Points learnt from Java mock exams

Java Certification, Programming, JavaBean and Object Oriented Reference Books Points learnt from Java mock exams

a) To get the width of an applet within the init method:
int width = this.getSize().w;
or
int width=getSize().w;

b) int x = evt.getX();
returns the horizontal location to the varaible x at the time of the event.

c) You can't add ActionListener to a TextArea but can be added to TextField. You can't add WindowListener and ContainerListener to TextField

d) class Test{
static int a = 1;
public static void main(String arg[]) {
int a;
System.out.println(a);
}
}

Even though there is a static variable named a, the local variable takes the precedence. So a compilation error occurs saying, 'a' may not be initialized

e) void loopTest() {
int x=0;
one: while(x < 10) {
two : System.out.println(++x);
if(x>3) break two;
}
}

will not compile because "two" is not associated with any loop

f) Panel p = new Applet(); //this is true
JPanel p = new JApplet(); //this is not true

see API for details.

g) byte x=-1;
x = x>>>5;
illegal because result of x>>>5 is int and can't be assigned to x which is byte.
it can be corrected as
byte x=-1;
x=(byte)(x>>>5)//legal

int x=100; float y = 100.0f;
if(x==y) {System.out.println("equal");}//converts int to float
output :
equal

byte b=2;
byte b1=3;
b=b*b1;//illegal

will not compile because before multipying both b and b1 will be converted to int and int can't be assigned to byte.

h)String s="Hello there";
String sub = s.substring(6);//sub="there"
String seg = s.substring(7,10);//seg="her";

creating a character array :
String s = "Hello there";
char[] arr;
arr=s.toCharArray();
System.out.println(arr[1]);//prints ""e"

creating String from character array :
char[] c={'H','e','l','l','o'};
String s = String.copyValueOf(c);

i)You can have any number of main method inside a class. You can also overload it. After all it is also a method. But remember that main is a static method.

class test {

   static int i[];

   public static void main(String arg[]) {

     System.out.println(i[2]); //gives NullPointerException

   }

}

k)
int k[][]=new int[10][20];
System.out.println(k.length); //prints 10
int k[][]={{1,2,3,4},{1,2,3},{5,4,6,4}};
System.out.println(k.length); //prints 3

l)Another way of initializing array
int num[] = new int[]{1,2,3};

m)Assigning one object to another (example from Thinking in Java)
1 Class Number {
2 int i;
3 }
4 public class Assignment {
5 public static void main(String[] args) {
6 Number n1 = new Number();
7 Number n2 = new Number();
8 n1.i = 9;
9 n2.i = 47;
10 System.out.println("1: n1.i: " + n1.i +
", n2.i: " + n2.i);
11 n1 = n2;
12 System.out.println("2: n1.i: " + n1.i +
", n2.i: " + n2.i);
13 n1.i = 27;
14 System.out.println("3: n1.i: " + n1.i +
", n2.i: " + n2.i);
15 }
16 }

Output:
1: n1.i: 9, n2.i: 47
2: n1.i: 47, n2.i: 47
3: n1.i: 27, n2.i: 27

Changing the n1 object, changes the n2 object. This is because both n1 and n2 contain the same reference, which is pointing to the same object.
Instead, if u change line number 11 to n1.i = n2.i;
The output will be
1: n1.i: 9, n2.i: 47
2: n1.i: 47, n2.i: 47
3: n1.i: 27, n2.i: 47

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