$(2)$ Given:

public class X

Eprivate int $a$;

public int $b$;$.$

public static int $c$;

$3$

Given:

Class Y has main$(...$

Valid or invalid:

$x$@$b_j=$ new $)$;

$-b_j*a={20}_j$

$0b{j}^{\text{'}}*b={30}^{\text{'}},$

$(3)$ main$(...)($Valid or invalid$)$

boolean $flg=$ objl $$ obj$2$;

$(4)$ From #$2$:

main $(...)$

$X$ obj$=$ new $X()$;

Obj.$c=10$;

$X.c=20$;

Valid or invalid