What is the difference between a static and a non-static initialization code block

Question

My question is about one particular usage of static keyword  It is possible to use static keyword to cover a code block within a class which does not belong to any function  For example following code compiles   public class Test       private static final int a          static           a   5          doSomething a             private static int doSomething int x            return  x 5             If you remove the static keyword it complains because the variable a is final  However it is possible to remove both final and static keywords and make it compile    It is confusing for me in both ways  How am I supposed to have a code section that does not belong to any method  How is it possible to invoke it  In general  what is the purpose of this usage  Or better  where can I find documentation about this

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The static code block can be used to instantiate or initialize class variables  as opposed to object variables    So declaring  a  static means that is only one shared by all Test objects  and the static code block initializes  a  only once  when the Test class is first loaded  no matter how many Test objects are created

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The static block is a  static initializer    It s automatically invoked when the class is loaded  and there s no other way to invoke it  not even via Reflection    I ve personally only ever used it when writing JNI code   class JNIGlue       static           System loadLibrary  foo

User · Answer

The static code block can be used to instantiate or initialize class variables  as opposed to object variables    So declaring  a  static means that is only one shared by all Test objects  and the static code block initializes  a  only once  when the Test class is first loaded  no matter how many Test objects are created

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The code block with the static modifier signifies a class initializer  without the static modifier the code block is an instance initializer     Class initializers are executed in the order they are defined  top down  just like simple variable initializers  when the class is loaded  actually  when it s resolved  but that s a technicality    Instance initializers are executed in the order defined when the class is instantiated  immediately before the constructor code is executed  immediately after the invocation of the super constructor   If you remove static from int a  it becomes an instance variable  which you are not able to access from the static initializer block  This will fail to compile with the error  non-static variable a cannot be referenced from a static context    If you also remove static from the initializer block  it then becomes an instance initializer and so int a is initialized at construction

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You will not write code into a static block that needs to be invoked anywhere in your program  If the purpose of the code is to be invoked then you must place it in a method   You can write static initializer blocks to initialize static variables when the class is loaded but this code can be more complex    A static initializer block looks like a method with no name  no arguments  and no return type   Since you never call it it doesn t need a name   The only time its called is when the virtual machine loads the class

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This is directly from http   www programcreek com 2011 10 java-class-instance-initializers  1  Execution Order Look at the following class  do you know which one gets executed first  public class Foo           instance variable initializer     String s    quot abc quot           constructor     public Foo             System out println  quot constructor called quot                  static initializer     static           System out println  quot static initializer called quot                  instance initializer               System out println  quot instance initializer called quot                public static void main String   args            new Foo            new Foo             Output   static initializer called instance initializer called constructor called instance initializer called constructor called  2  How do Java instance initializer work  The instance initializer above contains a println statement  To understand how it works  we can treat it as a variable assignment statement  e g   b   0  This can make it more obvious to understand  Instead of int b   0  you could write int b  b   0   Therefore  instance initializers and instance variable initializers are pretty much the same  3  When are instance initializers useful  The use of instance initializers are rare  but still it can be a useful alternative to instance variable initializers if   Initializer code must handle exceptions Perform calculations that can   t be expressed with an instance variable initializer   Of course  such code could be written in constructors  But if a class had multiple constructors  you would have to repeat the code in each constructor  With an instance initializer  you can just write the code once  and it will be executed no matter what constructor is used to create the object   I guess this is just a concept  and it is not used often   Another case in which instance initializers are useful is anonymous inner classes  which can   t declare any constructors at all   Will this be a good place to place a logging function   Thanks to Derhein  Also note that Anonymous classes that implement interfaces  1  have no constructors  Therefore instance initializers are needed to execute any kinds of expressions at construction time

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final  guarantees that a variable must be initialized before end of object initializer code  Likewise  static final  guarantees that a variable will be initialized by the end of class initialization code  Omitting the  static  from your initialization code turns it into object initialization code  thus your variable no longer satisfies its guarantees

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This is directly from http   www programcreek com 2011 10 java-class-instance-initializers  1  Execution Order Look at the following class  do you know which one gets executed first  public class Foo           instance variable initializer     String s    quot abc quot           constructor     public Foo             System out println  quot constructor called quot                  static initializer     static           System out println  quot static initializer called quot                  instance initializer               System out println  quot instance initializer called quot                public static void main String   args            new Foo            new Foo             Output   static initializer called instance initializer called constructor called instance initializer called constructor called  2  How do Java instance initializer work  The instance initializer above contains a println statement  To understand how it works  we can treat it as a variable assignment statement  e g   b   0  This can make it more obvious to understand  Instead of int b   0  you could write int b  b   0   Therefore  instance initializers and instance variable initializers are pretty much the same  3  When are instance initializers useful  The use of instance initializers are rare  but still it can be a useful alternative to instance variable initializers if   Initializer code must handle exceptions Perform calculations that can   t be expressed with an instance variable initializer   Of course  such code could be written in constructors  But if a class had multiple constructors  you would have to repeat the code in each constructor  With an instance initializer  you can just write the code once  and it will be executed no matter what constructor is used to create the object   I guess this is just a concept  and it is not used often   Another case in which instance initializers are useful is anonymous inner classes  which can   t declare any constructors at all   Will this be a good place to place a logging function   Thanks to Derhein  Also note that Anonymous classes that implement interfaces  1  have no constructors  Therefore instance initializers are needed to execute any kinds of expressions at construction time

User · Answer

The static block is a  static initializer    It s automatically invoked when the class is loaded  and there s no other way to invoke it  not even via Reflection    I ve personally only ever used it when writing JNI code   class JNIGlue       static           System loadLibrary  foo

User · Answer

The static block is a  static initializer    It s automatically invoked when the class is loaded  and there s no other way to invoke it  not even via Reflection    I ve personally only ever used it when writing JNI code   class JNIGlue       static           System loadLibrary  foo

User · Answer

The static code block can be used to instantiate or initialize class variables  as opposed to object variables    So declaring  a  static means that is only one shared by all Test objects  and the static code block initializes  a  only once  when the Test class is first loaded  no matter how many Test objects are created

User · Answer

You will not write code into a static block that needs to be invoked anywhere in your program  If the purpose of the code is to be invoked then you must place it in a method   You can write static initializer blocks to initialize static variables when the class is loaded but this code can be more complex    A static initializer block looks like a method with no name  no arguments  and no return type   Since you never call it it doesn t need a name   The only time its called is when the virtual machine loads the class

User · Answer

final  guarantees that a variable must be initialized before end of object initializer code  Likewise  static final  guarantees that a variable will be initialized by the end of class initialization code  Omitting the  static  from your initialization code turns it into object initialization code  thus your variable no longer satisfies its guarantees

User · Answer

when a developer use an initializer block  the Java Compiler copies the initializer into each constructor of the current class   Example   the following code   class MyClass        private int myField   3                myField   myField   2            myField is worth 5 for all instance            public MyClass             myField   myField   4            myField is worth 20 for all instance initialized with this construtor            public MyClass int  myParam            if   myParam  gt  0                myField   myField   4                myField is worth 20 for all instance initialized with this construtor               if  myParam is greater than 0           else               myField   myField   5                myField is worth 10 for all instance initialized with this construtor               if  myParam is lower than 0 or if  myParam is worth 0                      public void setMyField int  myField            myField    myField              public int getMyField             return myField           public class MainClass       public static void main String   args            MyClass myFirstInstance    new MyClass            System out println myFirstInstance  getMyField      20         MyClass mySecondInstance    new MyClass 1           System out println mySecondInstance  getMyField      20         MyClass myThirdInstance    new MyClass -1           System out println myThirdInstance  getMyField      10           is equivalent to   class MyClass        private int myField   3       public MyClass             myField   myField   2          myField   myField   4            myField is worth 20 for all instance initialized with this construtor            public MyClass int  myParam            myField   myField   2          if   myParam  gt  0                myField   myField   4                myField is worth 20 for all instance initialized with this construtor               if  myParam is greater than 0           else               myField   myField   5                myField is worth 10 for all instance initialized with this construtor               if  myParam is lower than 0 or if  myParam is worth 0                      public void setMyField int  myField            myField    myField              public int getMyField             return myField           public class MainClass       public static void main String   args            MyClass myFirstInstance    new MyClass            System out println myFirstInstance  getMyField      20         MyClass mySecondInstance    new MyClass 1           System out println mySecondInstance  getMyField      20         MyClass myThirdInstance    new MyClass -1           System out println myThirdInstance  getMyField      10           I hope my example is understood by developers

User · Answer

The code block with the static modifier signifies a class initializer  without the static modifier the code block is an instance initializer     Class initializers are executed in the order they are defined  top down  just like simple variable initializers  when the class is loaded  actually  when it s resolved  but that s a technicality    Instance initializers are executed in the order defined when the class is instantiated  immediately before the constructor code is executed  immediately after the invocation of the super constructor   If you remove static from int a  it becomes an instance variable  which you are not able to access from the static initializer block  This will fail to compile with the error  non-static variable a cannot be referenced from a static context    If you also remove static from the initializer block  it then becomes an instance initializer and so int a is initialized at construction

User · Answer

You will not write code into a static block that needs to be invoked anywhere in your program  If the purpose of the code is to be invoked then you must place it in a method   You can write static initializer blocks to initialize static variables when the class is loaded but this code can be more complex    A static initializer block looks like a method with no name  no arguments  and no return type   Since you never call it it doesn t need a name   The only time its called is when the virtual machine loads the class

User · Answer

Uff  what is static initializer   The static initializer is a static    block of code inside java class  and run only one time before the constructor or main method is called   OK  Tell me more      is a block of code static         inside any java class  and executed by virtual machine when class is called  No return statements are supported  No arguments are supported  No this or super are supported    Hmm where can I use it   Can be used anywhere you feel ok    that simple  But I see most of the time it is used when doing database connection  API init  Logging and etc   Don t just bark  where is example   package com example learnjava   import java util ArrayList   public class Fruit        static           System out println  Inside Static Initializer                 fruits array         ArrayList lt String gt  fruits   new ArrayList lt  gt             fruits add  Apple            fruits add  Orange            fruits add  Pear                print fruits         for  String fruit   fruits                System out println fruit                     System out println  End Static Initializer  n               public static void main String   args            System out println  Inside Main Method               Output        Inside Static Initializer       Apple      Orange      Pear      End Static Initializer       Inside Main Method    Hope this helps

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Uff  what is static initializer   The static initializer is a static    block of code inside java class  and run only one time before the constructor or main method is called   OK  Tell me more      is a block of code static         inside any java class  and executed by virtual machine when class is called  No return statements are supported  No arguments are supported  No this or super are supported    Hmm where can I use it   Can be used anywhere you feel ok    that simple  But I see most of the time it is used when doing database connection  API init  Logging and etc   Don t just bark  where is example   package com example learnjava   import java util ArrayList   public class Fruit        static           System out println  Inside Static Initializer                 fruits array         ArrayList lt String gt  fruits   new ArrayList lt  gt             fruits add  Apple            fruits add  Orange            fruits add  Pear                print fruits         for  String fruit   fruits                System out println fruit                     System out println  End Static Initializer  n               public static void main String   args            System out println  Inside Main Method               Output        Inside Static Initializer       Apple      Orange      Pear      End Static Initializer       Inside Main Method    Hope this helps

User · Answer

final  guarantees that a variable must be initialized before end of object initializer code  Likewise  static final  guarantees that a variable will be initialized by the end of class initialization code  Omitting the  static  from your initialization code turns it into object initialization code  thus your variable no longer satisfies its guarantees

User · Answer

final  guarantees that a variable must be initialized before end of object initializer code  Likewise  static final  guarantees that a variable will be initialized by the end of class initialization code  Omitting the  static  from your initialization code turns it into object initialization code  thus your variable no longer satisfies its guarantees

User · Answer

The code block with the static modifier signifies a class initializer  without the static modifier the code block is an instance initializer     Class initializers are executed in the order they are defined  top down  just like simple variable initializers  when the class is loaded  actually  when it s resolved  but that s a technicality    Instance initializers are executed in the order defined when the class is instantiated  immediately before the constructor code is executed  immediately after the invocation of the super constructor   If you remove static from int a  it becomes an instance variable  which you are not able to access from the static initializer block  This will fail to compile with the error  non-static variable a cannot be referenced from a static context    If you also remove static from the initializer block  it then becomes an instance initializer and so int a is initialized at construction

User · Answer

The static block is a  static initializer    It s automatically invoked when the class is loaded  and there s no other way to invoke it  not even via Reflection    I ve personally only ever used it when writing JNI code   class JNIGlue       static           System loadLibrary  foo

User · Answer

You will not write code into a static block that needs to be invoked anywhere in your program  If the purpose of the code is to be invoked then you must place it in a method   You can write static initializer blocks to initialize static variables when the class is loaded but this code can be more complex    A static initializer block looks like a method with no name  no arguments  and no return type   Since you never call it it doesn t need a name   The only time its called is when the virtual machine loads the class

User · Answer

The code block with the static modifier signifies a class initializer  without the static modifier the code block is an instance initializer     Class initializers are executed in the order they are defined  top down  just like simple variable initializers  when the class is loaded  actually  when it s resolved  but that s a technicality    Instance initializers are executed in the order defined when the class is instantiated  immediately before the constructor code is executed  immediately after the invocation of the super constructor   If you remove static from int a  it becomes an instance variable  which you are not able to access from the static initializer block  This will fail to compile with the error  non-static variable a cannot be referenced from a static context    If you also remove static from the initializer block  it then becomes an instance initializer and so int a is initialized at construction

User · Answer

when a developer use an initializer block  the Java Compiler copies the initializer into each constructor of the current class   Example   the following code   class MyClass        private int myField   3                myField   myField   2            myField is worth 5 for all instance            public MyClass             myField   myField   4            myField is worth 20 for all instance initialized with this construtor            public MyClass int  myParam            if   myParam  gt  0                myField   myField   4                myField is worth 20 for all instance initialized with this construtor               if  myParam is greater than 0           else               myField   myField   5                myField is worth 10 for all instance initialized with this construtor               if  myParam is lower than 0 or if  myParam is worth 0                      public void setMyField int  myField            myField    myField              public int getMyField             return myField           public class MainClass       public static void main String   args            MyClass myFirstInstance    new MyClass            System out println myFirstInstance  getMyField      20         MyClass mySecondInstance    new MyClass 1           System out println mySecondInstance  getMyField      20         MyClass myThirdInstance    new MyClass -1           System out println myThirdInstance  getMyField      10           is equivalent to   class MyClass        private int myField   3       public MyClass             myField   myField   2          myField   myField   4            myField is worth 20 for all instance initialized with this construtor            public MyClass int  myParam            myField   myField   2          if   myParam  gt  0                myField   myField   4                myField is worth 20 for all instance initialized with this construtor               if  myParam is greater than 0           else               myField   myField   5                myField is worth 10 for all instance initialized with this construtor               if  myParam is lower than 0 or if  myParam is worth 0                      public void setMyField int  myField            myField    myField              public int getMyField             return myField           public class MainClass       public static void main String   args            MyClass myFirstInstance    new MyClass            System out println myFirstInstance  getMyField      20         MyClass mySecondInstance    new MyClass 1           System out println mySecondInstance  getMyField      20         MyClass myThirdInstance    new MyClass -1           System out println myThirdInstance  getMyField      10           I hope my example is understood by developers

User · Answer

The static code block can be used to instantiate or initialize class variables  as opposed to object variables    So declaring  a  static means that is only one shared by all Test objects  and the static code block initializes  a  only once  when the Test class is first loaded  no matter how many Test objects are created

[java] What is the difference between a static and a non-static initialization code block

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