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1. Examples of GoF Design Patterns
I am learning GoF Java Design Patterns and I want to see some real life examples of them. Can you guys point to some good usage of these Design Patterns.(preferably in Java's core libraries). Thank you
2. Deep compare sets in Java
I have two sets in Java that compare Item objects. Is there a method to compare the sets so that Item's equals method is invoked and not just compare references?
3. Remove repeated vaules _stack&array
I want to write a program to implement an array-based stack, which accept integer numbers entered by the user.the program will then identify any occurrences of a given value from user and remove the repeated values from the stack,(using Java programming language). I just need your help of writing (removing values method) e.g. input:6 2 3 4 3 8 output:6 2 4 8
4. What is a appropriate way to compare two objects with hashsets?
I have a person class which has a name and a list of friends in HashSet. I want to override an equals method for this Person class. Below is what I have written so far. But I am unsure about this because, I know that HashSet does not necessarily have to be in order, and I also heard that I have to override HashCode method. What modifications should I make in below codes to correctly implem...
5. Java Collections API HashSet remove method
I encountered this issue while working with the Java Collections API. Basically this is a support method for an implementation of Kruskal's algorithm for finding an MST. I created this class for implementing the union/find algorithm. My question, as I was able to find a work around, is that does anybody know of any reason why the remove method in the "union" method would not work consistently....
6. Can't instantiate AbstractSet Class
Ok I've been searching for this problem for a while. I keep getting an error that I can't instantiate abstractSet. It keeps asking for a generic. I add the generic but still no dice. import java.util.AbstractSet; import java.util.Set; public class UnorderedTree{ private Object root; private Set subtrees; //Switched to AbstractSet private int size; public UnorderedTree(Object root){ thi... 
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  /*
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   * Copyright 1997-2006 Sun Microsystems, Inc.  All Rights Reserved.
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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   *
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   * This code is free software; you can redistribute it and/or modify it
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   * under the terms of the GNU General Public License version 2 only, as
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   * published by the Free Software Foundation.  Sun designates this
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   * particular file as subject to the "Classpath" exception as provided
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   * by Sun in the LICENSE file that accompanied this code.
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  *
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  * This code is distributed in the hope that it will be useful, but WITHOUT
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  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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  * version 2 for more details (a copy is included in the LICENSE file that
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  * accompanied this code).
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  *
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  * You should have received a copy of the GNU General Public License version
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  * 2 along with this work; if not, write to the Free Software Foundation,
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  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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  *
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  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
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  * CA 95054 USA or visit www.sun.com if you need additional information or
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  * have any questions.
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  */
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 package java.util;

This class provides a skeletal implementation of the Set interface to minimize the effort required to implement this interface.

The process of implementing a set by extending this class is identical to that of implementing a Collection by extending AbstractCollection, except that all of the methods and constructors in subclasses of this class must obey the additional constraints imposed by the Set interface (for instance, the add method must not permit addition of multiple instances of an object to a set).

Note that this class does not override any of the implementations from the AbstractCollection class. It merely adds implementations for equals and hashCode.

This class is a member of the Java Collections Framework.

Parameters:
<E> the type of elements maintained by this set
Author(s):
Josh Bloch
Neal Gafter
Since:
1.2
See also:
Collection
AbstractCollection
Set
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 public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {
    
Sole constructor. (For invocation by subclass constructors, typically implicit.) 
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     protected AbstractSet() {
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     }
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     // Comparison and hashing
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Compares the specified object with this set for equality. Returns true if the given object is also a set, the two sets have the same size, and every member of the given set is contained in this set. This ensures that the equals method works properly across different implementations of the Set interface.

This implementation first checks if the specified object is this set; if so it returns true. Then, it checks if the specified object is a set whose size is identical to the size of this set; if not, it returns false. If so, it returns containsAll((Collection) o).

Parameters:
o object to be compared for equality with this set
Returns:
true if the specified object is equal to this set
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     public boolean equals(Object o) {
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         if (o == this)
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             return true;
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         if (!(o instanceof Set))
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             return false;
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         Collection c = (Collection) o;
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         if (c.size() != size())
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             return false;
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         try {
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             return containsAll(c);
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         } catch (ClassCastException unused)   {
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             return false;
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         } catch (NullPointerException unused) {
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             return false;
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        }
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    }

    
Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a null element is defined to be zero. This ensures that s1.equals(s2) implies that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of java.lang.Object.hashCode().

This implementation iterates over the set, calling the hashCode method on each element in the set, and adding up the results.

Returns:
the hash code value for this set
See also:
java.lang.Object.equals(java.lang.Object)
Set.equals(java.lang.Object)
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    public int hashCode() {
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        int h = 0;
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        Iterator<E> i = iterator();
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        while (i.hasNext()) {
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            E obj = i.next();
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            if (obj != null)
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                h += obj.hashCode();
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        }
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        return h;
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    }

    
Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.

This implementation determines which is the smaller of this set and the specified collection, by invoking the size method on each. If this set has fewer elements, then the implementation iterates over this set, checking each element returned by the iterator in turn to see if it is contained in the specified collection. If it is so contained, it is removed from this set with the iterator's remove method. If the specified collection has fewer elements, then the implementation iterates over the specified collection, removing from this set each element returned by the iterator, using this set's remove method.

Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method.

Parameters:
c collection containing elements to be removed from this set
Returns:
true if this set changed as a result of the call
Throws:
java.lang.UnsupportedOperationException if the removeAll operation is not supported by this set
java.lang.ClassCastException if the class of an element of this set is incompatible with the specified collection (optional)
java.lang.NullPointerException if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
See also:
AbstractCollection.remove(java.lang.Object)
AbstractCollection.contains(java.lang.Object)
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    public boolean removeAll(Collection<?> c) {
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        boolean modified = false;
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        if (size() > c.size()) {
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            for (Iterator<?> i = c.iterator(); i.hasNext(); )
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                modified |= remove(i.next());
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        } else {
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            for (Iterator<?> i = iterator(); i.hasNext(); ) {
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                if (c.contains(i.next())) {
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                    i.remove();
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                    modified = true;
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                }
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            }
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        }
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        return modified;
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    }
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