Digits-doWhile.java

public Digits( int num ) {
    digitList = new ArrayList<>();
    int n = num; 
    do {
        digitList.add(0, n % 10);
        n /= 10;
    } while (n > 0); 
}

Digits.java

public Digits( int num ) {
    digitList = new ArrayList<>();

    if ( num == 0 ) {
        digitList.add( 0 );
    } else {
        int n = num;
        while ( n > 0 ) {
            digitList.add( 0, n % 10 );
            n /= 10;
        }
    }
}

isStrictlyIncreasing.java

public boolean isStrictlyIncreasing() {
    for ( int i = 0; i < digitList.size() - 1; i++ ) {
        if ( digitList.get( i ) >= digitList.get( i + 1 ) ) {
            return false;
        }
    }
    return true;
}