Default values and initialization in Java

Question

Based on my reference  primitive types have default values and Objects are null  I tested a piece of code  public class Main       public static void main String   args            int a          System out println a            The line System out println a   will be an error pointing at the variable a that says variable a might not have been initialized whereas in the given reference  integer will have 0 as a default value  However  with the given code below  it will actually print 0  public class Main       static int a      public static void main String   args            System out println a            What could possibly go wrong with the first code  Do class variables behave different from local variables

User · Answer

All member variables have to load into the heap, so they have to be initialized with default values when an instance of class is created.

In case of local variables, they don't get loaded into the heap. They are stored on the stack until they are being used. This is before Java 7, so we need to explicitly initialize them.

User · Answer

Read your reference more carefully   Default Values It s not always necessary to assign a value when a field is declared  Fields that are declared but not initialized will be set to a reasonable default by the compiler  Generally speaking  this default will be zero or null  depending on the data type  Relying on such default values  however  is generally considered bad programming style  The following chart summarizes the default values for the above data types        Local variables are slightly different  the compiler never assigns a default value to an uninitialized local variable  If you cannot initialize your local variable where it is declared  make sure to assign it a value before you attempt to use it  Accessing an uninitialized local variable will result in a compile-time error

User · Answer

Local variables do not get default values  Their initial values are undefined without assigning values by some means  Before you can use local variables they must be initialized  There is a big difference when you declare a variable at class level  as a member  i e   as a field  and at the method level  If you declare a field at the class level they get default values according to their type  If you declare a variable at the method level or as a block  means any code inside     do not get any values and remain undefined until somehow they get some starting values  i e   some values assigned to them

User · Answer

I wrote following function to return a default representation 0 or false of a primitive or Number         Retrieves the default value 0   false for any primitive representative or      link Number  type         param type        return      SuppressWarnings  quot unchecked quot   public static  lt T gt  T getDefault final Class lt T gt  type        if  type equals Long class     type equals Long TYPE           return  T  new Long 0       else if  type equals Integer class     type equals Integer TYPE           return  T  new Integer 0       else if  type equals Double class     type equals Double TYPE           return  T  new Double 0       else if  type equals Float class     type equals Float TYPE           return  T  new Float 0       else if  type equals Short class     type equals Short TYPE           return  T  new Short  short  0       else if  type equals Byte class     type equals Byte TYPE           return  T  new Byte  byte  0       else if  type equals Character class     type equals Character TYPE           return  T  new Character  char  0       else if  type equals Boolean class     type equals Boolean TYPE           return  T  new Boolean false       else if  type equals BigDecimal class           return  T  BigDecimal ZERO      else if  type equals BigInteger class           return  T  BigInteger ZERO      else if  type equals AtomicInteger class           return  T  new AtomicInteger        else if  type equals AtomicLong class           return  T  new AtomicLong        else if  type equals DoubleAdder class           return  T  new DoubleAdder        else         return null     I use it in hibernate ORM projection queries when the underlying SQL query returns null instead of 0         Retrieves the unique result or zero   lt code gt false lt  code gt  if it is     lt code gt null lt  code gt  and represents a number        param criteria        return zero if result is  lt code gt null lt  code gt      public static  lt T gt  T getUniqueResultDefault final Class lt T gt  type  final Criteria criteria        final T result    T  criteria uniqueResult         if  result    null          return result      else         return Utils getDefault type      One of the many unnecessary complex things about Java making it unintuitive to use  Why instance variables are initialized with default 0 but local are not is not logical  Similar why enums dont have built in flag support and many more options  Java lambda is a nightmare compared to C  and not allowing class extension methods is also a big problem  Java ecosystem comes up with excuses why its not possible but me as the user   developer i dont care about their excuses  I want easy approach and if they dont fix those things they will loose big in the future since C  and other languages are not waiting to make life of developers more simple  Its just sad to see the decline in the last 10 years since i work daily with Java

User · Answer

Yes  an instance variable will be initialized to a default value  For a local variable  you need to initialize before use  public class Main        int instaceVariable     An instance variable will be initialized to the default value      public static void main String   args            int localVariable   0     A local variable needs to be initialized before use

User · Answer

These are the main factors involved   member variable  default OK  static variable  default OK  final member variable  not initialized  must set on constructor  final static variable  not initialized  must set on a static block     local variable  not initialized   Note 1  you must initialize final member variables on every implemented constructor  Note 2  you must initialize final member variables inside the block of the constructor itself  not calling another method that initializes them  For instance  this is not valid  private final int memberVar   public Foo            Invalid initialization of a final member     init       private void init         memberVar   10     Note 3  arrays are Objects in Java  even if they store primitives  Note 4  when you initialize an array  all of its items are set to default  independently of being a member or a local array  I am attaching a code example  presenting the aforementioned cases  public class Foo          Static and member variables are initialized to default values         Primitives     private int a     Default 0     private static int b     Default 0         Objects     private Object c     Default NULL     private static Object d     Default NULL         Arrays  note  they are objects too  even if they store primitives      private int   e     Default NULL     private static int   f     Default NULL         What if declared as final          Primitives     private final int g     Not initialized  MUST set in the constructor     private final static int h     Not initialized  MUST set in a static            Objects     private final Object i     Not initialized  MUST set in constructor     private final static Object j     Not initialized  MUST set in a static            Arrays     private final int   k     Not initialized  MUST set in constructor     private final static int   l     Not initialized  MUST set in a static            Initialize final statics     static           h   5          j   new Object            l   new int 5      Elements of l are initialized to 0               Initialize final member variables     public Foo             g   10          i   new Object            k   new int 10      Elements of k are initialized to 0               A second example constructor        You have to initialize final member variables to every constructor      public Foo boolean aBoolean            g   15          i   new Object            k   new int 15      Elements of k are initialized to 0            public static void main String   args               Local variables are not initialized         int m     Not initialized         Object n     Not initialized         int   o     Not initialized             We must initialize them before use         m   20          n   new Object            o   new int 20      Elements of o are initialized to 0

User · Answer

There are a few things to keep in mind while declaring primitive type values  They are   Values declared inside a method will not be assigned a default value  Values declared as instance variables or a static variable will have default values assigned which is 0   So in your code  public class Main       int instanceVariable      static int staticVariable      public static void main String   args            Main mainInstance   new Main           int localVariable          int localVariableTwo   2          System out println mainInstance instanceVariable           System out println staticVariable             System out println localVariable      Will throw a compilation error         System out println localVariableTwo

User · Answer

In the first code sample  a is a main method local variable  Method local variables need to be initialized before using them  In the second code sample  a is class member variable  hence it will be initialized to the default value

User · Answer

In Java  the default initialization is applicable for only instance variable of class member  It isn t applicable for local variables

User · Answer

In the first case you are declaring  quot int a quot  as a local variable as declared inside a method  and local varible do not get default value  But instance variable are given default value both for static and non-static  Default value for instance variable  int   0 float double   0 0 reference variable   null char   0  space character  boolean   false

[java] Default values and initialization in Java

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