Why Transient In Arraylist

/** 
     * Save the state of the <tt>ArrayList</tt> instance to a stream (that 
     * is, serialize it). 
     * 
     * @serialData The length of the array backing the <tt>ArrayList</tt> 
     *             instance is emitted (int), followed by all of its elements 
     *             (each an <tt>Object</tt>) in the proper order. 
     */  
    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();  
        }  

    }  

/**
     * Save the state of the {@code Vector} instance to a stream (that
     * is, serialize it).
     * This method performs synchronization to ensure the consistency
     * of the serialized data.
     */
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        final java.io.ObjectOutputStream.PutField fields = s.putFields();
        final Object[] data;
        synchronized (this) {
            fields.put("capacityIncrement", capacityIncrement);
            fields.put("elementCount", elementCount);
            data = elementData.clone();
        }
        fields.put("elementData", data);
        s.writeFields();
    }