classes.md

Java is weird.

Inheritance

Parent constructor call

class A {

  A(){ System.out.println('1'); }

}

class B extends A {

  B(){ System.out.println('2'); }

}

class C {

  C(int x){ System.out.println('3'); }

}

class D extends C {                 // (Compile Error, no default constructor)

  D(){ System.out.println('4'); }

}

class E extends C {                 // (Valid, manually calls constructor)

  E(){ super(0); System.out.println('5'); }

}

new B(); // 12
new E(); // 35

Accessing Instance Vars

class A {

  private   int x = 1;
  protected int y = 2;
  public    int z = 3;

  A(){ }

}

class B extends A {

  B(){ }

  void set() { x = 99; }          // (Compile Error)

  void set2() { super.x = 99; }   // (Valid)

  void set3() {  
	   y = 99; z = 99;              // (Valid)
  }

  void set4() {
    super.y = 99; super.z = 99;   // (Valid)
  }

}

B test = new B();
test.x;            // (Compile Error, private)
test.y;            // 2
test.z             // 3

Changing Instance Vars

class A {

  int x = 1;

  A() { }

}

class B extends A {

  int x = 2;

  B() { }

}

B test = new B(); test.x; // 2
A test = new A(); test.x; // 1
A test = new B(); test.x; // 1

B test = new B();
test.x;                   // 2
((A)(test)).x;            // 1
((A)(test)).x = 3;
text.x;                   // 2
A test2 = text;
test2.x;                  // 3

Interface Vars

interface Dog {
  int a;               // (Compile Error, expects '=')
  int b = 1;           // (Valid, made public static final)
  private int c = 2;   // (Compile Error, not allowed - only public)
}

Blocks

class Blocker {

	static {
		System.out.print('A');
	}

	{
		System.out.print('B');
	}

	{
		System.out.print('C');
	}

	Blocker(){
		System.out.print('D');
	}

}

new Blocker();  // ABCD
new Blocker();  // BCD
new Blocker();  // BCD