tij0093.html

学习java的经典书籍

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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean
removeAll(Collection)
</B></FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">*Removes
all the elements that are contained in the argument. Returns true if any
removals occurred.
</FONT><P></DIV>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>boolean
retainAll(Collection)
</B></FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">*Retains
only elements that are contained in the argument (an &#8220;intersection&#8221;
from set theory). Returns true if any changes occurred.
</FONT><P></DIV>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int
size(&#160;)
</B></FONT><P></DIV>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Returns
the number of elements in the Collection. 
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object[]
toArray(&#160;)
</B></FONT><P></DIV>
</TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Returns
an array containing all the elements in the Collection.
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<P></TD>
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<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">*This
is an &#8220;optional&#8221; method, which means it might not be implemented by
a particular Collection. If not, that method throws an
UnsupportedOperationException. Exceptions will be covered in Chapter 9.
</FONT><P></DIV>
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</TR>
</TABLE></DIV>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
following example demonstrates all of these methods. Again, these work with
anything that inherits from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Collection</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">;
an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ArrayList</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is used as a kind of &#8220;least-common denominator&#8221;:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Collection1.java</font>
<font color="#009900">// Things you can do with all Collections</font>
<font color="#0000ff">package</font> c08.newcollections;
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Collection1 {
  <font color="#009900">// Fill with 'size' elements, start</font>
  <font color="#009900">// counting at 'start':</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> Collection 
  fill(Collection c, <font color="#0000ff">int</font> start, <font color="#0000ff">int</font> size) {
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = start; i &lt; start + size; i++)
      c.add(Integer.toString(i));
    <font color="#0000ff">return</font> c;
  }
  <font color="#009900">// Default to a "start" of 0:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> Collection 
  fill(Collection c, <font color="#0000ff">int</font> size) {
    <font color="#0000ff">return</font> fill(c, 0, size);
  }
  <font color="#009900">// Default to 10 elements:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> Collection fill(Collection c) {
    <font color="#0000ff">return</font> fill(c, 0, 10);
  }
  <font color="#009900">// Create &amp; upcast to Collection:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> Collection newCollection() {
    <font color="#0000ff">return</font> fill(<font color="#0000ff">new</font> ArrayList());
    <font color="#009900">// ArrayList is used for simplicity, but it's</font>
    <font color="#009900">// only seen as a generic Collection </font>
    <font color="#009900">// everywhere else in the program.</font>
  }
  <font color="#009900">// Fill a Collection with a range of values:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> Collection 
  newCollection(<font color="#0000ff">int</font> start, <font color="#0000ff">int</font> size) {
    <font color="#0000ff">return</font> fill(<font color="#0000ff">new</font> ArrayList(), start, size);
  }
  <font color="#009900">// Moving through a List with an iterator:</font>
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> print(Collection c) {
    <font color="#0000ff">for</font>(Iterator x = c.iterator(); x.hasNext();)
      System.out.print(x.next() + " ");
    System.out.println();
  }    
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    Collection c = newCollection();
    c.add("ten");
    c.add("eleven");
    print(c);
    <font color="#009900">// Find max and min elements; this means</font>
    <font color="#009900">// different things depending on the way</font>
    <font color="#009900">// the Comparable interface is implemented:</font>
    System.out.println("Collections.max(c) = " +
      Collections.max(c));
    System.out.println("Collections.min(c) = " +
      Collections.min(c));
    <font color="#009900">// Add a Collection to another Collection</font>
    c.addAll(newCollection());
    print(c);
    c.remove("3"); <font color="#009900">// Removes the first one</font>
    print(c);
    c.remove("3"); <font color="#009900">// Removes the second one</font>
    print(c);
    <font color="#009900">// Remove all components that are in the</font>
    <font color="#009900">// argument collection:</font>
    c.removeAll(newCollection());
    print(c);
    c.addAll(newCollection());
    print(c);
    <font color="#009900">// Is an element in this Collection?</font>
    System.out.println(
      "c.contains(\"4\") = " + c.contains("4"));
    <font color="#009900">// Is a Collection in this Collection?</font>
    System.out.println(
      "c.containsAll(newCollection()) = " + 
      c.containsAll(newCollection()));
    Collection c2 = newCollection(5, 3);
    <font color="#009900">// Keep all the elements that are in both</font>
    <font color="#009900">// c and c2 (an intersection of sets):</font>
    c.retainAll(c2);
    print(c);
    <font color="#009900">// Throw away all the elements in c that</font>
    <font color="#009900">// also appear in c2:</font>
    c.removeAll(c2);
    System.out.println("c.isEmpty() = " +
      c.isEmpty());
    c = newCollection();
    print(c);
    c.clear(); <font color="#009900">// Remove all elements</font>
    System.out.println("after c.clear():");
    print(c);
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
first methods provide a way to fill any 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Collection</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
with test data, in this case just 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>int</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
converted to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s.
The second method will be used frequently throughout the rest of this chapter.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
two versions of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>newCollection(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
create 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ArrayList</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
containing different sets of data and return them as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Collection</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects, so it&#8217;s clear that nothing other than the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Collection</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface is being used.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>print(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method will also be used throughout the rest of this section. Since it moves
through a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Collection</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
using an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Iterator</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which any 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Collection</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
can produce, it will work with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>List</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Set</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s
and any 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Collection</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Map</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
produces.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
uses simple exercises to show all of the methods in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Collection</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">The
following sections compare the various implementations of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>List</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Set,</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Map</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and indicate in each case (with an asterisk) which one should be your default
choice. You&#8217;ll notice that the legacy classes 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Stack</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and