Getting hold of the outer class object from the inner class object

Question

I have the following code  I want to get hold of the outer class object using which I created the inner class object inner  How can I do it   public class OuterClass        public class InnerClass           private String name    Peakit              public static void main String   args            OuterClass outer   new OuterClass            InnerClass inner   outer new InnerClass              How to get the same outer object which created the inner object back          OuterClass anotherOuter                 if anotherOuter    outer                 System out println  Was able to reach out to the outer object via inner                 else                System out println  No luck  -                          EDIT  Well  some of you guys suggested of modifying the inner class by adding a method   public OuterClass outer        return OuterClass this      But what if I don t have control to modify the inner class  then  just to confirm  do we have some other way of getting the corresponding outer class object from the inner class object

User · Answer

Have been edited in 2020-06-15  public class Outer        public Inner getInner            return new Inner this              static class Inner            public final Outer Outer           public Inner Outer outer                this Outer outer                       public static void main String   args            Outer outer   new Outer            Inner inner   outer getInner            Outer anotherOuter inner Outer           if anotherOuter    outer                System out println  Was able to reach out to the outer object via inner                 else               System out println  No luck  -

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OuterClass this references the outer class

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The more general answer to this question involves shadowed variables and how they are accessed   In the following example  from Oracle   the variable x in main   is shadowing Test x   class Test       static int x   1      public static void main String   args            InnerClass innerClassInstance   new InnerClass                         public void printX                                 System out print  x     x                   System out println    Test this x     Test this x                                   innerClassInstance printX               public abstract static class InnerClass               int x   0           public InnerClass                public abstract void printX              Running this program will print   x 0  Test this x 1   More at  http   docs oracle com javase specs jls se7 html jls-6 html jls-6 6

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if you don t have control to modify the inner class  the refection may help you  but not recommend   this 0 is reference in Inner class which tells which instance of Outer class was used to create current instance of Inner class

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Here s the example      Test public void foo         C c   new C        A s      s     A B c  get        System out println s getR           classes class C     class A      public class B extends C       A get    return A this           public String getR          return  This is string

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You could  but you shouldn t  use reflection for the job   import java lang reflect Field   public class Outer       public class Inner              public static void main String   args  throws Exception               Create the inner instance         Inner inner   new Outer   new Inner                Get the implicit reference from the inner to the outer instance                make it accessible  as it has default visibility         Field field   Inner class getDeclaredField  this 0            field setAccessible true               Dereference and cast it         Outer outer    Outer  field get inner           System out println outer             Of course  the name of the implicit reference is utterly unreliable  so as I said  you shouldn t  -

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Not applicable to Static Inner Class  nested class      public static Object getDeclaringTopLevelClassObject Object object        if  object    null            return null            Class cls   object getClass        if  cls    null            return object            Class outerCls   cls getEnclosingClass        if  outerCls    null               this is top-level class         return object               get outer class object     Object outerObj   null      try           Field   fields   cls getDeclaredFields            for  Field field   fields                if  field    null  amp  amp  field getType      outerCls                      amp  amp  field getName      null  amp  amp  field getName   startsWith  this                       field setAccessible true                   outerObj   field get object                   break                                catch  Exception e            e printStackTrace              return getDeclaringTopLevelClassObject outerObj       Of course  the name of the implicit reference is unreliable  so you shouldn t use reflection for the job

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Within the inner class itself  you can use OuterClass this  This expression  which allows to refer to any lexically enclosing instance  is described in the JLS as Qualified this   I don t think there s a way to get the instance from outside the code of the inner class though  Of course  you can always introduce your own property   public OuterClass getOuter         return OuterClass this      EDIT  By experimentation  it looks like the field holding the reference to the outer class has package level access - at least with the JDK I m using    EDIT  The name used  this 0  is actually valid in Java  although the JLS discourages its use      The   character should be used only in   mechanically generated source code or    rarely  to access pre-existing names on   legacy systems

[java] Getting hold of the outer class object from the inner class object

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