Your sayig you want to check that a does not equal c?
If so then:

if(!a.equals(c))
{
//code This will be executed if a does _not_ equal c
}

Notice the '!', that is the inverse operator, which is will turn true to
false, and false  to true, therefore the above logic would work.
Hope I understood you correctly, and vice versa

should be if(a!=c)..........................

Remember that will only work if the Strings are created literally, and not
with new String(..);

i think this should be work
if( !(a.equals(c))

The equals method will check with the contents of the
String are the same while the == will check if the
references are to the same object

so if(a == c) will equate to false.

Use the unary ! operator.
if( !a.equals(c))