAME="Index162"></A><A NAME="Index163"></A><A NAME="Index164"></A>
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