CS230 ObjectList Contract

The ObjectList class describes immutable linked lists of objects. An immutable object list is defined recursively as either

• the empty list, or
• a (non-empty) immutable list node with a head component that is an object and a tail component that is another object list. The list node is immutable in the sense that its head and tail components cannot be changed.

Object lists are immutable in the sense that the head or tail of a list node cannot be changed after the list node has been created (via the prepend() method).

There are no public constructor methods for immutable object lists, and only one public instance method (toString()). Instead, object lists are created and manipulated via the class methods specified below. By default, an ObjectList class method is invoked via ObjectList.methodName(args), but this can be abbreviated to OL.methodName(args) by including the following "magical" declaration in the body of a class:

public static ObjectList OL; // declare but never initialize this variable.

Class Methods

public static ObjectList empty()
Returns an empty object list.

public static ObjectList prepend (Object x, ObjectList L)
Returns a new object list node whose head is x and whose tail is L. From another perspective, returns a list whose length is one more than the length of L and whose elements are x followed by the elements of L, in order.

public static boolean isEmpty (ObjectList L)
Returns true if L is an empty object list and false if L is an object list node.

public static Object head (ObjectList L)
Returns the object that is the head component of the object list node L. Signals an exception if L is empty.

public static ObjectList tail (ObjectList L)
Returns the object list that is the tail component of the object list node L. Signals an exception if L is empty.

public static String toString (ObjectList L)
Returns a string representation of the object list L in which the string representations of the list elements are separated by commas and delimited by square brackets. For example, if L1 is a list containing the sequence of strings "Sam", "I", "am", then toString(L1) returns "(Sam,I,am)".

public static ObjectList fromString (String s)
If s is the printed representation of an object list (i.e., comma separated elements delimited by parentheses), returns an ObjectList containing the strings of each element. For example, fromString("(Sam,I,am)") returns a 3-element object list with elements "Sam", "I", and "am". If s is not the printed representation of an object list, a RuntimeException is thrown.