ArrayLists

1
ArrayLists
2
ArrayLists and arrays

• A ArrayList is like an array of Objects
• Differences between arrays and ArrayLists
• Arrays have special convenience syntax
  ArrayLists dont
• An array is a fixed size, but a ArrayList expands
  as you add things to it
• This means you dont need to know the size
  beforehand
• Arrays can hold primitives or objects, but
  ArrayLists can only hold objects
• However, autoboxing can make it appear that an
  ArrayList can hold primitives
• We will discuss autoboxing in some other lecture

3
Creating a ArrayList

• Specify, in angle brackets after the name, the
  type of object that the class will hold
• Examples
• ArrayListltStringgt vec1 new ArrayListltStringgt()
• ArrayListltStringgt vec2 new ArrayListltStringgt(10)
  
• (10 is the initial size you want)
• This is sort of like an array, where you would
  say String ary new String10instead
  of ArrayListltStringgt ary new
  ArrayListltStringgt(10)

4
Adding elements to a ArrayList

• boolean add(Type obj)
• Appends the object obj to the end of this
  ArrayList
• The obj must be of the correct type, or you get a
  compile-time (syntax) error
• Always returns true
• This is for consistency with other, similar
  classes
• void add(int index, Type element)
• Inserts the element at position index in this
  ArrayList
• The index must be greater than or equal to zero
  and less than or equal to the number of elements
  in the ArrayList
• The obj must be of the correct type, or you get a
  compile-time (syntax) error

5
Removing elements

• boolean remove(Object obj)
• Removes the first occurrence of obj from this
  ArrayList
• Returns true if an element was removed
• Uses equals to test if it has found the correct
  element
• void remove(int index)
• Removes the element at position index from this
  ArrayList
• void clear()
• Removes all elements

6
Getting values out

• Type get(int index)
• Returns the component at position index
• Using get
• ArrayListltStringgt myList new ArrayListltStringgt()
  myList.add("Some string")String s
  myList.get(0)

7
Searching a ArrayList

• boolean contains(Object element)
• Tests if element is a component of this ArrayList
• Uses equals to test if it has found the correct
  element
• int indexOf(Object element)
• Returns the index of the first occurrence of
  element in this ArrayList
• Uses equals to test if it has found the correct
  element
• Returns -1 if element was not found in this
  ArrayList
• int lastIndexOf(Object element)
• Returns the index of the last occurrence of
  element in this ArrayList
• Uses equals to test if it has found the correct
  element
• Returns -1 if element was not found in this
  ArrayList

8
Getting information

• boolean isEmpty()
• Returns true if this ArrayList has no elements
• int size()
• Returns the number of elements currently in this
  ArrayList
• Type toArray(Type )
• Returns an array containing all the elements of
  this ArrayList in the correct order
• The parameter is usually a zero-length array of
  the correct type
• Example String names nameList.toArray(new
  String0)

9
More about equals

• There are many different notions of equality
• Example two sets are equal if they contain the
  same elements order of elements is irrelevant
• Java defines public boolean equals(Object) in the
  Object class, but
• equals is defined to be the same as
• Its often a good idea to override equals for
  your own objects
• If you do this, note that the argument should be
  a general Object
• The String class (and some others) override
  equals

10
Conclusion

• A ArrayList is like an array of Objects
• The advantage of a ArrayList is that you dont
  need to know beforehand how big to make it
• The disadvantage of a ArrayList is that you cant
  use the special syntax for arrays
• You should never use an array that you hope is
  big enoughuse a ArrayList instead

11
The End
Where a calculator on the ENIAC is equipped with
18 000 vacuum tubes and weighs 30 tons, computers
of the future may have only 1 000 vacuum tubes
and perhaps weigh 1½ tons.
--Popular Mechanics, March 1949