Question

//This is a small code to study how collection works but when I create an Mountain object in main method it gives an error. Explained further in the end.

import java. util. * ;
    public class sortMountains {
     class Mountain {
               String name;
               int height;
               Mountain(String n, int h) {
                 name      = n;

                 height = h;
               }
               public String toString( ) {
                 return name +  + height;
               }
            }
    List<Mountain> mtn  = new ArrayList<Mountain> ();
      class NameCompare implements Comparator <Mountain> {
        public int compare(Mountain one, Mountain two) {
          return one.name. compareTo(two. name);
      }
      }
      class HeightCompare implements Comparator <Mountain> {
        public int compare(Mountain one, Mountain two) {
    return (two. height - one. height) ;
    }

      }
      public void go() {
            mtn.add(new Mountain("Longs ", 14255));
            mtn.add(new Mountain("Elbert ", 14433));
            mtn.add(new Mountain("Maroon " , 14156));
            mtn.add(new Mountain("Castle ", 14265));

            System.out.println("as entered:\n" + mtn);
            NameCompare nc = new NameCompare();
            Collections.sort(mtn, nc);
            System.out.println("by name:\n'" + mtn);
            HeightCompare hc = new HeightCompare();
            Collections.sort(mtn, hc);
            System.out.println("by height:\n " + mtn);
          }
    public static void main(String args[]){

        sortMountains   sorting=new sortMountains();
        sorting.go();
         //error line   Mountain a=new Mountain("Everest",12121);            
    }

    }

The above when compiled without error line works fine but when I want to create an object of Mountain in main method it gives an error “non-static cannot be referenced from static method”

Answer 1

Make the Mountain class static, like so:

static class Mountain {
   ...

If you don’t, every instance of Mountain has to have an instance of sortMountains associated with it. Since there’s no such instance in main() (the method itself is static), the compiler won’t allow you to instantiate Mountain there.

For further discussion, see:

• Java: Static vs non static inner class
• Java inner class and static nested class