1. 25-Jun-15 Vectors

2. 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. 3 Creating a Vector import java.util.*; Vector vec1 = new Vector(); Vector vec2 = new Vector( initialSize ); Vector vec3 = new Vector( initialSize, increment );

4. 4 Adding elements to a Vector boolean add(Object obj ) Appends the object obj to the end of this Vector Always returns true This is for consistency with other, similar classes void add(int index, Object element ) Inserts the element at position index in this Vector The index must be greater than or equal to zero and less than or equal to the number of elements in the Vector

5. 5 Removing elements from a Vector boolean remove(Object obj ) Removes the first occurrence of obj from this Vector Returns true if an element was removed void remove(int index ) Removes the element at position index from this Vector void removeAllElements() Removes all elements

6. 6 Accessing elements of a Vector Object elementAt(int index ) or Object get(int index ) Returns the component at position index elementAt is an older method, retained for compatibility with older programs Object firstElement() Returns the component at location 0 Object lastElement() Returns the last component

7. 7 Searching a Vector I boolean contains(Object element ) Tests if element is a component of this Vector int indexOf(Object element ) Returns the index of the first occurrence of element in this Vector Returns -1 if element was not found in this Vector int indexOf(Object element, int index ) Returns the index of the first occurrence of element in this Vector, beginning the search at index Returns -1 if element was not found in this Vector

8. 8 Searching a Vector II int lastIndexOf(Object element ) Returns the index of the last occurrence of element in this Vector Returns -1 if element was not found in this Vector int lastIndexOf(Object element, int index ) Returns the index of the last occurrence of element in this Vector, searching backward from index Returns -1 if element was not found in this Vector All searching is done using equals

9. 9 Getting information about a Vector boolean isEmpty() Returns true if this Vector has no elements int size() Returns the number of elements currently in this Vector Object[ ] toArray() Returns an array containing all the elements of this Vector in the correct order

10. 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 public 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 If you do this, note that the argument should be a general Object The String class (and some others) override equals

11. 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.get(0); Instead, you have to do bunny = (Rabbit)vec.get(0); Vectors are defined to hold Object s; when you get something out, Java doesn’t know what kind you expect it to be

12. 12 Fixing the nuisance You can extend Vector and override whatever methods you choose class RabbitVector extends Vector { Rabbit elementAt(int i) { return (Rabbit)super.get(i); } } Now you can do Vector vec = new RabbitVector(); vec.add(bunny); bunny = vec.get(0);

13. 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. 14 The End