A student is trying to convert the string present in x to a numerical value, so that he can find the square root of the converted value. However the code has an error. Name the error (syntax / logical / runtime). Correct the code so that it compiles and runs correctly.

String x= "25";
int y=Double.parseDouble(x); 
double r=Math.sqrt(y); 
System.out.println(r);

Give the output of the following code:

String P = "20", Q ="19";
int a = Integer.parseInt(P);
int b = Integer.valueOf(Q);
System.out.println(a+""+b);

Differentiate between boxing and unboxing.