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+/* AbstractCollection.java -- Abstract implementation of most of Collection
+   Copyright (C) 1998 Free Software Foundation, Inc.
+
+This file is part of GNU Classpath.
+
+GNU Classpath is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2, or (at your option)
+any later version.
+ 
+GNU Classpath is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Classpath; see the file COPYING.  If not, write to the
+Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
+02111-1307 USA.
+
+As a special exception, if you link this library with other files to
+produce an executable, this library does not by itself cause the
+resulting executable to be covered by the GNU General Public License.
+This exception does not however invalidate any other reasons why the
+executable file might be covered by the GNU General Public License. */
+
+
+package java.util;
+
+import java.lang.reflect.Array;
+
+/**
+ * A basic implementation of most of the methods in the Collection interface to
+ * make it easier to create a collection. To create an unmodifiable Collection,
+ * just subclass AbstractCollection and provide implementations of the
+ * iterator() and size() methods. The Iterator returned by iterator() need only
+ * provide implementations of hasNext() and next() (that is, it may throw an
+ * UnsupportedOperationException if remove() is called). To create a modifiable
+ * Collection, you must in addition provide an implementation of the
+ * add(Object) method and the Iterator returned by iterator() must provide an
+ * implementation of remove(). Other methods should be overridden if the
+ * backing data structure allows for a more efficient implementation. The
+ * precise implementation used by AbstractCollection is documented, so that
+ * subclasses can tell which methods could be implemented more efficiently.
+ */
+public abstract class AbstractCollection implements Collection {
+
+  /**
+   * Return an Iterator over this collection. The iterator must provide the
+   * hasNext and next methods and should in addition provide remove if the
+   * collection is modifiable.
+   */
+  public abstract Iterator iterator();
+
+  /**
+   * Return the number of elements in this collection.
+   */
+  public abstract int size();
+
+  /**
+   * Add an object to the collection. This implementation always throws an
+   * UnsupportedOperationException - it should be overridden if the collection
+   * is to be modifiable.
+   *
+   * @param o the object to add
+   * @return true if the add operation caused the Collection to change
+   * @exception UnsupportedOperationException if the add operation is not
+   *   supported on this collection
+   */
+  public boolean add(Object o) {
+    throw new java.lang.UnsupportedOperationException();
+  }
+
+  /**
+   * Add all the elements of a given collection to this collection. This
+   * implementation obtains an Iterator over the given collection and iterates
+   * over it, adding each element with the add(Object) method (thus this method
+   * will fail with an UnsupportedOperationException if the add method does).
+   *
+   * @param c the collection to add the elements of to this collection
+   * @return true if the add operation caused the Collection to change
+   * @exception UnsupportedOperationException if the add operation is not
+   *   supported on this collection
+   */
+  public boolean addAll(Collection c) {
+    Iterator i = c.iterator();
+    boolean modified = false;
+    while (i.hasNext()) {
+      modified |= add(i.next());
+    }
+    return modified;
+  }
+
+  /**
+   * Remove all elements from the collection. This implementation obtains an
+   * iterator over the collection and calls next and remove on it repeatedly
+   * (thus this method will fail with an UnsupportedOperationException if the
+   * Iterator's remove method does) until there are no more elements to remove.
+   * Many implementations will have a faster way of doing this.
+   *
+   * @exception UnsupportedOperationException if the Iterator returned by
+   *   iterator does not provide an implementation of remove
+   */
+  public void clear() {
+    Iterator i = iterator();
+    while (i.hasNext()) {
+      i.next();
+      i.remove();
+    }
+  }
+
+  /**
+   * Test whether this collection contains a given object. That is, if the
+   * collection has an element e such that (o == null ? e == null :
+   * o.equals(e)). This implementation obtains an iterator over the collection
+   * and iterates over it, testing each element for equality with the given
+   * object. If it is equal, true is returned. Otherwise false is returned when
+   * the end of the collection is reached.
+   *
+   * @param o the object to remove from this collection
+   * @return true if this collection contains an object equal to o
+   */
+  public boolean contains(Object o) {
+    Iterator i = iterator();
+
+    // This looks crazily inefficient, but it takes the test o==null outside
+    // the loop, saving time, and also saves needing to store the result of
+    // i.next() each time.
+    if (o == null) {
+      while (i.hasNext()) {
+        if (i.next() == null) {
+          return true;
+        }
+      }
+    } else {
+      while (i.hasNext()) {
+        if (o.equals(i.next())) {
+          return true;
+        }
+      }
+    }
+    return false;
+  }
+
+  /**
+   * Tests whether this collection contains all the elements in a given
+   * collection. This implementation iterates over the given collection,
+   * testing whether each element is contained in this collection. If any one
+   * is not, false is returned. Otherwise true is returned.
+   *
+   * @param c the collection to test against
+   * @return true if this collection contains all the elements in the given
+   *   collection
+   */
+  public boolean containsAll(Collection c) {
+    Iterator i = c.iterator();
+    while (i.hasNext()) {
+      if (!contains(i.next())) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  /**
+   * Test whether this collection is empty. This implementation returns
+   * size() == 0.
+   *
+   * @return true if this collection is empty.
+   */
+  public boolean isEmpty() {
+    return size() == 0;
+  }
+
+  /**
+   * Remove a single instance of an object from this collection. That is,
+   * remove one element e such that (o == null ? e == null : o.equals(e)), if
+   * such an element exists. This implementation obtains an iterator over the
+   * collection and iterates over it, testing each element for equality with
+   * the given object. If it is equal, it is removed by the iterator's remove
+   * method (thus this method will fail with an UnsupportedOperationException
+   * if the Iterator's remove method does). After the first element has been
+   * removed, true is returned; if the end of the collection is reached, false
+   * is returned.
+   *
+   * @param o the object to remove from this collection
+   * @return true if the remove operation caused the Collection to change, or
+   *   equivalently if the collection did contain o.
+   * @exception UnsupportedOperationException if this collection's Iterator
+   *   does not support the remove method
+   */
+  public boolean remove(Object o) {
+    Iterator i = iterator();
+
+    // This looks crazily inefficient, but it takes the test o==null outside
+    // the loop, saving time, and also saves needing to store the result of
+    // i.next() each time.
+    if (o == null) {
+      while (i.hasNext()) {
+        if (i.next() == null) {
+          i.remove();
+	  return true;
+        }
+      }
+    } else {
+      while (i.hasNext()) {
+        if (o.equals(i.next())) {
+          i.remove();
+          return true;
+        }
+      }
+    }
+    return false;
+  }
+
+  /**
+   * Remove from this collection all its elements that are contained in a given
+   * collection. This implementation iterates over this collection, and for
+   * each element tests if it is contained in the given collection. If so, it
+   * is removed by the Iterator's remove method (thus this method will fail
+   * with an UnsupportedOperationException if the Iterator's remove method
+   * does).
+   *
+   * @param c the collection to remove the elements of
+   * @return true if the remove operation caused the Collection to change
+   * @exception UnsupportedOperationException if this collection's Iterator
+   *   does not support the remove method
+   */
+  public boolean removeAll(Collection c) {
+    Iterator i = iterator();
+    boolean changed = false;
+    while (i.hasNext()) {
+      if (c.contains(i.next())) {
+        i.remove();
+        changed = true;
+      }
+    }
+    return changed;
+  }
+
+  /**
+   * Remove from this collection all its elements that are not contained in a
+   * given collection. This implementation iterates over this collection, and
+   * for each element tests if it is contained in the given collection. If not,
+   * it is removed by the Iterator's remove method (thus this method will fail
+   * with an UnsupportedOperationException if the Iterator's remove method
+   * does).
+   *
+   * @param c the collection to retain the elements of
+   * @return true if the remove operation caused the Collection to change
+   * @exception UnsupportedOperationException if this collection's Iterator
+   *   does not support the remove method
+   */
+  public boolean retainAll(Collection c) {
+    Iterator i = iterator();
+    boolean changed = false;
+    while (i.hasNext()) {
+      if (!c.contains(i.next())) {
+        i.remove();
+        changed = true;
+      }
+    }
+    return changed;
+  }
+
+  /**
+   * Return an array containing the elements of this collection. This
+   * implementation creates an Object array of size size() and then iterates
+   * over the collection, setting each element of the array from the value
+   * returned by the iterator.
+   *
+   * @return an array containing the elements of this collection
+   */
+  public Object[] toArray() {
+    Object[] a = new Object[size()];
+    Iterator i = iterator();
+    for (int pos = 0; pos < a.length; pos++) {
+      a[pos] = i.next();
+    }
+    return a;
+  }
+
+  /**
+   * Copy the collection into a given array if it will fit, or into a
+   * dynamically created array of the same run-time type as the given array if
+   * not. If there is space remaining in the array, the first element after the
+   * end of the collection is set to null (this is only useful if the
+   * collection is known to contain no null elements, however). This
+   * implementation first tests whether the given array is large enough to hold
+   * all the elements of the collection. If not, the reflection API is used to
+   * allocate a new array of the same run-time type. Next an iterator is
+   * obtained over the collection and the elements are placed in the array as
+   * they are returned by the iterator. Finally the first spare element, if
+   * any, of the array is set to null, and the created array is returned.
+   *
+   * @param a the array to copy into, or of the correct run-time type
+   * @return the array that was produced
+   * @exception ClassCastException if the type of the array precludes holding
+   *   one of the elements of the Collection
+   */
+  public Object[] toArray(Object[] a) {
+    final int n = size();
+    if (a.length < n) {
+      a = (Object[])Array.newInstance(a.getClass().getComponentType(), n);
+    }
+    Iterator i = iterator();
+    for (int pos = 0; pos < n; pos++) {
+      a[pos] = i.next();
+    }
+    if (a.length > n) {
+      a[n] = null;
+    }
+    return a;
+  }
+
+  /**
+   * Creates a String representation of the Collection. The string returned is
+   * of the form "[a, b, ...]" where a and b etc are the results of calling
+   * toString on the elements of the collection. This implementation obtains an
+   * Iterator over the Collection and adds each element to a StringBuffer as it
+   * is returned by the iterator.
+   *
+   * @return a String representation of the Collection
+   */
+  public String toString() {
+    StringBuffer s = new StringBuffer();
+    s.append('[');
+    Iterator i = iterator();
+    boolean more = i.hasNext();
+    while(more) {
+      s.append(i.next());
+      if (more = i.hasNext()) {
+        s.append(", ");
+      }
+    }
+    s.append(']');
+    return s.toString();
+  }
+}