Can we rely on String.isEmpty for checking null condition on a String in Java?

No, absolutely not - because if acct is null, it won't even get to isEmpty... it will immediately throw a NullPointerException.

Your test should be:

if (acct != null && !acct.isEmpty())

Note the use of && here, rather than your || in the previous code; also note how in your previous code, your conditions were wrong anyway - even with && you would only have entered the if body if acct was an empty string.

Alternatively, using Guava:

if (!Strings.isNullOrEmpty(acct))

Use StringUtils.isEmpty instead, it will also check for null.

Examples are:

 StringUtils.isEmpty(null)      = true
 StringUtils.isEmpty("")        = true
 StringUtils.isEmpty(" ")       = false
 StringUtils.isEmpty("bob")     = false
 StringUtils.isEmpty("  bob  ") = false

See more on official Documentation on String Utils.