ArrayLists

자바의 ArrayList에 대해 정리하였습니다.

 

 

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< Introduction >

When we work with arrays in Java, we’ve been limited by the fact that once an array is created, it has a fixed size. We can’t add or remove elements.

But with ArrayList, we can create mutable and dynamic lists.

 

ArrayList allow us to:

• Store object references as elements
• Store elements of the same type (just like arrays)
• Access elements by index (just like arrays)
• Add elements
• Remove elements

 

To use an ArrayList, we need to import them from Java’s util package as well:

import java.util.ArrayList;

 

 

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< Creating ArrayLists >

< Syntax >

// Declaring:
ArrayList<DataType> VariableName;
// Initializing:
VariableName = new ArrayList<DataType>();
 
// Declaring and initializing in one line:
ArrayList<DataType> VariableName = new ArrayList<DataType>();

• Use angle brackets < > to declare the type of the ArrayList.
  : These symbols are used for generics. Generics are a Java construct that allows us to define classes and objects as parameters(data-type) of an ArrayList.
• For this reason, we can’t use primitive types in an ArrayList
  : alternatively, use <Integer>, <Double>, and <Character> generic, etc.

 

Example)

import java.util.ArrayList;

class ToDos {
  public static void main(String[] args) {
    ArrayList<String> toDoList = new ArrayList<String>();
  }
}

 

 

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< Adding an Item >

To insert an element using add( ) method.

 

• add( ) method need two arguments
  1. index of the new element (optional)
  2. value of the new element

 

Example)

ArrayList<Car> carShow = new ArrayList<Car>();
 
carShow.add(ferrari);
// carShow now holds [ferrari]
carShow.add(thunderbird);
// carShow now holds [ferrari, thunderbird]
carShow.add(1, corvette);
// carShow now holds [ferrari, corvette, thunderbird]

 

+. It is possible to create an ArrayList that holds values of different types.

ArrayList assortment = new ArrayList<>();
assortment.add("Hello"); // String
assortment.add(12); // Integer
assortment.add(ferrari); // reference to Car
// assortment holds ["Hello", 12, ferrari]

: In the above snippet, assortment is an ArrayList that can store different values because we do not specify its type during initialization.

: In this case, the items stored in this ArrayList will be considered Objects

 

 

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< ArrayList Size >

To know ArrayList's length using size( ) method.

 

Example)

ArrayList<String> SangHunsToDos = new ArrayList<String>();

SangHunsToDos.add("Study Java");
SangHunsToDos.add("Study English");
SangHunsToDos.add("Go to Travel!");

SangHunsToDos.size();   //   3

 

 

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< Accessing an Index >

Use the method get( ) to access an index instead of [ ]

 

Example)

ArrayList<String> SangHunsToDos = new ArrayList<String>();

SangHunsToDos.add("Study Java");
SangHunsToDos.add("Study English");
SangHunsToDos.add("Go to Travel!");

SangHunsToDos.get(2);   //   Go to Travel!

 

 

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< Changing a Value >

Use the set( ) method.

 

• set( ) method need two arguments
  1. index of the element
  2. value of the element

 

Example)

ArrayList<String> SangHunsToDos = new ArrayList<String>();

SangHunsToDos.add("Study Java");
SangHunsToDos.add("Study English");
SangHunsToDos.add("Go to Travel!");
SangHunsToDos.set(0, "Study HTML");

// ["Study HTML", "Study English", "Go to Travel!"]

 

 

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< Removing an Item >

To remove an element using remove( ) method.

 

• remove( ) method need one parameter :
  1. index : the element's index to remove. OR
  2. element : the element's value to remove.

 

Example)

ArrayList<String> SangHunsToDos = new ArrayList<String>();

SangHunsToDos.add("Study Java");
SangHunsToDos.add("Study English");
SangHunsToDos.add("Go to Travel!");
SangHunsToDos.remove(0);
SangHunsToDos.remove("Study English");

// ["Go to Travel!"]

 

 

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< Getting an Item's Index >

To know the position of a certain element using indexOf( ) method.

 

Example)

// ["Holmes", "Poirot", "Marple", "Spade", "Fletcher", "Conan", "Ramotswe"];
System.out.println(detectives.indexOf("Fletcher"));   //   4