I created a non-static inner class like this:
class Sample {
public void sam() {
System.out.println("hi");
}
}
I called it in main method like this:
Sample obj = new Sample();
obj.sam();
It gave a compilation error: non-static cannot be referenced from a static context When I declared the non-static inner class as static, it works. Why is that so?
For a non-static inner class, the compiler automatically adds a hidden reference to the "owner" object instance. When you try to create it from a static method (say, the main method), there is no owning instance. It is like trying to call an instance method from a static method - the compiler won't allow it, because you don't actually have an instance to call.
So the inner class must either itself be static (in which case no owning instance is required), or you only create the inner class instance from within a non-static context.
A non-static inner class has the outer class as an instance variable, which means it can only be instantiated from such an instance of the outer class:
public class Outer{
public class Inner{
}
public void doValidStuff(){
Inner inner = new Inner();
// no problem, I created it from the context of *this*
}
public static void doInvalidStuff(){
Inner inner = new Inner();
// this will fail, as there is no *this* in a static context
}
}
An inner class needs an instance of the outer class, because there is an implicit constructor generated by compiler. However you can get around it like the following:
public class A {
public static void main(String[] args) {
new A(). new B().a();
}
class B {
public void a() {
System.err.println("AAA");
}
}
}
Maybe this will help : http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html
the non-static inner class cannot be called in a static context (in your example there is no instance of the outer class).
Related
I know how to make an instance of an inner class. But I want to know why we can not do that in the following way:
class outerclass{
public static void main(String[] args){
innerclass in=new innerclass();
}
class innerclass{
}
}
If I do this then I get the following error:
No enclosing instance of type outerclass is accessible. Must qualify the allocation with an enclosing instance of type outerclass (e.g. x.new A() where x is an instance of outerclass).
Why?
class Demo{
public static void main(String[] args){
System.out.println(innerclass.a);
}
static class innerclass{
static int a=1;
}
}
Gives the output 1.
See here while making the inner class as static You can easily access in your outer class,In order to create an instance of the Nested class you must reference it by prefixing it with the Outer class name, like this:
Outer.Nested instance = new Outer.Nested();
Non-static Nested Classes (Inner Classes)
Non-static nested classes in Java are also called inner classes. Inner classes are associated with an instance of the enclosing class. Thus, you must first create an instance of the enclosing class to create an instance of an inner class. Here is an example inner class definition:
public class Outer {
public class Inner {
}
}
Here is how you create an instance of the Inner class:
Outer outer = new Outer();
Outer.Inner inner = outer.new Inner();
Notice how you put new after the reference to the outer class in order to create an instance of the inner class.
Non-static nested classes (inner classes) have access to the fields of the enclosing class, even if they are declared private. Here is an example of that:
public class Outer {
private String text = "I am private!";
public class Inner {
public void printText() {
System.out.println(text);
}
}
}
Your innerclass is not static. This means it must have a reference to the outerclass. main is static and has no implicit outerclass.
The simple solution is to make your inner class a static nested class.
You must either make your inner class static (as already mentioned) or create your inner class from a non-static context, e.g. from a non-static method.
I.e. either this:
class outerclass{
void myMethod() {
innerclass in = new innerclass();
}
class innerclass{
}
}
or this
class outerclass{
public static void main(String[] args){
innerclass in=new innerclass();
}
static class innerclass{
}
}
outerclass thats encapsulates innerclass is not instantiated, hence, calling innerclass directly would throw an error since there is no outerclass to attach innerclass.
Therefore as suggested by the previous answers, making innerclass static would resolve the problem, allowing access to the innerclass without instantiation.
There are lot of existing answers with regards to this topic. A quick google brings this up.
Java - No enclosing instance of type Foo is accessible
I read that an instance of an inner class cannot be created without an instance of outer class. But when I tried to create an instance of my inner class using it as an instance member of my outer class, it worked.
I understand that it is creating an inner object through a reference to my outer class object, but is it the right way to do it?
Below is my code snippet:
public class TestInner {
private Nonstatic non = null;
private static int access = 4;
public class Nonstatic {
void hello() {
access = 90;
}
}
public static void main(String[] args) {
TestInner outer = new TestInner();
TestInner.Nonstatic innern= outer.new Nonstatic();
System.out.println("Non static obj1 is "+innern);
outer.testinnerObj();
}
public void testinnerObj() {
non = new Nonstatic();
System.out.println("Non static obj2 is "+non);
non.hello();
}
}
You're writing "An instance of Inner class cannot be created without an instance of outer class". And that's exactly what you are doing.
First, you create an instance of the "outer" class:
TestInner outer = new TestInner();
Then, you create an instance of the "inner" class - it only lives
in the scope of outer:
TestInner.Nonstatic innern= outer.new Nonstatic();
So, the question maybe boils down to this: yes, you are creating the object in the static main method. But that does not matter, because you are using the syntax outer.newwhich creates it in the scope of outer.
Hope that helps.
On changing non-static inner class to static why there are compile time error on running code says-
Illegal enclosing instance specification
public class TestingInnerStatic{
public static void main(String args[]) {
InnerSame innerSame = new TestingInnerStatic().new InnerSame();//compile fail
Outer.InnerDiff innerDiff = new Outer().new InnerDiff();//compile fail
}
public void main() {
InnerSame innerSame = new InnerSame();
Outer.InnerDiff innerDiff = new Outer().new InnerDiff();//compile fail
}
static class InnerSame{}
}
class Outer{
static class InnerDiff{}
}
take a example of other member,this is only a convention and a good practice to call a static member on reference of class but if U call them on object they works not show compile fail.So why there is a compile fail?
If an inner class is non-static then you require an instance of the outer class to make an instance of the inner class. But that is not the case for static classes, an instance of an inner static class can exist without an instance of the outer class.
static example:
InnerClass ic = new Outer.InnerClass();
Notice I am not making a new instance of the outer class.
EDIT: reference
I have the following problem. Two nested anonymous types. I want to access "this" reference of the outer anonymous class inside the most inner class. Usually if one has anonymous nested class in a named outer class (lets call it "class Outer") he/she would type inside the nested class Outer.this.someMethod(). How do I refer the outer class if it's anonymous ?
Example Code:
public interface Outer {
void outerMethod();
}
public interface Inner {
void innerMethod();
}
...
public static void main(String[] args) {
...
new Outer() {
public void outerMethod() {
new Inner() {
public void innerMethod() {
Outer.this.hashCode(); // this does not work
} // innerMethod
}; // Inner
} // outerMethod
}; // Outer
...
} // main
The error I get is
No enclosing instance of the type Outer is accessible in scope
I know that I can copy the reference like this:
final Outer outerThisCopy = this;
just before instantiating the Inner object and then refer to this variable. The real goal is that I want to compare the hashCodes of outerThisCopy and the object accessed inside the new Inner object (i.e the Outer.this) for debugging purposes. I have some good arguments to think that this two objects are different (in my case).
[Context: The argument is that calling a getter implemented in the "Outer" class which is not shadowed in the "Inner" class returns different objects]
Any ideas how do I access the "this" reference of the enclosing anonymous type ?
Thank you.
You cannot access an instance of anonymous class directly from inner class or another anonymous class inside it, since the anonymous class doesn't have a name. However, you can get a reference to the outer class via a method:
new Outer()
{
public Outer getOuter()
{
return this;
}
public void outerMethod()
{
new Inner()
{
public void innerMethod()
{
getOuter().hashCode();
}
};
}
};
public class InnerClass {
class Inner
{
public void method()
{
System.out.println("Innerclass");
}
}
}
class Sample extends InnerClass.Inner
{
public static void main(String [] arg)
{
Sample s = new Sample(new InnerClass());
s.method();
}
//why is this mandatory???
Sample(InnerClass i) {
i.super();
}
#Override
public void method() {
System.out.println("derived class");
}
}
when i make a class that derives from an innerclass (Innerclass.Inner) default constructor doesn't works. later i came to know that it requires to include a constructor taking Enclosing class reference why is it so?
Non static inner classes in Java have an implicit reference to the enclosing instance. You can solve your problem with:
public class InnerClass {
static class Inner // can make it public too
{
public void method()
{
System.out.println("Innerclass");
}
}
}
Just don't expect to be able to call any methods on InnerClass without a specific instance.
Because non-static inner classes have an implicit member that points back to their outer class, and you can't create an instance of the inner class without giving it that pointer. If you directly create an instance of an inner class, you have to use new outer.Inner() (or it might be outer.new Inner(), I can never remember). But Sample isn't an inner class, it just inherits one, so the outer instance must be passed in its constructor to the base constructor. Thus, it needs to have some instance of outer available, or create it itself.