Java boolean Variable Default Value

I have read in a few places that the default value of a boolean variable is false so I decided to check for myself. My first attempt is shown in prog58 below:

andrew@UBUNTU:~/Java$ cat prog58.java
public class prog58
{
public static void main (String args[])
  {
  boolean b1;
  System.out.println("b1 is " + b1);
  }
}
andrew@UBUNTU:~/Java$ javac prog58.java
prog58.java:6: variable b1 might not have been initialized
  System.out.println("b1 is " + b1);
                                ^
1 error
andrew@UBUNTU:~/Java$

My second attempt is shown in prog59 below, which does suggest that the default value of a boolean variable is false:

andrew@UBUNTU:~/Java$ cat prog59.java
public class prog59
{
static boolean b1;
public static void main (String args[])
  {
  System.out.println("b1 is " + b1);
  }
}
andrew@UBUNTU:~/Java$ javac prog59.java
andrew@UBUNTU:~/Java$ java prog59
b1 is false
andrew@UBUNTU:~/Java$

The two programs are slightly different. Once I have worked out why one seems to work and the other does not, I will let you know.

Use Java to Work Out Your Lottery Numbers

To enter the UK lottery, you need to choose six different numbers between 1 and 49.

 

This program uses a for loop to create 100 lines of lottery numbers.

 

It stores each line’s values in an array then uses a boolean and a while loop to ensure that each number is only used once per line.

 

The individual numbers are chosen using Math.random(). In common with many other programming languages, this generates a random value between 0 and 1 so it has to be converted to give a value between 1 and 49:

 

Solaris > cat prog44.java

public class prog44

{

public static void main (String args[])

  {

  int x;

  for(x=1;x<=100;x++)

    {

    int[] numbers_used = new int[6];

    int y;

    for (y=0;y<=5;y++)

      {

      boolean number_found = false;

      while (!number_found)

        {

        int lottery_number =(int)(Math.random()*49)+1;

        number_found = true;

        int z;

        for (z=0;z<y;z++)

          {

          if (lottery_number == numbers_used[z])

            {

            number_found = false;

            break;

            }

          }

        if (number_found)

          {

          numbers_used [y] = lottery_number;

          if (lottery_number <10) System.out.print(" ");

          System.out.print(lottery_number+" ");

          }

        }

      }

    System.out.print("\n");

    }

  }

}

Solaris > javac prog44.java

Solaris > java prog44

38 15  8 36 40 32

47 31  8 22 25 29

34 24 35 19 42 28

 4  8 10  6 40 43

35  8 20 19 34 15

27  2 31 42 43 39

13  5 47  1 40 36

22  5  6  1 42  3

41  9 45 29  4  3

27 11 18 26 43 22

34 41 18 30  1 38

11 44  4 42 16 48

21  4 47 41 40 19

49 30 38 12 31 24

35 24 37 49 38 36

36  2  9 30 32 17

17 37 27 32  3 20

31  5 24 44 20 45

22 15 19  9 20 44

24 35 48 36  4  6

41  8 28 42 14 19

34  5 12  2 43 13

23 21 48 11  9 42

39 18 38 34  9 10

10 40 25 49 23 31

 7 11  5 17 37 45

15 20 21  9  6 40

14  6 18 15 22  8

 5 27 11  8 22 13

46 47 27  9 20 18

17  7 14 18 11 41

44  7 32  8 27  1

33 15 34 19 45 31

44 34  2  7 24 15

49 32  6 41 20 16

38 34 48 25 39 17

 4 33 46 43 49 28

26  5 22 46  1 16

30 12 25 19 40  2

49 25  3 31 39 18

48 30  1 13 39 16

25 14 24 37 18 19

 3  1  7 43 25 29

46 10 13 22 40 16

 8  1 39 21 48 23

37  2 39 14 20 23

11 44 15 21 12 17

27  4 40  3  1 10

36 47 34 13 11 19

43 31 13 29 45 42

 1 27  5 33 35 26

 1 36 45 49 48 27

22 16 43 17 10 32

44 35 17 15  2 40

 1 42 45 14 12 13

29  7 31 16 48 17

 3  4 13 42  5 23

20  7 17 28 45 47

22  6 49 48 15 18

12 24 20 44 28 38

 3 11 14 25 44 17

10 12 46  5 22 11

45  5 41 39 47 14

49 46 24 43  1 15

40 30 45 35 29 25

18 30 12 38 28 20

 7  2 45 49 15 22

25 45 42  6 37 43

43  7 29 19 44 12

 4 24 40 41 20 35

17 11 25 31 40 19

 9 48 24 33 43 17

43 25 27 37 13 18

 5  9 34 12 29 15

23 48 26 31 36 27

15  4 11 29 18 26

49 24  5 44 30 27

15 40 20 37 46 27

17 12 33 10 31  5

36 17 23  4 44 16

29 38  9  4 26 47

17 47  4 19 14 27

11 31  4 25 23 44

27 45 26  6 20  7

29 42 40 37  1 22

38 12 42  3 22 20

40 15 23 17 18 22

27 22  8 29 48 34

43  9 34 46 30  6

26 35 24  5  3 16

 4 19 49 28 25 39

21 40  9 37 22 32

 8 42 35 12 43 37

38 21 44 18 46 48

 1 40 41 48  3 13

19 48  1 27 18 33

26 31 43 19 46 25

40  2 47 26 21 49

20 46 10  8 34 24

22 15 18 33 39 41

Solaris >

Java boolean Variables

These store the values true or false. There are two in the example below.

The first is called one_greater_than_two and stores the value obtained by evaluating the condition 1 > 2, which is clearly false.

The second is called two_greater_than_one and stores the value obtained by evaluating the condition 2 > 1, which is clearly true.

The tests at the end of the program check that the two booleans have been assigned the correct values:

UBUNTU > cat prog20.java
public class prog20
{
public static void main (String args[])
  {
  boolean one_greater_than_two = (1 > 2);
  boolean two_greater_than_one = (2 > 1);

  if (one_greater_than_two)
    {System.out.println ("1 > 2");}

  if (two_greater_than_one)
    {System.out.println ("2 > 1");}
  }
}
UBUNTU > javac prog20.java
UBUNTU > java prog20
2 > 1
UBUNTU >

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