Initializing ArrayList with some predefined values

Question

I have an sample program as shown   I want my ArrayList symbolsPresent to be initialized with some predefined symbols  ONE  TWO  THREE  and FOUR   symbolsPresent add  ONE    symbolsPresent add  TWO    symbolsPresent add  THREE    symbolsPresent add  FOUR        import java util ArrayList   public class Test        private ArrayList lt String gt  symbolsPresent   new ArrayList lt String gt          public ArrayList lt String gt  getSymbolsPresent             return symbolsPresent             public void setSymbolsPresent ArrayList lt String gt  symbolsPresent            this symbolsPresent   symbolsPresent             public static void main String args                  Test t   new Test            System out println  Symbols Present is    t symbolsPresent                  Is that possible

User · Answer

How about using overloaded ArrayList constructor    private ArrayList lt String gt  symbolsPresent   new ArrayList lt String gt  Arrays asList new String     One   Two   Three   Four

User · Answer

Double brace initialization is an option   List lt String gt  symbolsPresent   new ArrayList lt String gt          add  ONE       add  TWO       add  THREE       add  FOUR          Note that the String generic type argument is necessary in the assigned expression as indicated by JLS   15 9     It is a compile-time error if a class instance creation expression declares an anonymous class using the   lt    form for the class s type arguments

User · Answer

I would suggest to use Arrays asList   for single line initialization  For different ways of declaring and initializing a List you can also refer Initialization of ArrayList in Java

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Personnaly I like to do all the initialisations in the constructor  public Test       symbolsPresent   new ArrayList lt String gt       symbolsPresent add  ONE      symbolsPresent add  TWO      symbolsPresent add  THREE      symbolsPresent add  FOUR        Edit   It is a choice of course and others prefer to initialize in the declaration  Both are valid  I have choosen the constructor because all type of initialitions are possible there  if you need a loop or parameters        However I initialize the constants in the declaration on the top on the source  The most important is to follow a rule that you like and be consistent in our classes

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public static final List lt String gt  permissions   new ArrayList lt String gt           add  public profile        add  email        add  user birthday        add  user about me        add  user location        add  user likes        add  user posts

User · Answer

If you just want to initialize outside of any method then use the initializer blocks    import java util ArrayList   public class Test   private ArrayList lt String gt  symbolsPresent   new ArrayList lt String gt         All you need is this block    symbolsPresent   new ArrayList lt String gt     symbolsPresent add  ONE    symbolsPresent add  TWO    symbolsPresent add  THREE    symbolsPresent add  FOUR        public ArrayList lt String gt  getSymbolsPresent         return symbolsPresent     public void setSymbolsPresent ArrayList lt String gt  symbolsPresent        this symbolsPresent   symbolsPresent     public static void main String args              Test t   new Test        System out println  Symbols Present is    t symbolsPresent

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Also  if you want to enforce the List to be read-only  throws a UnsupportedOperationException if modified    List lt String gt  places   Collections unmodifiableList Arrays asList  One    Two    Three

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Java 9 allows you to create an unmodifiable list with a single line of code using List of factory   public class Main       public static void main String   args            List lt String gt  examples   List of  one    two    three            System out println examples             Output    one  two  three

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try this  new String     One   Two   Three   Four      or  List lt String gt  places   Arrays asList  One    Two    Three      ARRAYS

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You can use Java 8 Stream API  You can create a Stream of objects and collect them as a List     private List lt String gt  symbolsPresent   Stream of  ONE    TWO    THREE    FOUR        collect Collectors toList

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import com google common collect Lists         ArrayList lt String gt  getSymbolsPresent   Lists newArrayList  item 1    item 2

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You can also use the varargs syntax to make your code cleaner   Use the overloaded constructor   ArrayList lt String gt  list   new ArrayList lt String gt  Arrays asList  a    b    c       Subclass ArrayList in a utils module   public class MyArrayList lt T gt  extends ArrayList lt T gt        public MyArrayList T    values            super Arrays asList values             ArrayList lt String gt  list   new MyArrayList lt String gt   a    b    c      Or have a static factory method  my preferred approach    public class Utils     public static  lt T gt  ArrayList lt T gt  asArrayList T    values        return new ArrayList lt T gt  Arrays asList values           ArrayList lt String gt  list   Utils asArrayList  a    b    c

User · Answer

I use a generic class that inherit from ArrayList and implement a constructor with a parameter with variable number or arguments    public class MyArrayList lt T gt  extends ArrayList lt T gt        public MyArrayList T   items           for  T item   items                this add item                       Example   MyArrayList lt String gt myArrayList new MyArrayList lt String gt   s1   s2   s2

[java] Initializing ArrayList with some predefined values

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