Vectors

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Vectors
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Vectors and arrays

• A Vector is like an array of Objects
• Differences between arrays and Vectors
• Arrays have special syntax Vectors dont
• You can have an array of any type, but a Vector
  holds Objects
• An array is a fixed size, but a Vector expands as
  you add things to it
• This means you dont need to know the size
  beforehand

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Creating a Vector

• import java.util.
• Vector vec1 new Vector()
• Vector vec2 new Vector(initialSize)
• Vector vec3 new Vector(initialSize,
  increment)

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Adding elements to a Vector

• boolean add(Object o)
• Appends the specified element to the end of this
  Vector. Always returns true.
• void add(int index, Object element)
• Inserts the specified element at the specified
  position in this Vector. The index must be 0
  and

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Removing elements from a Vector

• boolean remove(Object o)
• Removes the first occurrence of the specified
  element from this Vector returns true if an
  element was removed
• void remove(int index)
• Removes the element at the specified position in
  this Vector
• void removeAllElements()
• Removes all elements

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Accessing elements of a Vector

• Object elementAt(int index) orObject get(int
  index)
• Returns the component at the specified index
• Object firstElement()
• Returns the component at location 0
• Object lastElement()
• Returns the last component

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Searching a Vector I

• boolean contains(Object elem)
• Tests if the specified object is a component of
  this Vector
• int indexOf(Object elem)
• Returns the index of the first occurrance of elem
  in this Vector, or -1 if not found
• int indexOf(Object elem, int index)
• Returns the index of the first occurrance of elem
  in this Vector, beginning the search at index or
  -1 if not found

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Searching a Vector II

• int lastIndexOf(Object elem)
• Returns the index of the last occurrance of elem
  in this Vector, or -1 if not found
• int lastIndexOf(Object elem, int index)
• Returns the index of the last occurrance of elem
  in this Vector, searching backward from index or
  -1 if not found
• All searching is done using equals

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Getting information about a Vector

• boolean isEmpty()
• Tests if this Vector has no elements
• int size()
• Returns the number of elements in this Vector
• Object toArray()
• Returns an array containing all the elements of
  this Vector in the correct order

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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 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
• The String class (and some others) override
  equals

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A minor nuisance

• Suppose you define
• Vector vec new Vector()
• Rabbit bunny new Rabbit()
• You can do
• vec.add(bunny)
• But you cannot do
• bunny vec.elementAt(0)
• Instead, you have to do
• bunny (Rabbit)vec.elementAt(0)

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Fixing the nuisance

• class RabbitVector extends Vector Rabbit
  elementAt(int i) return
  (Rabbit)super.elementAt(i)
• Now you can do
• Vector vec new RabbitVector()
• vec.add(bunny)
• bunny vec.elementAt(0)

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Conclusion

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

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The End