Vectors 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 Creating a Vector import java util Vector vec1 new Vector Vector vec2 new Vector initialSize Vector vec3 new Vector initialSize increment Adding elements to a Vector boolean add Object o Appends the object o 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 Removing elements from a Vector boolean remove Object o Removes the first occurrence of o 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 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 Searching a Vector I boolean contains Object elem Tests if elem is a component of this Vector int indexOf Object elem Returns the index of the first occurrence of elem in this Vector Returns 1 if elem was not found in this Vector int indexOf Object elem int index Returns the index of the first occurrence of elem in this Vector beginning the search at index Returns 1 if elem was not found in this Vector Searching a Vector II int lastIndexOf Object elem Returns the index of the last occurrence of elem in this Vector Returns 1 if elem was not found in this Vector int lastIndexOf Object elem int index Returns the index of the last occurrence of elem in this Vector searching backward from index Returns 1 if elem was not found in this Vector All searching is done using equals Getting information about a Vector boolean isEmpty Returns true 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 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 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 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 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 The End
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