Vectors Jan 14 2019 Vectors and arrays A Vector is like an array of Objects Differences between arrays and Vectors Arrays have special syntax Vectors don t 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 don t need to know the size beforehand 2 Creating a Vector import java util Vector vec1 new Vector Vector vec2 new Vector initialSize Vector vec3 new Vector initialSize increment 3 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 4 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 5 Accessing elements of a Vector Object elementAt int index or Object get int index Object firstElement Returns the component at position index elementAt is an older method retained for compatibility with older programs Returns the component at location 0 Object lastElement Returns the last component 6 Searching a Vector I boolean contains Object element int indexOf Object element Tests if element is a component of this Vector 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 7 Searching a Vector II int lastIndexOf Object element int lastIndexOf Object element int index Returns the index of the last occurrence of element in this Vector Returns 1 if element was not found in this Vector 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 8 Getting information about a Vector boolean isEmpty int size Returns true if this Vector has no elements 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 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 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 10 A minor nuisance Suppose you define You can do bunny vec get 0 Instead you have to do vec add bunny But you cannot do Vector vec new Vector Rabbit bunny new Rabbit bunny Rabbit vec get 0 Vectors are defined to hold Objects when you get something out Java doesn t know what kind you expect it to be 11 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 12 Conclusion A Vector is like an array of Objects 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 13 The End 14
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