Re: str.equals(null) or str==null ?

I think the following two programs explain all:
public class Test{
    public static void main(String[] args){
        String a = "hallo";
        System.out.println("Comparison with null: " + a.equals(null));
        System.out.println("Comparison with different String: " +
a.equals("ciao"));
        System.out.println("Comparison with same String: " +
a.equals("hallo"));
    }
}

public class Test{
    public static void main(String[] args){
        String a = null;
        System.out.println("Comparison with null: " + a.equals(null));
        System.out.println("Comparison with different String: " +
a.equals("ciao"));
        System.out.println("Comparison with same String: " +
a.equals("hallo"));
    }
}

Clearly in the second case a NullPointerException will be launched,
since "a" does not hold an actual reference to an object and therefore
no comparison can be invoked.

In general for reference types the "==" compares the operands on both
sides for the same reference value, i.e. if not both operance are
referencing to the same object in memory the result will be false.
Example:
public class Test{
    public static void main(String[] args){
        String a = new String("hallo");
        String b = new String("hallo");
        System.out.println(a==b);
    }
}
It will return false, even if both strings contain the same value.

The "equals()" on the other side is an instance method. The equals()
method implementation of the String class checks whether the content,
i.e. the string of the receiver String object is the same as that of
the passed String object.
Same example as above with equals():
public class Test{
    public static void main(String[] args){
        String a = new String("hallo");
        String b = new String("hallo");
        System.out.println(a.equals(b));
    }
}
Now it will return true, since it doesn't check for the references but
for the content.

But ATTENTION:
If you declare the Strings like this
         String a = "hallo";
Then also the "==" operator will return true!
Example:
public class Test{
    public static void main(String[] args){
        String a = "hallo";
        String b = "hallo";
        System.out.println(a==b);
    }
}
This is because the JVM manages this kind of String a little different.
It don't threats them as objects but as some kind of "native" datatype
(but I'm not really sure here)

In general it is to say, that whenever Strings have to be compared, the
equals() method should be used.
The "== null" should only be used to check whether the variable really
references to a String object to be sure not to get a
NullPointerException

kito

David Segall wrote: