pass value in Boolean Object

Question

public class BuildStuff { 

    public static void main(String[] args) {     
        Boolean test = new Boolean(true);     
        Integer x = 343;     
        Integer y = new BuildStuff().go(test, x);        
        System.out.println(y); 
    }  

    int go(Boolean b, int i)   {  
        if(b) 
            return (i/7);  
        return (i/49);  
    } 
}

This is from SCJP , I understand that answer is "49", but I have a doubt. When we create an object and pass a value in that object. Let's say: Dog d = new Dog("tommy"); in this statement d is reference to object Dog and Dog has "name" instance variable set to "tommy". But it doesn't mean d = tommy .

However, in case of Boolean, when we passed true in Boolean Object. It sets reference to true. Why it is in case of Boolean? Is there any other class too?

Answer 1

The passed-in Boolean b can be converted to a primitive boolean with an unboxing conversion, resulting in the expression true for the if statement.

All primitive types have corresponding built-in wrapper classes for them, and Java will unbox/box as necessary.

• Boolean <=> boolean
• Byte <=> byte
• Character <=> char
• Short <=> short
• Integer <=> int
• Long <=> long
• Float <=> float
• Double <=> double

(An unboxing conversion will fail with a NullPointerException if the corresponding wrapper class instance is null :

Boolean test = null;
// NPE
boolean b = test;

)

Here's Java's tutorial on boxing and unboxing .

Answer 2

From the javadoc : The Boolean class wraps a value of the primitive type boolean in an object. An object of type Boolean contains a single field whose type is boolean.

If you have

Boolean a = new Boolean(true);
Boolean b = new Boolean(true);

a and b are different objects, but a.booleanValue() and b.booleanValue() return the same primitive. This can be verified by testing a == b versus a.equals(b) .

There are other classes like this, viz. Integer, Double, etc. As others have already mentioned, you should look into autoboxing and unboxing.