1. Vectors

2. Vectors and arrays A Vector is like an array of Object s Differences between arrays and Vector s: –Arrays have special syntax; Vector s don’t –You can have an array of any type, but a Vector holds Object s –An array is a fixed size, but a Vector expands as you add things to it This means you don’t need to know the size beforehand

3. Creating a Vector import java.util.*; Vector vec1 = new Vector(); Vector vec2 = new Vector(initialSize); Vector vec3 = new Vector(initialSize, increment);

4. 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 <= number of elements in the Vector.

5. 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

6. Accessing elements of a Vector Object elementAt(int index) or Object get(int index) –Returns the component at the specified index Object firstElement() –Returns the component at location 0 Object lastElement() –Returns the last component

7. 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

8. 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

9. 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

10. 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 == –It’s often a good idea to override equals for your own objects –The String class (and some others) override equals

11. 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);

12. 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);

13. Conclusion A Vector is like an array of Object s The advantage of a Vector is that you don’t need to know beforehand how big to make it The disadvantage of a Vector is that you can’t use the special syntax for arrays You should never use an array that you hope is “big enough”--use a Vector instead

14. The End