How to make a maxium number of the position of the decimal point, in java

There are various options for this. Given that your method can only return int, there aren't very many options available, so you could just write:

private static final int[] results = { 9, 99, 999, 9999, ... };

public static int getMax(int a) {
    // TODO: Validate argument
    return results[a - 1];
}

Or you could loop:

public static int getMax(int a) {
    // TODO: Validate argument
    int result = 9;
    for (int i = 1; i < a; i++) {
        result = result * 10 + 9;
    }
}

Or you could use Math.pow(), given that each result is 10^a - 1:

public static int getMax(int a) {
    // TODO: Validate argument
    return (int) (Math.pow(10, a) - 1);
}