Java

Data structures

• List - abstract list
  • ArrayList - concrete list implemented as an array
• Map - abstract map
  • Hashmap - map using hash table
• Set - set interface

When creating an object which is inherited from an abstract type (e.g., Map), make sure to initialize using the abstract class, instantiate (w/ ‘new’) with the concrete type (e.g., HashMap). This allows the flexibility of using, for example, a Map as a parameter, but being able to pass either a TreeMap or a HashMap in. Always use the highest-level interface possible.

Watch out for terms like “returns a view” or “backed by”. For instance, the sublist function, which returns part of an array, doesn’t return a copy of part of the array, it returns a “view” of part of the array. This is faster, but watch for mutability/concurrency issues.

Program structure

Classpaths

• Classpaths specify what libraries are available at runtime
  • Easiest just to append to the system/user variable “CLASSPATH”, as in CLASSPATH = “C:\Program Files (x86)\Weka-3-7\*”
  • Can also use at the command line like “java -cp “Test.jar;lib/*” my.package.MainClass”

File types

• .jar: library
• .class: virtual machine compiled code (java bytecode)
• .java: source code

Java streams and readers

• Stream = flow of data
• Byte stream is lowest level
• Can use a filereader to analyze character-by-character
• Buffered reader allows line-by-line access (takes filereader as input). It also buffers the reads so that each read or readline doesn’t cause a read to/from disk.
• Can then wrap a scanner around a buffered reader to look for individual tokens (can use scanner.useDelimiter)
• Verbose summary:
  • A stream is a way of sequentially accessing a file. A byte stream access the file byte by byte. A byte stream is suitable for any kind of file, however not quite appropriate for text files. For example, if the file is using a unicode encoding and a character is represented with two bytes, the byte stream will treat these separately and you will need to do the conversion yourself.
  • A character stream will read a file character by character. A character stream needs to be given the file’s encoding in order to work properly.

Iterating through hashes

// Just the keys:
for (String key : map.keySet()) {
    // ...
}

// Just the values:
for (Object value : map.values()) {
    // ...
}

// Keys and values:
for (Map.Entry<String, Object> entry : map.entrySet()) {
    String key = entry.getKey();
    Object value = entry.getValue();
    // ...
}