java.lang.Object
    java.util.AbstractCollection<E>
        java.util.AbstractList<E>
            java.util.ArrayList<E>

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
}

java.lang.Object
    java.lang.String

public final class String
extends Object
implements Serializable, Comparable<String>, CharSequence

/** The value is used for character storage. */
private final char value[];

    public String(String original) {
        this.value = original.value;
        this.hash = original.hash;
    }

public String(char value[]) {
    this.value = Arrays.copyOf(value, value.length);
}

    public String(StringBuffer buffer) {
        synchronized(buffer) {
            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
        }
    }

   public String(StringBuilder builder) {
        this.value = Arrays.copyOf(builder.getValue(), builder.length());
    }

java.lang.Object
    java.util.AbstractCollection<E>
        java.util.AbstractSet<E>
            java.util.HashSet<E>

public class HashSet<E>
extends AbstractSet<E>
implements Set<E>, Cloneable, Serializable

// Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

/**
 * Adds the specified element to this set if it is not already present.
 * More formally, adds the specified element <tt>e</tt> to this set if
 * this set contains no element <tt>e2</tt> such that
 * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
 * If this set already contains the element, the call leaves the set
 * unchanged and returns <tt>false</tt>.
 *
 * @param e element to be added to this set
 * @return <tt>true</tt> if this set did not already contain the specified
 * element
 */
public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
}

    private static final long serialVersionUID = 876323262645176354L;

    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out array length
        s.writeInt(elementData.length);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++)
            s.writeObject(elementData[i]);

        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }

    }

    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out size
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (Node<E> x = first; x != null; x = x.next)
            s.writeObject(x.item);
    }

    private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }