K&B-6-pg-591 - Backed Collections - exam watch

[Leonid Shchervinsky]

Quote from K&B-6-pg-591 - Backed Collections - exam watch:
==========================================================
"... . So it's most likely that invoking pollFirstXxx() on the copy (assuming - on the backed collection) will remove an entry from both collections, but invoking pollFirstXxx() on the original will remove only the entry form the original."

Since I created a backed map using say, sumMap() method on a TreeMap instance wich returns a SortedMap, and SortedMap does not have pollFirstXxx() method, that would not be a good example. What would be an example of a backed collection which does have pollFirstXxx() methods?

Thanks
Leonid

[Ankit Garg]

Well to actually use pollFirstXXX() method, first of all you need to have JDK 1.6..

Then you will have to retrieve a backed set or map using the new subSet or subMap method which takes 4 arguments. They will return a TreeSet and TreeMap respectively, then you can use pollFirstXXX and pollLastXXX methods on them...

The list of methods is given on page 588 and 590...

[Leonid Shchervinsky]

except that submap returns SortedMap and SortedMap does not have pollxxxxEntry()...

[Ankit Garg]

Originally posted by Leonid Shchervinsky:
except that submap returns SortedMap and SortedMap does not have pollxxxxEntry()...

I didn't understand what you said...but what I am saying is that you must use

NavigableMap subMap(start, boolean, end, boolean);
and
NavigableSet subSet(start, boolean, end, boolean);

then you can call pollFirstXXX and pollLastXXX methods on the set or map returned...

[Leonid Shchervinsky]

Originally posted by Ankit Garg:


I didn't understand what you said...but what I am saying is that you must use

NavigableMap subMap(start, boolean, end, boolean);
and
NavigableSet subSet(start, boolean, end, boolean);

then you can call pollFirstXXX and pollLastXXX methods on the set or map returned...

NavigableMap is implemented by SortedMap, but SortedMap does not implement pollFirstXXX and pollLastXXX methods.

If you think this should work, can you please show a code snippet?

Thanks
Leonid

[Ankit Garg]

If you see the documentation(which obviously you didn't), NavigableMap implements SortedMap and not the reverse.

TreeSet t = new TreeSet();
...
NavigableSet ns = t.subSet("start",true,"end",true);
ns.pollFirst();

[Leonid Shchervinsky]

Originally posted by Ankit Garg:
If you see the documentation(which obviously you didn't), NavigableMap implements SortedMap and not the reverse.

TreeSet t = new TreeSet();
...
NavigableSet ns = t.subSet("start",true,"end",true);
ns.pollFirst();

import java.util.SortedMap;
import java.util.TreeMap;
 
public class Backed1Test {
 
    public static void main(String[] args) {
        TreeMap<String, String> map = new TreeMap<String, String>();
        map.put("a", "ant");
        map.put("d", "dog");
        map.put("h", "horse");
 
        SortedMap<String, String> submap;
        submap = map.subMap("a", "h");
        System.out.println(map + " " + submap);
 
        map.put("b", "bat");
        submap.put("f", "fish");
        System.out.println(map + " " + submap);
 
        map.put("r", "raccoon");
        //submap.put("p", "pig");           // out of range
        System.out.println(map + " " + submap);
        
        // --------------
        map.headMap("a");
        map.headMap("a", true);
        
        map.tailMap("h");
        map.tailMap("h", true);
        
        map.subMap("a", "r");
        map.subMap("a", true, "f", false);
        
        map.pollFirstEntry();
        map.pollLastEntry();
 
        submap.pollFirstEntry();  //compile time error: "cannot find symbol"
    }
    
}

[Ankit Garg]

Leonid why don't you try to understand. There are two subMap methods in TreeMap and two subSet in TreeSet class.

TreeMap subMap(Object start, boolean inclusive, Object end, boolean inclusive)
SortedMap subMap(Object start, Object end)

TreeSet subSet(Object start, boolean inclusive, Object end, boolean inclusive)
SortedSet subSet(Object start, Object end)

In your code just change these lines

SortedMap<String, String> submap;
submap = map.subMap("a", "h");

to

TreeMap<String, String> subMap;
subMap = map.subMap("a", true, "h", true);

[Leonid Shchervinsky]

Originally posted by Ankit Garg:
Leonid why don't you try to understand. There are two subMap methods in TreeMap and two subSet in TreeSet class.

TreeMap subMap(Object start, boolean inclusive, Object end, boolean inclusive)
SortedMap subMap(Object start, Object end)

TreeSet subSet(Object start, boolean inclusive, Object end, boolean inclusive)
SortedSet subSet(Object start, Object end)

In your code just change these lines

SortedMap<String, String> submap;
submap = map.subMap("a", "h");

to

TreeMap<String, String> subMap;
subMap = map.subMap("a", true, "h", true);

Thanks, Ankit:

I am trying :-)

I have just changed the two lines as you recommended:

TreeMap<String, String> submap;
submap = map.subMap("a", true, "h", true); // error

Getting: "incompatible types: found NavigatableMap, required: TreeMap"

Can you please run this code and see what is up?

Thanks
Leonid

[Leonid Shchervinsky]

Thanks, Ankit for trying to help me out.

Am I totally off base here?

Are there any bartenders on this forum?

Thanks
Leonid

[Ankit Garg]

Just change the code to

NavigableMap<String, String> submap;
submap = map.subMap("a", true, "h", true); // error

[Terence Gronowski]

I wonder what kind of backed (or related) situations exist. It seems to be a very important Issue at SCJP 6.

Here some situations I know they exist (does somebody know other?) :?:

a) TreeSet (Methods headSet, tailSet, subSet): Adding within range allowed, out of range => key out of range Exception

Example:

TreeSet<String> ts= new TreeSet<String>();
  ts.add("A"); //in die Queue schreiben
  ts.add("B");
  ts.add("C");
  ts.add("E");
  ts.add("H");

  SortedSet<String> ts1=ts.subSet("B",true,"G",true);
  ts.add("D");  //within Rangeof ts1
  ts.add("Z");  //within Range of ts1
  
  ts1.add("F");  //within Range of ts1, ok

  //ts1.add("G"); //  IllegalArgumentExeption: key out of range

  for (String s:ts1)
    System.out.print(s+" ");

  }
}

b) TreeMap (Methods headMap, tailMap, subMap): Adding within range allowed, out of range => key out of range Exception (same as above)

Example:

TreeMap<String,String> m= new TreeMap<String,String>();
  m.put("a","A"); //in die Queue schreiben
  m.put("b","B");
  m.put("c","C");
  m.put("e","E");
  m.put("f","F");

  SortedMap<String,String> m1=m.subMap("b","e");
  m.put("d","D");
  m.put("z","Z");
  m.put("g","G");
  m.put("h","H");  //no change in m1, as out of range
  for (Object i:m1.keySet())
    System.out.println(i+" "+m1.get(i));
  System.out.println("==========");
  m1.put("d","Z"); //Adding within range allowed
  //m1.put("z","Y");  //IllegalArgumentException:key out of range: Changing out of range not allowed
  for (Object i:m1.keySet())
    System.out.println(i+" "+m1.get(i));

//output: b c d

c) toArray() results in a fixed sized array (arrays are always fixed!). Altering of array-element allowed. Changing of ArrayList has no influence on array.

ArrayList<String> ar= new ArrayList<String>();
  ar.add("a"); 
  ar.add("b");
  ar.add("c");  
  ar.add("e");
  ar.add("f");
  String[] iarr = new String[ar.size()]; //take size from ArrayList
  iarr=ar.toArray(iarr);

  iarr[0]="b";     //Changing of Array allowed
                        //no influence on List
  
  ar.set(0,"z"); //No influence on Array
  
  for (String i:iarr)
    System.out.print(i+" ");
  System.out.println();
  for (String i:ar)
    System.out.print(i+" ");
  }

//output: 
//b b c e f
//a b c e f

d) Arrays.asList(): Not allowed to add new element to list, as list comes from fixed array. Changing of list allowed, changes array as it is backed.

String[] sarr = new String[]{"a","b","c","e" };
  List<String> l=Arrays.asList(sarr);
  //l.add("f"); //UnsupportedOperationException, adding not allowed
   l.set(0,"z");  //changing allowed
  for (String i:l)
    System.out.print(i+" ");
  System.out.println();
 for (String i:sarr)
    System.out.print(i+" ");

//Output
//z b c e
//z b c e

Answers welcome to have more systematics in this topic!

[Bert Bates]

Hey Leonid,

I haven't followed this thread in detail, but you're in good hands with Ankit!

Bert

[Jesper de Jong]

Please use code tags when you post source code.