New to Java

I want to check that the content of a text field is numeric. When I try:

if (isnum(answerString)==false) {
//my code
}

I am told the operator == cannot be used with isnum, boolean. When I try:

if (isnum(answerString)) {
//my code reversed
}

I am advised that a boolean was required and isnum was found.

Is there a quick and easy way of doing this correctly?