Simple things that we forget

Create a few simple classes (one extends the other)

   1: class A {

   2:     public String prop_a1 = "property a1";

   3:  

   4:     public String method_a2() {

   5:         return "return from method_a2";

   6:     }

   7: }

   1: class B extends A {

   2:     public String prop_b1 = "property b1";

   3:     public String method_b2() {

   4:         return "return from method_b2";

   5:     }

   6: }

Now test the methods from inheritance chain...

   1: class C {

   2:     public static void main(String [] args) {

   3:         A a = new B(); // this is upcasting

   4:  

   5: System.out.println(a.prop_a1);

   6:         System.out.println(a.method_a2());

   7:  

   8:  

   9:         /**

  10:         ** The following code is compile-time error

  11:         ** System.out.println(a.prop_b1);

  12:         ** System.out.println(a.method_b2());

  13: 

  14:                 C:\Java\test4>javac A.java

  15:                 A.java:24: cannot find symbol

  16:                 symbol : variable prop_b1

  17:                 location: class A

  18:                 System.out.println(a.prop_b1);

  19:                                    ^

  20:                 A.java:25: cannot find symbol

  21:                 symbol : method method_b2()

  22:                 location: class A

  23:                 System.out.println(a.method_b2());

  24:                                    ^

  25:                 2 errors

  26:             **/

  27:  

  28:  

  29:  

  30:  

  31:         B b = new A();

  32:         /**

  33:          ** B b = new A();

  34:          ** The above code is "downcasting" [I stand corrected]

  35:          * C:\Java\test4>javac A.java

  36:                 A.java:45: incompatible types

  37:                 found : A

  38:                 required: B

  39:                 B b = new A();

  40:                 ^

  41:                 1 error

  42:             **/

  43:  

  44:             B b = (B)new A();

  45:                 /**

  46:                   * Casting will compile

  47:                   * But causes a runtime error

  48:                  **/

  49:  

  50:         }

  51: }

 

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[Edit]

Looking at the comments below, I feel compelled to write the reason for these snippets of code. In ActionScript 3.0, the following works in standard mode (deferred type checking)

   1: class ClassBase

   2: {

   3: }

You can subsequently create a subclass of ClassBase named ClassExtender, which has one property named someString, as follows:

   1: class ClassExtender extends ClassBase

   2: {

   3:     var someString:String;

   4: }

and finally:

   1: var myClass:ClassBase = new ClassExtender();

   2: myClass.someString = 

   3: "hello";

   4: // no error in ActionScript 3.0 standard mode

 

[Edit]

Few more: public static void main(String[] args) { int i = 10; Integer I = new Integer(10); System.out.println("I == i : "+ (I == i)); // true System.out.println("I.equals(i) : "+ I.equals(i)); // true long l = 10L; System.out.println("I == l : " + (I == l)); // true Long L = new Long((long)10); System.out.println("I == L : Compiler Error...Incompatible datatypes"); System.out.println("I.equals(L) : " + I.equals(L)); // false }