如何迭代比Java中列表大小指定的次数更多的次数

Question

I have a list of class type which holds value from object of class type in following way

public  List<Vacc_vs6> refarray_vac1(String fdate,String ldate) throws SQLException, ParseException {
    st_jsp.clear();
    try {  
        con = getConnection();
        stmt = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_READ_ONLY);
        String vs1 = "sql query";
        stmt.executeQuery(vs1);
        rs = stmt.getResultSet();

        while (rs.next()) {
            Vacc_vs6 ref = new Vacc_vs6();

            ref.setLogtime(rs.getString(1));
            ref.setBeam_current(rs.getString(2));
            ref.setBeam_energy(rs.getString(3));
            ref.setst4_vs6_bag1_rb(rs.getString(4));
            ref.setst4_vs6_bag2_rb(rs.getString(5));
            ref.setst4_vs6_bag3_rb(rs.getString(6));
            ref.setst4_vs6_bag4_rb(rs.getString(7));
            ref.setst4_vs6_bag5_rb(rs.getString(8));
            ref.setst4_vs6_bag6_rb(rs.getString(9));
            ref.setst4_vs6_bag7_rb(rs.getString(10));
            ref.setst4_vs6_bag8_rb(rs.getString(11));
            ref.setst4_vs6_bag9_rb(rs.getString(12));
            st_jsp.add(ref);                  
        }
    } catch (Exception e) {
        System.out.println("\nException in refarray_vac1 " + e);
    }          
    return st_jsp;
}

st_jsp is the list. Now I take another list of of class type and want to add values into it corresponding to every column of list st_jsp. I also want to add only those values in the second list which satisfy the given if condition otherwise go to else and put null at particular columns. For this the code is-

List<Vacc_vs6> new_list=new ArrayList<Vacc_vs6>();
double is = 9.5;
double js = 10.5;
int no=0;
for (no=0;no<st_jsp.size();no++) {
    String i= st_jsp.get(no).getBeam_current();
    double im = Double.parseDouble(i) ;
    if(im>is && im<js) {
        new_list.addAll(st_jsp);
    } else  {
        new_list.addAll(null);
    }
    is +=10;js +=10; 
}

1. Problem-1--> how to add all the 12 columns corresponding to that index in new_list.addAll(???) ; Did I have to add column where I put ??

   ref.setLogtime(rs.getString(1)); ref.setBeam_current(rs.getString(2));

2. Problem-2 for(no=0;no<st_jsp.size();no++) loop will go till the size of st_jsp but if I do so then all values of st_jsp list will not be displayed because of else condition as I have to add 0 or null to those rows which do not satisfy the above if condition. What to do for this?

Answer 1

1 Problem-1--> **how to add all the 12 columns corresponding to that index in new_list.addAll(???);**Did I have to add column where I put ??

To add only the object that satisfies your condition to the new list you should do:

if (im > is && im < js) {
    new_list.add(st_jsp.get(no));
}

The method addAll() adds all elements from one collection into another. So, the way you are doing it, the first time your condition is satisfied you are adding every ref in st_jsp to new_list . And then you will keep re-adding the elements everytime new_list.addAll(st_jsp) is called. Using add() will add only the object you want.

2) Problem-2 for(no=0;no<st_jsp.size();no++) loop will go till the size of st_jsp but if I do so then all values of st_jsp list will not be displayed because of else condition as I ahve to add 0 or null to those rows whcih do not satisfy the above if condition.What to do for this

You are doing the same action in else that you are doing when the if condition is satisfied, that is, adding all elements from st_jsp to new_list :

if (im > is && im < js) {
    new_list.addAll(st_jsp);
} else {
    new_list.addAll(st_jsp);
}

This means that, after your loop, you will end with st_jsp.size() copies of st_jsp in new_list .

If you need to add an object containing null in all fields to new_list everytime an object does not satisfy your condition you first need to create this object, and then add it when the condition is not met. Also, if you need to test for each element in st_jsp if it satisfies the conditions for all values of is and js , you need to place another loop inside the for loop.

List<Vacc_vs6> new_list = new ArrayList<Vacc_vs6>();
for (int no=0; no < st_jsp.size(); no++) {
    double is = 9.5;
    double js = 10.5;
    double im = Double.parseDouble(st_jsp.get(no).getBeam_current());
    // flag to check if an element was added to new_list
    boolean added = false;
    while (is < 209.9 && js < 210.5) {
        if (im > is && im < js) {
            new_list.add(st_jsp.get(no));
            added = true;
            break;
        }
        is += 10;
        js += 10;
    }
    if (!added) {
        // If the element was not added, add an empty object,
        // assuming that no field is set when creating the object.
        new_list.add(new Vacc_vs6());
    }
}