Java Collections - Part 2: Object Ordering

This article is a part of the Java Programming Language article series.

• List<String>: alphabetical
• List<Date>: chronological
• String and Date implements Comparable interface that provides natural ordering

Classes Implementing Comparable

Throws a ClassCastException

• Collections.sort(list): objects that do not implement Comparable
• Collections.sort(list, comparator): objects cannot be compared to one another using the comparator

Custom Comparable Types

The Comparable Interface

Custom Type

Program

Comparators

• Sort objects in an order other than their natural ordering
• That don’t implement Comparable

Comparator Interface

Implementation

Program

SortedSet

• A Set that maintains its elements in ascending order
• Provides additional ordering operations: range view, endpoints, comparator access
• Range view: arbitrary range operations
• Endpoints: first or last element
• Comparator access: returns the Comparator

Range-view Operations

Endpoint Operations

Comparator Accessor

• Copy a sorted set with the same ordering

SortedMap

• A Map that maintains its mappings in ascending key order
• Dictionaries, telephone directories
• Provides additional ordering operations: range view, endpoints, comparator access
• Range view: arbitrary range operations
• Endpoints: first or last element
• Comparator access: returns the Comparator