Validator for mathematical expressions in infix notation
You can already reuse operators in each methods by making it a constant. You can create a Set and use the containsmethod instead of this.isIn.

You can simplify validateEndCharacters, if ( condition ) { return true } else { return false } can always be simplified by return condition.

The two else if looks like the second block will never be executed :

...
} else if (this.isIn(c, operators) &amp;&amp; prevOperator == '\0') {
    prevOperator = c;
} else if (this.isIn(c, operators) &amp;&amp; prevOperator == '\0') {
    isValid &amp;= false;
    break; 
}