Why doesn’t Java allow you to create instances of inner classes?

See English answers > 25 Java inner classes and static nested classes

public class m
{
  String n="n";
  static String s="s";
  public class sub1
  {
    public void fn(){System.out.println(n);}
    //static public void fs(){System.out.println(s);}
  }
  static class sub2
  {
    //public void fn(){System.out.println(n);}
    static public void fs(){System.out.println(s);}
  }
  public void f()
  {
    sub1 s1=new sub1();//OK,no error
    sub2 s2=new sub2();//OK
  }

  public static void main(String[] args)
  {
    m obj=new m();
    sub1 s1=new sub1();//Error
    s1.fn();
    //s1.fs();
    sub2 s2=new sub2();//OK
    //s2.fn();
    s2.fs();
  }
}

I use openjdk to compile it under Linux, and it reports errors

$java -version
openjdk version "1.8.0_91"
OpenJDK Runtime Environment (build 1.8.0_91-8u91-b14-3ubuntu1~16.04.1-b14)
OpenJDK 64-Bit Server VM (build 25.91-b14,mixed mode)

$javac m.java 
m.java:24: Error: Cannot reference non-static variable this in a static context.
    sub1 s1=new sub1();//Error
            ^
1 Errors

This is strange to me: 1 In the m.f() member function, we can "sub1 S1 = new sub1();", But in main, we can't 2 Can static class sub2 have instances instead of static sub1?

Is this a java design? Why?

Solution

>A non - static inner class is treated as a member of an outer class

So you have to do something like this,

OuterClass outer = new OuterClass();
InnerClass inner = outer.new InnerClass();

So, in your case,

m obj = new m();
sub1 s1 = obj.new Sub1();