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class SuperClass{
    static void display(){
         System.out.println("hello super class");
    }
 
} 
public class SubClass extends SuperClass{
    static void display(){
         System.out.println("hello sub class");
    }
 
    public static void main(String[] args){
        SuperClass s = new SuperClass();
        s.display();
            SubClass sub = new SubClass();
        sub.display();
 
        s=sub;
        s.display();
        sub=s;
        sub.display();
 
    }
 
}

Main.java:7: error: class SubClass is public, should be declared in a file named SubClass.java
public class SubClass extends SuperClass{
       ^
Main.java:20: error: incompatible types: SuperClass cannot be converted to SubClass
        sub=s;
            ^
2 errors

Standard output is empty