Java handling of Boolean and NULL

Question

Boolean object in JAVA can have 3 values True, False, NULL

public class First {

    public static void main(String args[])
    {
        System.out.println("equals(new Boolean(\"True\"),True) :: " + isEqual(new Boolean("True"), true));
        System.out.println("equals(new Boolean(\"False\"), new Boolean(null)) :: " + isEqual(new Boolean("False"), new Boolean(null)));
        System.out.println("equals(new Boolean(\"False\"), null)) :: " + isEqual(new Boolean("False"), null));
    }

    static boolean isEqual(Boolean a, Boolean b)
    {
        return a.equals(b);
    }
}

Output for above code is

equals(new Boolean("True"),True) :: true
equals(new Boolean("False"), new Boolean(null)) :: true
equals(new Boolean("False"), null)) :: false

Please explain why Case 2 returns true but Case 3 returns false

Obviously null cannot be equal to any valid, non-null reference. — duffymo
– duffymo, Commented Aug 4, 2015 at 9:41
How can be null equal to any other value ? and in second you are creating object new Boolean(null) — Subodh Joshi
– Subodh Joshi, Commented Aug 4, 2015 at 9:42
new Boolean(null) means a Boolean object created and its value is false. — kswaughs
– kswaughs, Commented Aug 4, 2015 at 9:46

Pramod Karandikar · Accepted Answer · 2015-08-04 09:45:40Z

9

That's because the constructor for Boolean, if provided with null will allocate a Boolean object representing the value false

Read here: http://docs.oracle.com/javase/7/docs/api/java/lang/Boolean.html

public Boolean(String s)

Allocates a Boolean object representing the value true if the string argument is not null and is equal, ignoring case, to the string "true". Otherwise, allocate a Boolean object representing the value false. Examples: new Boolean("True") produces a Boolean object that represents true. new Boolean("yes") produces a Boolean object that represents false. Parameters:s - the string to be converted to a Boolean.

answered Aug 4, 2015 at 9:45

Pramod Karandikar

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Comments

Pramod Karandikar · Accepted Answer · 2017-10-05 13:09:26Z

4

If you look at the source code of Booleanclass, you can se that passing a nullvalue returns false:

private static boolean toBoolean(String name) { 
    return ((name != null) && name.equalsIgnoreCase("true"));
 }

edited Oct 5, 2017 at 13:09

Pramod Karandikar

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answered Aug 4, 2015 at 9:44

Fran Montero

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1 Comment

Adam Over a year ago

I don't know what version you were using then but in 1.8 there is public static boolean parseBoolean(String s) { return ((s != null) && s.equalsIgnoreCase("true")); }

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