Can I call methods in constructor in Java

Question

I have situation  where I want to read configuration file only one time  when class is instantiated   Suppose I have a method named readConfig    that reads configuration and puts it into a Map object  When the program is required to use configuration value it reads object with it s define key  I want to know that constructor calls only once it s life cycle  Can I put my method readConfig   into constructor  which would give me benefit of one time calling or is there another mechanism to do that

User · Answer

You can  But by placing this in the constructor you are making your object hard to test   Instead you should    provide the configuration with a setter have a separate init   method   Dependency injection frameworks give you these options   public class ConfigurableObject      private Map lt String  String gt  configuration     public ConfigurableObject               public void setConfiguration                   simply set the configuration          An example of the 2nd option  best used when the object is managed by a container    public class ConfigurableObject      private File configFile     private Map lt String  String gt  configuration     public ConfigurableObject File configFile           this configFile   configFile           public void init            this configuration   parseConfig       implement          This  of course  can be written by just having the constructor  public ConfigurableObject File configfile        this configuration   parseConfig configFile       But then you won t be able to provide mock configurations   I know the 2nd opttion sounds more verbose and prone to error  if you forget to initialize   And it won t really hurt you that much if you do it in a constructor  But making your code more dependency-injection oriented is generally a good practice   The 1st option is best - it can be used with both DI framework and with manual DI

User · Answer

Why not to use Static Initialization Blocks   Additional details here  Static Initialization Blocks

User · Answer

Can I put my method readConfig   into constructor    Invoking a not overridable method in a constructor is an acceptable approach  While if the method is only used by the constructor you may wonder if extracting it into a method  even private  is really required        If you choose to extract some logic done by the constructor into a method  as for any method you have to choose a access modifier that fits to the method requirement but in this specific case it matters further as protecting the method against the overriding of the method has to be done at risk of making the super class constructor inconsistent      So it should be private if it is used only by the constructor s   and instance methods  of the class  Otherwise it should be both package-private and final if the method is reused inside the package or in the subclasses            which would give me benefit of one time calling or is there another mechanism to do that     You don t have any benefit or drawback to use this way  I don t encourage to perform much logic in constructors but in some cases it may make sense to init multiple things in a constructor  For example the copy constructor may perform a lot of things  Multiple JDK classes illustrate that  Take for example the HashMap copy constructor that constructs a new HashMap with the same mappings as the specified Map parameter    public HashMap Map lt   extends K    extends V gt  m        this loadFactor   DEFAULT LOAD FACTOR      putMapEntries m  false      final void putMapEntries Map lt   extends K    extends V gt  m  boolean evict        int s   m size        if  s  gt  0            if  table    null       pre-size             float ft     float s   loadFactor    1 0F              int t     ft  lt   float MAXIMUM CAPACITY                          int ft   MAXIMUM CAPACITY               if  t  gt  threshold                  threshold   tableSizeFor t                     else if  s  gt  threshold              resize            for  Map Entry lt   extends K    extends V gt  e   m entrySet                  K key   e getKey                V value   e getValue                putVal hash key   key  value  false  evict                       Extracting the logic of the map populating in putMapEntries   is a good thing because it allows        reusing the method in other contexts  For example clone   and putAll   use it too  minor but interesting  giving a meaningful name that conveys the performed logic

User · Answer

The constructor is called only once  so you can safely do what you want  however the disadvantage of calling methods from within the constructor  rather than directly  is that you don t get direct feedback if the method fails   This gets more difficult the more methods you call   One solution is to provide methods that you can call to query the  health  of the object once it s been constructed   For example the method isConfigOK   can be used to see if the config read operation was OK   Another solution is to throw exceptions in the constructor upon failure  but it really depends on how  fatal  these failures are   class A       Map  lt String String gt  config   null      public A                 readConfig               protected boolean readConfig                                public boolean isConfigOK                    Check config here         return true

User · Answer

You can  this is what constructors are for  Also you make it clear that the object is never constructed in an unknown state  without configuration loaded    You shouldn t  calling instance method in constructor is dangerous because the object is not yet fully initialized  this applies mainly to methods than can be overridden   Also complex processing in constructor is known to have a negative impact on testability

User · Answer

Singleton pattern  public class MyClass          private static MyClass instance   null                Get instance of my class  Singleton             public static MyClass getInstance             if instance    null                instance   new MyClass                      return instance                      Private constructor            private MyClass               This will only be called once  by calling getInstanse   method

User · Answer

Better design would be  public static YourObject getMyObject File configFile         process and create an object configure it and return it      Factory design pattern

[java] Can I call methods in constructor in Java?

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