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Data Structures

-- Additional Documentation

Collections class
Iterator interface
Comparator interface

Array - Array Documentation

In Java Arrays are orderable unresizeable collections of data of the same type

Declaration

String[] stringArr = [15]; <-- Type (String arr), name, assignment (=), size of array [15];

Access

String str = stringArr[1];
stringArr[2] = "newString";

Iterate

for(String s: stringArr) (for element of type String in the stringArr Array)
{
   System.out.println;
}

ArrayList ArrayList Documentation

In Java ArrayLists are orderable resizable collections of data of the same type

Required Imports

  • java.util.List
  • java.util.ArrayList

Declaration

List<int> numList = new ArrayList<>(); /* Type (List of type int), name, (=), instantiate ArrayList class object
                                       (shares type implicitly) */

Access

numList.get(index);

Append

numList.add(1);  // add to end of list 
numList.add(index, 1); // insert at specific index

Change

numList.set(index, newValue);

Iterate

numList.forEach(ele -> System.out.println(ele)); /* forEach element in numList print ele 
                                                 uses a lambda expression to facilitate iteration */

SIDE NOTE - Lambda Expression

The simplest Lambda expression contains a single parameter and an expression
parameter -> expression

To use more then one parameter wrap them in parentheses
(param1, param2) -> expression

expressions must immediatley return a value if you need to perform additional operations in a Lambda expression encapsulate it inside of a code block {}
parameter -> {...code}

The first parameter in the ArrayList forEach method takes a Consumer interface class that effectively returns each element in the list and allows you to perform operations on each element for each cycle. This interface's functionality can be utilized via a Lambda Expression formatted in the same form as the interface (ie. same number of parameters, same return type) (more on interfaces later)

Sort (Low -> High)

import java.util.Collections

    .....

    Collections.sort(numList);

HashMaps - HashMap Documentation

Hashmaps are collections of pieces of data organized in <key,value> (Entries) pairs this data structure is of flexible size

Required Imports

  • java.util.Map;
  • java.util.HashMap;

Declare

``java Map<String, Integer> hashMap = new HashMap<>(): /* Type(Map key of type String value of type Integer) name (=) new HashMap class object */

#### Append

```java
hashMap.put("key" value);

Search for keys or values

hashMap.containsKey("key");
hashMap.containsValue(value);

Remove <key,value> pair (Entry)

hashMap.remove("key");

Empty HashMap

hashMap.clear();

Iterate

for(HashMap.Entry mapElem: hashMap.entrySet()) /* Sets type of iterand to Entry, 
                                                grabs set of entries from hashMap  */
{
  System.out.println(mapElem.getKey()); //reference key for each entry in hashMap
  System.out.println(mapElem.getValue()); //reference value for each entry in hashMap
}

Sort

HashMaps are unsortable by themselves the entries must be transferred to another data structure in order to be sorted

import java.util.Comparator;
import java.util.ArrayList;
import java.util.List;

....

List<HashMap.Entry> entryList = new ArrayList<>(hashMap.entrySet()); 

entryList.sort(Comparator.comparing(entry -> entry.getKey().toString()));


/* NOTES FOR ABOVE CODE

  Create an ArrayList class object of type HashMap.Entry initilize with the set 
  of entries from your hashMap object

  Call ArrayList's sort method passing in a Comparator interface (facilitates 
  sorting by comparing elements in the list) call the comparing method and in that 
  method use a Lambda expression to set what the sort will compare against 
  whilst sorting in this case the key of each entry cast to a String type   */

HashSets - HashSet documentation

Hashsets are a collection of pieces of data stored as values of flexible size where each value is unique

Required Imports

  • Java.util.Set;
  • Java.util.HashSet;

Declare

Set<String> stringHashSet = new HashSet<>(); /* (type set with elements type string)
                                                 name = new HashSet class object */

Append

stringHashSet.add("value");

Remove

stringHashSet.remove("value");

Empty HashSet

stringHashSet.clear();

Iterate

import java.util.Iterator;

....

Iterator<String> iter = stringHashSet.iterator(); /* declare Iterator of same type as HashSet
                                                  name = iterator returned by HashSet iterator method */

/* Iterator is an interface which takes an iterable data structure and provides methods to move through its elements 
   one at at time */

while(iter.hasNext()) // while there is another element in the data structure
{
  String next = i.next(); /* grabs next value in data structure starts at first value 
                              moves the iterator to the next element in the data structure 
                              in each loop (moves while loop to end condition) */
  System.out.println(next);
}

Sort

HashSets are not sortable on their own and must be converted to another data structure in order to be sorted

import java.util.List;
import java.util.ArrayList;
import java.util.Collections;

...

List<String> setList = new ArrayList<>(stringHashSet); /* List of same type as HashSet
                                                       name = new ArrayList class object 
                                                       initilized with values in HashSet */

Collections.sort(setList); /* utilizes the Collections class's sort method to sort the list
                               in ascending order */