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Chapter 12



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  • 1. Collections Chapter 12
  • 2. 12 Java Collection Frameworks The Java collection framework is a set of utility classes and interfaces. Designed for working with collections of objects A collection is a group of objects, each of which is known as an element in the collection.
  • 3. 12 Types of Collections Simple Collections Sets have no duplicate elements. Lists are ordered collections that can have duplicate elements. Maps Map uses key/value pairs to associate an object (the value) with a key. Both sets and maps can be sorted or unsorted.
  • 4. 12 The Collection Interface Root interface of collection classes Common methods defined in Collection: add() contains() isEmpty() iterator() remove() size()
  • 5. 12 The ArrayList ArrayList is a concrete implementation of the List interface. A concrete implementation is a class that can be instantiated.
  • 6. 12 ArrayList vs. an Array An ArrayList has several advantages: Can be resized dynamically as more objects are added Elements can be removed from an ArrayList Simpler to use than an array
  • 7. 12 Controlling ArrayList size Important methods: size() returns the number of elements ensureCapacity() sets the capacity of the ArrayList to a certain number of elements trimToSize() trims an ArrayList to only the number of elements it currently holds to reduce memory requirements
  • 8. 12 Using Collections Although objects in a collection do not have to be of the same type: It is not good practice to mix object types in a collection. You must check object type and cast to appropriate class if heterogeneous collections are used.
  • 9. 12 Adding Elements to a Collection add() method is overloaded myList.add(myObject) adds myObject to end of collection myList.add(3, myObject) inserts myObject at offset 3 in a collection
  • 10. 12 Replacing and Removing Elements To replace and remove elements in an ArrayList: set() method replaces an element at a specified position remove() method removes an element at a specified position