Java - No enclosing instance of type Foo is accessible

Question

I have the following code   class Hello       class Thing           public int size           Thing                 size   0                       public static void main String   args            Thing thing1   new Thing            System out println  Hello  World               I know Thing does nothing  but my Hello  World program compiles just fine without it  It s only my defined classes that are failing on me   And it refuses to compile  I get No enclosing instance of type Hello is accessible   at the line that creates a new Thing  I m guessing either    I have system level problems  either in DrJava or my Java install  or I have some basic misunderstanding of how to construct a working program in java    Any ideas

User · Answer

Thing is an inner class with an automatic connection to an instance of Hello  You get a compile error because there is no instance of Hello for it to attach to  You can fix it most easily by changing it to a static nested class which has no connection   static class Thing

User · Answer

Declare the INNER class Thing as a static and it will work with no issues  I remember I have the same issue with the inner class Dog when I declared it as class Dog   only  I got the same issue as you did  There were two solutions  1- To declare the inner class Dog as static  Or 2- To move the inner class Dog to a new class by itself  Here is the Example  public class ReturnDemo   public static void main String   args             int z   ReturnDemo calculate 10  12       System out println  quot z    quot    z            ReturnDemo Dog dog   new Dog  quot Bosh quot    quot  Doggy quot        System out println  dog getDog         public static int calculate  int x  int y        return x   y     public void print          System out println  quot void method quot        return     public String getString         return  quot Retrun String type value quot       static class Dog        private String breed  private String name   public Dog String breed  String name        super        this breed   breed      this name   name     public Dog getDog            return Dog type      return this          public String toString         return  quot breed quot    breed concat  quot name   quot    name

User · Answer

Lets understand it with the following simple example   This happens because this is NON-STATIC INNER CLASS  You should need the instance of outer class    public class PQ        public static void main String   args                create dog object here         Dog dog   new PQ   new Dog              OR         PQ pq   new PQ            Dog dog1   pq new Dog               abstract class Animal           abstract void checkup               class Dog extends Animal            Override         void checkup                 System out println  Dog checkup                          class Cat extends Animal            Override         void checkup                 System out println  Cat Checkup

User · Answer

static class Thing will make your program work   As it is  you ve got Thing as an inner class  which  by definition  is associated with a particular instance of Hello  even if it never uses or refers to it   which means it s an error to say new Thing    without having a particular Hello instance in scope    If you declare it as a static class instead  then it s a  nested  class  which doesn t need a particular Hello instance

User · Answer

Well    so many good answers but i wanna to add more on it  A brief look on Inner class in Java- Java allows us to define a class within another class and Being able to nest classes in this way has certain advantages    It can hide It increases encapsulation  the class from other classes - especially relevant if the class is only being used by the class it is contained within  In this case there is no need for the outside world to know about it  It can make code more maintainable as the classes are logically grouped together around where they are needed  The inner class has access to the instance variables and methods of its containing class    We have mainly three types of  Inner Classes     Local inner  Static Inner Class Anonymous Inner Class     Some of the important points to be remember   We need class object to access the Local Inner Class in which it exist  Static Inner Class get directly accessed same as like any other static method of the same class in which it is exists  Anonymous Inner Class are not visible to out side world as well as to the other methods or classes of the same class in which it is exist  and it is used on the point where it is declared     Let s try to see the above concepts practically   public class MyInnerClass    public static void main String args    throws InterruptedException          direct access to inner class method     new MyInnerClass StaticInnerClass   staticInnerClassMethod            static inner class reference object     StaticInnerClass staticInnerclass   new StaticInnerClass        staticInnerclass staticInnerClassMethod            access local inner class     LocalInnerClass localInnerClass   new MyInnerClass   new LocalInnerClass        localInnerClass localInnerClassMethod                   Pay attention to the opening curly braces and the fact that there s a        semicolon at the very end  once the anonymous class is created                      AnonymousClass anonymousClass   new AnonymousClass                 your code goes here                      static inner class static class StaticInnerClass       public void staticInnerClassMethod             System out println  Hay    from Static Inner class                 local inner class class LocalInnerClass       public void localInnerClassMethod             System out println  Hay    from local Inner class                   I hope this will helps to everyone  Please  refer for more

User · Answer

You ve declared the class Thing as a non-static inner class  That means it must be associated with an instance of the Hello class   In your code  you re trying to create an instance of Thing from a static context  That is what the compiler is complaining about   There are a few possible solutions  Which solution to use depends on what you want to achieve    Move Thing out of the Hello class  Change Thing to be a static nested class   static class Thing  Create an instance of Hello before creating an instance of Thing   public static void main String   args        Hello h   new Hello        Thing thing1   h new Thing       hope this syntax is right  typing on the fly  P      The last solution  a non-static nested class  would be mandatory if any instance of Thing depended on an instance of Hello to be meaningful  For example  if we had   public class Hello       public int enormous       public Hello int n            enormous   n             public class Thing           public int size           public Thing int m                if  m  gt  enormous                  size   enormous              else                 size   m                              any raw attempt to create an object of class Thing  as in   Thing t   new Thing 31     would be problematic  since there wouldn t be an obvious enormous value to test 31 against it  An instance h of the Hello outer class is necessary to provide this h enormous value       Hello h   new Hello 30       Thing t   h new Thing 31         Because it doesn t mean a Thing if it doesn t have a Hello   For more information on nested inner classes  Nested Classes  The Java Tutorials

[java] Java - No enclosing instance of type Foo is accessible

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