class Program {
	static String[] listaUsuarios = new String[] { "1", "12", "123" };
	public static void main (String[] args) throws java.lang.Exception {
		System.out.println(autentica("123"));
		System.out.println(autentica("456"));
	}
	public static boolean autentica (String usuario) {
	    for(int i = 0; i < listaUsuarios.length; i++) {
    	    if(usuario.equals(listaUsuarios[i])) {
        	    return true;
        	}
    	}
    	//opcionalmente faz alguma coisa aqui.
    	return false;
	}
}
 
//https://pt.stackoverflow.com/q/48512/101

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