.equals() vs ==
==================
public class Main {
    public static void main(String[] args) {

        String s1 = "red";
        String s2 = "";
        s2 += "r";
        s2 += "e";
        s2 += "d";

        System.out.println(s1.equals("red")); // return true
        System.out.println(s2.equals("red")); // return true

        System.out.println(s1 == "red"); // return ____ ^
        System.out.println(s2 == "red"); // return ____
    }
}

/*
It is wrong to compare string contents with "==" 
Reason:
"==" tests whether the LHS and RHS refer to the same object 
but not compare the contents. (Ref. Topic02 Page 9).

But for "why ^ returns true?"  
It is because JVM makes smart optimization for string literals 
("String Interning" - https://www.baeldung.com/java-string-pool)

** Similar thing happens in other compiled languages, e.g. C++.  ^_^
*/



this and super
==============
class A {
	
	public String tellMe() {return "a";}
	
	public void alg() {System.out.println(tellMe() + "*");}	
	
}

class B extends A {
	public String tellMe() {return "b";}

	public void alg() {System.out.println(tellMe() + "#");}	
}

class C extends A {
	public String tellMe() {return "c";}
}

public class Main {

	public static void main(String [] args)	
	{	
		A x1 = new B(); x1.alg(); //b#
		
		B x2 = new B(); x2.alg(); //b#
		
		A x0 = new A(); x0.alg(); //______________

		A x3 = new C(); x3.alg(); //______________

		C x4 = new C(); x4.alg(); //______________
	}		
}

/*
b#
b#
a*
c*
c*
*/


super, Object, toString
==============================
class X {  // the super class is Object

    public void alg() {        
        System.out.println(super.toString()); //like: X@28a418fc
        System.out.println(toString()); //like: X@28a418fc

        // Why the same?
        // Answer: 
        //    Similar to Employee and Manager:
        //    Inside Manager.java, 
        //          we can type: super.getName()
        //    Or    simply type: getName();
    }
}

public class Main {
    public static void main(String[] args) {
	X x = new X();
        x.alg();
    }
}


this, super, Object, toString
==============================
class A {	
	public String toString() {return "a";}
	
	public void alg() {System.out.println(super.toString() + "*");}	
}

class B extends A {
	public String toString() {return "b";}
	
	public void alg() {System.out.println(super.toString() + "#");}	
}

class C extends A {
	public String toString() {return "c";}
}

public class Main {

	public static void main(String [] args)	
	{	
		A x1 = new B(); x1.alg(); //b#
		
		B x2 = new B(); x2.alg(); //b#
		
		A x0 = new A(); x0.alg(); //______________

		A x3 = new C(); x3.alg(); //______________

		C x4 = new C(); x4.alg(); //______________
	}		
}

/*
a#
a#
A@2a139a55*
C@368239c8*
C@9e89d68*
*/


Dimension needed?
==================
import java.util.*;

public class Main {

	public static void main(String [] args)	
	{	
		ArrayList<int> arr1 = new ArrayList<>(); //Compilation error
		//ArrayList<int[]> arr2 = new ArrayList<>(); //OK
	}		
}