add a few classes and methods to the classpath

Add java.lang.CharSequence, java.util.AbstractSet,
java.util.AbstractCollection, Collections.unmodifiableSet,
 System.getProperty(String,String), etc.

classpath/java/util/AbstractCollection.java

@@ -0,0 +1,112 @@
+/* Copyright (c) 2008, Avian Contributors
+
+   Permission to use, copy, modify, and/or distribute this software
+   for any purpose with or without fee is hereby granted, provided
+   that the above copyright notice and this permission notice appear
+   in all copies.
+
+   There is NO WARRANTY for this software.  See license.txt for
+   details. */
+
+package java.util;
+
+/**
+ * @author zsombor
+ * 
+ */
+public abstract class AbstractCollection<T> implements Collection<T> {
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.util.Collection#add(java.lang.Object)
+	 */
+	public boolean add(T element) {
+		throw new UnsupportedOperationException("adding to "
+				+ this.getClass().getName());
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.util.Collection#addAll(java.util.Collection)
+	 */
+	public boolean addAll(Collection<? extends T> collection) {
+		boolean result = false;
+		for (T obj : collection) {
+			result |= add(obj);
+		}
+		return result;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.util.Collection#clear()
+	 */
+	public void clear() {
+		throw new UnsupportedOperationException("clear() in "
+				+ this.getClass().getName());
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.util.Collection#contains(java.lang.Object)
+	 */
+	public boolean contains(T element) {
+		if (element != null) {
+			for (Iterator<T> iter = iterator(); iter.hasNext();) {
+				if (element.equals(iter.next())) {
+					return true;
+				}
+			}
+		} else {
+			for (Iterator<T> iter = iterator(); iter.hasNext();) {
+				if (iter.next()==null) {
+					return true;
+				}
+			}
+			
+		}
+		return false;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.util.Collection#isEmpty()
+	 */
+	public boolean isEmpty() {
+		return size() == 0;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.util.Collection#remove(java.lang.Object)
+	 */
+	public boolean remove(T element) {
+		throw new UnsupportedOperationException("remove(T) in "
+				+ this.getClass().getName());
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.util.Collection#size()
+	 */
+	public abstract int size();
+
+	/*
+	 * (non-Javadoc)
+	 * 
+	 * @see java.util.Collection#toArray(S[])
+	 */
+	public <S> S[] toArray(S[] array) {
+		return Collections.toArray(this, array);
+	}
+
+	public abstract Iterator<T> iterator();
+
+}