forEach() for java で、ループカンタを使用する場合、配列で定義しましょう

public List<MymemberMst> findByCondition(BeanMap beanMap) {
    String[] atriKeys = new String[beanMap.size()];
    Object[] atriVals = new Object[beanMap.size()];

    int i = 0; //★ココ
    beanMap.forEach((key, value)-> {
        atriKeys[ i ]  = key;
        atriVals[ i ] = value;
        i++;
    });
    :
}

↑こうでななく、↓こう

public List<MymemberMst> findByCondition(BeanMap beanMap) {
    String[] atriKeys = new String[beanMap.size()];
    Object[] atriVals = new Object[beanMap.size()];

    int[] i = { 0 }; //★ココ
    beanMap.forEach((key, value)-> {
        atriKeys[ i[0] ]  = key;
        atriVals[ i[0] ] = value;
        i[0]++;
    });
    :
}