Boolean methods

[jignesh soni]

I am confused with boolean methods.

boolean frighten(int d){
System.out.println("arrrrgh");
return true;
}


boolean frighten(int x){
System.out.println("a bite");
return false;
}


What will be the output of these methods ?
first metho will print, because its return value is true and second method will not print because its return value is false. Is that right ?

My confusio is, the method will return something only if return value is true otherwise if the value is false it will not output anything. So for boolean method to return anything, the return value has to be true, am I right ?

Pls explain, and thanks for your co-operation.

[fred rosenberger]

what a method returns is unrelated to what it does. ideally there is some logical connection, but there doesn't have to be.

a method that is declared as boolean simply means that it will return a boolean - either true or false. The method could print something, could compute a 13th root of a 100 digit number, play a song... doesn't matter what it does, but when it's done, it sends a boolean back to the caller.

This value in no way effects what the method does. so, in your examples...

an int is passed into both methods. it happens that that int is never used, but it's still there if you need it.

both methods then print their respective strings. one then returns true to the caller, and the other returns false. those values in no way alter what the method does.

[Harish Kashyap]

consider this

if(x){
System.out.print(" x is true, so I am printing");
}else
{
// x is false, I don't print anything
}

this piece of code will do exactly what you said. but this is not a function.

All functions with a return type must return a value of that type.
So, both your functions have to return a boolean value i.e. either true or false.

so if you call your first function as
System.out.println(frighten(1));
it will print:
arrrrgh
true

and same if you do with your second function definition, it will print
a bite
false

the functions do what they are programmed to do and then return the value irrespective of what it is (as long as it confirms to the return type).

[jignesh soni]

Thanks for reply guys.

But still not clear.
I will put the whole issue here. This is taken fron Head First Java book


class Monster{
boolean frighten(int d){
System.out.println("arrrrrgh");
return true;
}
}

class Vampire extends Monster {
boolean frighten(int x){
System.out.println("a bite?");
return false ;
}
}


class Dragon extends Monster {
boolean frighten(int degree){
System.out.println("breath fire");
return true;
}
}

public class MonsterTestDrive{
public static void main(String[] args){
Monster[] ma = new Monster[3] ;
ma[0] = new Vampire();
ma[1] = new Dragon();
ma[2] = new Monster();

for(int x=0; x<3; x++){
ma[x].frighten(x);
}
}
}



will this be the output for this coding , when I run MonsterTestDrive at command prompt ?

a bite ? false
breath fire true
arrrrgh true

Pls explain.
Thanks for youe replies

[Andre Brito]

Bro,

I tried this

public class Test1
{
    public Test1() { }
 
    boolean test(int a) {
        System.out.println("False");
        return false; 
    }
    
    boolean test(int b) {
        System.out.println("True");
        return true;
    }
    
 
    
    public static void main(String[] args)
    {
        Test1 t = new Test1();
        if (t.test()) {
            System.out.println("The decision has already been showed, righ?");
        }
        else {
            System.out.println("The decision has already been showed, righ?");
        }
    }
        
}

And there is a syntax error at the line that the method is duplicated.
How did you compiled it?
Is anything wrong with my code?

Tnks.

[Christophe Verré]

will this be the output for this coding , when I run MonsterTestDrive at command prompt ?

a bite ? false
breath fire true
arrrrgh true

Where are you printing the returned value ? Nowhere. "return true;" does not print anything. It justs returns true. Same for "return false;", which returns false. Nothing printed here either.
If you want to print out the returned value, you could change your code like this :

for(int x=0; x<3; x++){
System.out.println(ma[x].frighten(x));
}

Here is another example (that does not compile):

boolean isTodayFriday() {
System.println("Checking if it is friday..."); // here, the message is printed
if ( today == FRIDAY ) {
return true; // yes it is.
} else {
return false; // no it's not
}
}
 
void checkDay() {
// Print the returned value of isTodayFriday method
System.out.println("Is it Friday today ? -> " + isTodayFriday() );
}

[ December 13, 2007: Message edited by: Christophe Verre ]

[jignesh soni]

Where are you printing the returned value ? Nowhere. "return true;" does not print anything. It justs returns true. Same for "return false;", which returns false. Nothing printed here either.
If you want to print out the returned value, you could change your code like this :

So my question is, if boolean method just prints whatever is on println irrespective of return value true or false then whats the meaning of return value ? what is the significance of it ?

[jignesh soni]

--------------------------------------------------------------------------
Where are you printing the returned value ? Nowhere. "return true;" does not print anything. It justs returns true. Same for "return false;", which returns false. Nothing printed here either.
If you want to print out the returned value, you could change your code like this :
--------------------------------------------------------------------------
So my question is, if boolean method just prints whatever is on println statement irrespective of boolean value true or false, then whats the significance of true or false value and why do we need it ?

[Balasubramanian Chandrasekaran]

Hi Andre,

Originally posted by Andre Brito Fonseca:
Bro,

I tried this

public class Test1
{
    public Test1() { }
 
    boolean test(int a) {
        System.out.println("False");
        return false; 
    }
    
    boolean test(int b) {
        System.out.println("True");
        return true;
    }
    
 
    
    public static void main(String[] args)
    {
        Test1 t = new Test1();
        if (t.test()) {
            System.out.println("The decision has already been showed, righ?");
        }
        else {
            System.out.println("The decision has already been showed, righ?");
        }
    }
        
}

And there is a syntax error at the line that the method is duplicated.
How did you compiled it?
Is anything wrong with my code?

Tnks.

What you are trying to do is making a duplicate method inside the same class itself,which compiler will not allow.

[Balasubramanian Chandrasekaran]

Hi Christophe Verre,

Originally posted by Christophe Verre:

...Here is another example (that does not compile):

boolean isTodayFriday() {
System.println("Checking if it is friday..."); // here, the message is printed
if ( today == FRIDAY ) {
return true; // yes it is.
} else {
return false; // no it's not
}
}
 
void checkDay() {
// Print the returned value of isTodayFriday method
System.out.println("Is it Friday today ? -> " + isTodayFriday() );
}

[ December 13, 2007: Message edited by: Christophe Verre ]

Just make a couple of changes in your code and it will surely work.Change you code like this

  
public class Express {
        
    static String today = ""; //Give today value here to check equality
    /**
     * Creates a new instance of <code>Express</code>.
     */
    public Express() {
    }
    
    boolean isTodayFriday() {
System.out.println("Checking if it is friday..."); // here, the message is printed
if ( today.equlas("FRIDAY") )  //Here one more thing is that never use == to compare Strings
        {
return true; // yes it is.
} else {
return false; // no it's not
}
}
 
void checkDay() {
// Print the returned value of isTodayFriday method
System.out.println("Is it Friday today ? -> " + isTodayFriday() );
}

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Express e = new Express();
        e.checkDay();
    }
}
  

[Christophe Verré]

Balasubramanian :

Just make a couple of changes in your code and it will surely work.

You don't seem to understand the problem here. It's not about compile errors, but about the meaning of boolean values. I was just posting a kind of algorithm here. Please read the first post to understand what is being discussed here.

jignesh:

So my question is, if boolean method just prints whatever is on println statement irrespective of boolean value true or false, then whats the significance of true or false value and why do we need it ?

Let's take it back to your first example :

boolean frighten(int x){
System.out.println("a bite");
return false;
}

As long as the frigthen method is called, you now understand that "a bite" will be printed out, don't you ? So what is the use of "return false;" ? Your method returns a boolean. You will use that returned value to do something else after this method has been called. You may want to do different things according to the returned value. For example, print another message only when the returned value is true :

if ( frighten(1) == true ) {
// Do something
}

This method is not very interesting, so let's change it a bit. Let's imagine that only kids under 5 years old will get frightened in the monster house :

boolean frighten(int age){
System.out.println("a bite");
if ( age < 5 ) {
// Young kid has been frightened by the vampire
return true;
} else {
// Too old to get frightened by the vampire
return false;
}
}
 
void enterMonsterHouse() {
int kevinAge = 4;// Kevin's age
int bobAge = 24;// Bob's age
// The following messages will be printed if the person has been scared by the vampire
if ( frighten(kevinAge) == true ) {
System.out.println("Kevin is scared");
}
if ( frighten(bobAge) == true ) {
System.out.println("Bob is scared");
}
}

[Balasubramanian Chandrasekaran]

K i got that point.

[jignesh soni]

Thanks Christopher , I guess now I have begu to understand boolean. That was good explanation.