How to call methods within Java Singleton enum constructor?
5
My sample enum Singleton class is:
public class Test
public enum MyClass
INSTANCE;
private static String name = "Hello";
MyClass()
test();
private static void test()
name = name + "World";
System.out.println(name);
public static void main(String a)
MyClass m1 = MyClass.INSTANCE;
Obtained output : nullWorld
Expected output : HelloWorld
In main(), if
MyClass m1 = MyClass.INSTANCE;
is replaced by
MyClass.INSTANCE.test();
then, the output is HelloWorld, as expected.
This shows that static fields are not initialized until the constructor has completed execution.
Question : How to achieve this functionality of calling a method within constructor that accesses static fields?
java enums constructor singleton
asked Nov 15 '18 at 4:05
Aadhirai RAadhirai R
818
add a comment |
5
My sample enum Singleton class is:
public class Test
public enum MyClass
INSTANCE;
private static String name = "Hello";
MyClass()
test();
private static void test()
name = name + "World";
System.out.println(name);
public static void main(String a)
MyClass m1 = MyClass.INSTANCE;
Obtained output : nullWorld
Expected output : HelloWorld
In main(), if
MyClass m1 = MyClass.INSTANCE;
is replaced by
MyClass.INSTANCE.test();
then, the output is HelloWorld, as expected.
This shows that static fields are not initialized until the constructor has completed execution.
Question : How to achieve this functionality of calling a method within constructor that accesses static fields?
java enums constructor singleton
asked Nov 15 '18 at 4:05
Aadhirai RAadhirai R
818
Check this: stackoverflow.com/questions/443980/…
– ernest_k
Nov 15 '18 at 5:03
2
@AadhiraiR the whole point of singleton ist to have a single instance that can hold state - why do you need a static variable in a singleton? Just make it an instance variable instead.
– Hulk
Nov 15 '18 at 7:24
@Hulk, true! I realized that and am removing all the static fields in the enum singleton class.
– Aadhirai R
Nov 15 '18 at 9:18
add a comment |
5
5
5
3
My sample enum Singleton class is:
public class Test
public enum MyClass
INSTANCE;
private static String name = "Hello";
MyClass()
test();
private static void test()
name = name + "World";
System.out.println(name);
public static void main(String a)
MyClass m1 = MyClass.INSTANCE;
Obtained output : nullWorld
Expected output : HelloWorld
In main(), if
MyClass m1 = MyClass.INSTANCE;
is replaced by
MyClass.INSTANCE.test();
then, the output is HelloWorld, as expected.
This shows that static fields are not initialized until the constructor has completed execution.
Question : How to achieve this functionality of calling a method within constructor that accesses static fields?
java enums constructor singleton
asked Nov 15 '18 at 4:05
Aadhirai RAadhirai R
818
My sample enum Singleton class is:
public class Test
public enum MyClass
INSTANCE;
private static String name = "Hello";
MyClass()
test();
private static void test()
name = name + "World";
System.out.println(name);
public static void main(String a)
MyClass m1 = MyClass.INSTANCE;
Obtained output : nullWorld
Expected output : HelloWorld
In main(), if
MyClass m1 = MyClass.INSTANCE;
is replaced by
MyClass.INSTANCE.test();
then, the output is HelloWorld, as expected.
This shows that static fields are not initialized until the constructor has completed execution.
Question : How to achieve this functionality of calling a method within constructor that accesses static fields?
java enums constructor singleton
java enums constructor singleton
asked Nov 15 '18 at 4:05
Aadhirai RAadhirai R
818
asked Nov 15 '18 at 4:05
Aadhirai RAadhirai R
818
asked Nov 15 '18 at 4:05
Aadhirai RAadhirai R
818
asked Nov 15 '18 at 4:05
Aadhirai RAadhirai R
818
asked Nov 15 '18 at 4:05
Aadhirai RAadhirai R
818
818
Check this: stackoverflow.com/questions/443980/…
– ernest_k
Nov 15 '18 at 5:03
2
@AadhiraiR the whole point of singleton ist to have a single instance that can hold state - why do you need a static variable in a singleton? Just make it an instance variable instead.
– Hulk
Nov 15 '18 at 7:24
@Hulk, true! I realized that and am removing all the static fields in the enum singleton class.
– Aadhirai R
Nov 15 '18 at 9:18
add a comment |
Check this: stackoverflow.com/questions/443980/…
– ernest_k
Nov 15 '18 at 5:03
2
@AadhiraiR the whole point of singleton ist to have a single instance that can hold state - why do you need a static variable in a singleton? Just make it an instance variable instead.
– Hulk
Nov 15 '18 at 7:24
@Hulk, true! I realized that and am removing all the static fields in the enum singleton class.
– Aadhirai R
Nov 15 '18 at 9:18
Check this: stackoverflow.com/questions/443980/…
– ernest_k
Nov 15 '18 at 5:03
Check this: stackoverflow.com/questions/443980/…
– ernest_k
Nov 15 '18 at 5:03
2
2
@AadhiraiR the whole point of singleton ist to have a single instance that can hold state - why do you need a static variable in a singleton? Just make it an instance variable instead.
– Hulk
Nov 15 '18 at 7:24
@AadhiraiR the whole point of singleton ist to have a single instance that can hold state - why do you need a static variable in a singleton? Just make it an instance variable instead.
– Hulk
Nov 15 '18 at 7:24
@Hulk, true! I realized that and am removing all the static fields in the enum singleton class.
– Aadhirai R
Nov 15 '18 at 9:18
@Hulk, true! I realized that and am removing all the static fields in the enum singleton class.
– Aadhirai R
Nov 15 '18 at 9:18
add a comment |
1 Answer
1
active
oldest
votes
2
This is because INSTANCE is declared before name, so it is created and initalized before name is initialized.
This works:
public enum MyClass{
INSTANCE;
private static final String name = "Hello";
MyClass()
test();
private static void test()
String name1 = name + "World";
System.out.println(name1);
answered Nov 15 '18 at 4:29
Evgeniy DorofeevEvgeniy Dorofeev
106k23143223
1
Why does it work by makingnamefinal? Arefinal staticvariables initialized beforestaticvariables even if they are declared after?
– Kartik
Nov 15 '18 at 4:36
My requirement is to have a static variable that is not final. I want to be able to modify it.
– Aadhirai R
Nov 15 '18 at 4:50
I know Josh Bloch has suggested this way to implement Singleton pattern in his Effective Java book. But when this approach doesn't work then one should fall back to old approach of making a singleton via class implementation where you can define the name variable before the INSTANCE variable and then not keep it final but still you get your expected output! Is there any force that you have to implement singleton via enum only?
– Ketan
Nov 15 '18 at 5:33
1
@Aadhirai R if it were not enum you could move name before INSTANCE then it would work.
– Evgeniy Dorofeev
Nov 15 '18 at 5:37
3
@Kartik Making itfinalchanges things because the variable is now a compile-time constant. This means the compiler replaces the use ofnamewithintest()with the constant value ofname. Since this happens at compile-time there is no issue with the order of initialization.
– Slaw
Nov 15 '18 at 9:06
|
show 3 more comments
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1 Answer
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oldest
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1 Answer
1
active
oldest
votes
active
oldest
votes
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oldest
votes
2
This is because INSTANCE is declared before name, so it is created and initalized before name is initialized.
This works:
public enum MyClass{
INSTANCE;
private static final String name = "Hello";
MyClass()
test();
private static void test()
String name1 = name + "World";
System.out.println(name1);
answered Nov 15 '18 at 4:29
Evgeniy DorofeevEvgeniy Dorofeev
106k23143223
1
Why does it work by makingnamefinal? Arefinal staticvariables initialized beforestaticvariables even if they are declared after?
– Kartik
Nov 15 '18 at 4:36
My requirement is to have a static variable that is not final. I want to be able to modify it.
– Aadhirai R
Nov 15 '18 at 4:50
I know Josh Bloch has suggested this way to implement Singleton pattern in his Effective Java book. But when this approach doesn't work then one should fall back to old approach of making a singleton via class implementation where you can define the name variable before the INSTANCE variable and then not keep it final but still you get your expected output! Is there any force that you have to implement singleton via enum only?
– Ketan
Nov 15 '18 at 5:33
1
@Aadhirai R if it were not enum you could move name before INSTANCE then it would work.
– Evgeniy Dorofeev
Nov 15 '18 at 5:37
3
@Kartik Making itfinalchanges things because the variable is now a compile-time constant. This means the compiler replaces the use ofnamewithintest()with the constant value ofname. Since this happens at compile-time there is no issue with the order of initialization.
– Slaw
Nov 15 '18 at 9:06
|
show 3 more comments
2
This is because INSTANCE is declared before name, so it is created and initalized before name is initialized.
This works:
public enum MyClass{
INSTANCE;
private static final String name = "Hello";
MyClass()
test();
private static void test()
String name1 = name + "World";
System.out.println(name1);
answered Nov 15 '18 at 4:29
Evgeniy DorofeevEvgeniy Dorofeev
106k23143223
1
Why does it work by makingnamefinal? Arefinal staticvariables initialized beforestaticvariables even if they are declared after?
– Kartik
Nov 15 '18 at 4:36
My requirement is to have a static variable that is not final. I want to be able to modify it.
– Aadhirai R
Nov 15 '18 at 4:50
I know Josh Bloch has suggested this way to implement Singleton pattern in his Effective Java book. But when this approach doesn't work then one should fall back to old approach of making a singleton via class implementation where you can define the name variable before the INSTANCE variable and then not keep it final but still you get your expected output! Is there any force that you have to implement singleton via enum only?
– Ketan
Nov 15 '18 at 5:33
1
@Aadhirai R if it were not enum you could move name before INSTANCE then it would work.
– Evgeniy Dorofeev
Nov 15 '18 at 5:37
3
@Kartik Making itfinalchanges things because the variable is now a compile-time constant. This means the compiler replaces the use ofnamewithintest()with the constant value ofname. Since this happens at compile-time there is no issue with the order of initialization.
– Slaw
Nov 15 '18 at 9:06
|
show 3 more comments
2
2
2
This is because INSTANCE is declared before name, so it is created and initalized before name is initialized.
This works:
public enum MyClass{
INSTANCE;
private static final String name = "Hello";
MyClass()
test();
private static void test()
String name1 = name + "World";
System.out.println(name1);
answered Nov 15 '18 at 4:29
Evgeniy DorofeevEvgeniy Dorofeev
106k23143223
This is because INSTANCE is declared before name, so it is created and initalized before name is initialized.
This works:
public enum MyClass{
INSTANCE;
private static final String name = "Hello";
MyClass()
test();
private static void test()
String name1 = name + "World";
System.out.println(name1);
answered Nov 15 '18 at 4:29
Evgeniy DorofeevEvgeniy Dorofeev
106k23143223
answered Nov 15 '18 at 4:29
Evgeniy DorofeevEvgeniy Dorofeev
106k23143223
answered Nov 15 '18 at 4:29
Evgeniy DorofeevEvgeniy Dorofeev
106k23143223
answered Nov 15 '18 at 4:29
Evgeniy DorofeevEvgeniy Dorofeev
106k23143223
106k23143223
1
Why does it work by makingnamefinal? Arefinal staticvariables initialized beforestaticvariables even if they are declared after?
– Kartik
Nov 15 '18 at 4:36
My requirement is to have a static variable that is not final. I want to be able to modify it.
– Aadhirai R
Nov 15 '18 at 4:50
I know Josh Bloch has suggested this way to implement Singleton pattern in his Effective Java book. But when this approach doesn't work then one should fall back to old approach of making a singleton via class implementation where you can define the name variable before the INSTANCE variable and then not keep it final but still you get your expected output! Is there any force that you have to implement singleton via enum only?
– Ketan
Nov 15 '18 at 5:33
1
@Aadhirai R if it were not enum you could move name before INSTANCE then it would work.
– Evgeniy Dorofeev
Nov 15 '18 at 5:37
3
@Kartik Making itfinalchanges things because the variable is now a compile-time constant. This means the compiler replaces the use ofnamewithintest()with the constant value ofname. Since this happens at compile-time there is no issue with the order of initialization.
– Slaw
Nov 15 '18 at 9:06
|
show 3 more comments
1
Why does it work by makingnamefinal? Arefinal staticvariables initialized beforestaticvariables even if they are declared after?
– Kartik
Nov 15 '18 at 4:36
My requirement is to have a static variable that is not final. I want to be able to modify it.
– Aadhirai R
Nov 15 '18 at 4:50
I know Josh Bloch has suggested this way to implement Singleton pattern in his Effective Java book. But when this approach doesn't work then one should fall back to old approach of making a singleton via class implementation where you can define the name variable before the INSTANCE variable and then not keep it final but still you get your expected output! Is there any force that you have to implement singleton via enum only?
– Ketan
Nov 15 '18 at 5:33
1
@Aadhirai R if it were not enum you could move name before INSTANCE then it would work.
– Evgeniy Dorofeev
Nov 15 '18 at 5:37
3
@Kartik Making itfinalchanges things because the variable is now a compile-time constant. This means the compiler replaces the use ofnamewithintest()with the constant value ofname. Since this happens at compile-time there is no issue with the order of initialization.
– Slaw
Nov 15 '18 at 9:06
1
1
Why does it work by making
name final? Are final static variables initialized before static variables even if they are declared after?– Kartik
Nov 15 '18 at 4:36
Why does it work by making
name final? Are final static variables initialized before static variables even if they are declared after?– Kartik
Nov 15 '18 at 4:36
My requirement is to have a static variable that is not final. I want to be able to modify it.
– Aadhirai R
Nov 15 '18 at 4:50
My requirement is to have a static variable that is not final. I want to be able to modify it.
– Aadhirai R
Nov 15 '18 at 4:50
I know Josh Bloch has suggested this way to implement Singleton pattern in his Effective Java book. But when this approach doesn't work then one should fall back to old approach of making a singleton via class implementation where you can define the name variable before the INSTANCE variable and then not keep it final but still you get your expected output! Is there any force that you have to implement singleton via enum only?
– Ketan
Nov 15 '18 at 5:33
I know Josh Bloch has suggested this way to implement Singleton pattern in his Effective Java book. But when this approach doesn't work then one should fall back to old approach of making a singleton via class implementation where you can define the name variable before the INSTANCE variable and then not keep it final but still you get your expected output! Is there any force that you have to implement singleton via enum only?
– Ketan
Nov 15 '18 at 5:33
1
1
@Aadhirai R if it were not enum you could move name before INSTANCE then it would work.
– Evgeniy Dorofeev
Nov 15 '18 at 5:37
@Aadhirai R if it were not enum you could move name before INSTANCE then it would work.
– Evgeniy Dorofeev
Nov 15 '18 at 5:37
3
3
@Kartik Making it
final changes things because the variable is now a compile-time constant. This means the compiler replaces the use of name within test() with the constant value of name. Since this happens at compile-time there is no issue with the order of initialization.– Slaw
Nov 15 '18 at 9:06
@Kartik Making it
final changes things because the variable is now a compile-time constant. This means the compiler replaces the use of name within test() with the constant value of name. Since this happens at compile-time there is no issue with the order of initialization.– Slaw
Nov 15 '18 at 9:06
|
show 3 more comments
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Check this: stackoverflow.com/questions/443980/…
– ernest_k
Nov 15 '18 at 5:03
2
@AadhiraiR the whole point of singleton ist to have a single instance that can hold state - why do you need a static variable in a singleton? Just make it an instance variable instead.
– Hulk
Nov 15 '18 at 7:24
@Hulk, true! I realized that and am removing all the static fields in the enum singleton class.
– Aadhirai R
Nov 15 '18 at 9:18