如何控制字符串迭代中定界符（，）的添加

Question

I'm using this code that gets the values from the arraylist. After a data has been retrieved, i want to concatenate a (,) after it. The concatenation of data will stop if retrieved data will make the length count of the whole string greater than 160. After which, i want to stop adding comma since no data will follow after. How can i do this?

 List<Pending> pending = db.getAllPending();
 List<String> resultingStrings = new ArrayList<String>();
 String a = "";
 for (Pending pn : pending) {
     if (a.length() + pn.getPm_str().length() <= 160) {
         a+=pn.getPm_str();
         a+=",";
     } else {
         resultingStrings.add(a);
          a = pn.getPm_str();
     }
 }
 resultingStrings.add(a);

Output

RBSN/C1/12/3/4/8,CVTE/C1/2/3/4/1,RBSN/C1/4/2131/331/12,RBSN/C1/45/67/12/44,RBSN/C1/44/231/123/23213,RBSN/C1/444/55522/222/11,RBSN/C1/123/123/213123/1231232,

There's a comma after the last concatenated data. Total string length: 146

Expected Output

RBSN/C1/12/3/4/8,CVTE/C1/2/3/4/1,RBSN/C1/4/2131/331/12,RBSN/C1/45/67/12/44,RBSN/C1/44/231/123/23213,RBSN/C1/444/55522/222/11,RBSN/C1/123/123/213123/1231232

No comma after the last data concatenated. Total string length: 145

Answer 1

Rather than adding the comma after you add the element add it before so you'll never have a comma at the end.

List<Pending> pending = db.getAllPending();
List<String> resultingStrings = new ArrayList<String>();
String a = "";
for (Pending pn : pending) {
  if (a.length() + pn.getPm_str().length() < 160) { // < to leave room for the comma as well
        if(a.length() != 0) { // don't add a comma before the first 1
          a+=",";
        }
        a+=pn.getPm_str();
  } else {
        resultingStrings.add(a);
        a = pn.getPm_str();
  }
}
resultingStrings.add(a);

Answer 2

Add one line to the else statement:

List<Pending> pending = db.getAllPending();
List<String> resultingStrings = new ArrayList<String>();
String a = "";

for (Pending pn : pending) {
    if (a.length() + pn.getPm_str().length() <= 160) {
        a+=pn.getPm_str();
        a+=",";
    }
    else {
        a = a.substring(0, a.length()-1)  <-----
        resultingStrings.add(a);
        a = pn.getPm_str();
    }
}
resultingStrings.add(a);

Answer 3

This is called the trailing comma problem. There are a lot of ways to solve this.

One way is to take the final string and just remove the last character:

s = s.substring(0, s.length()-1);

Another way is to convert your loop to a regular for loop and check whether it's not at the end before you add a comma.

 List<Pending> pending = db.getAllPending();
 List<String> resultingStrings = new ArrayList<String>();
 String a = "";
 for (int i = 0; i < pending.size(); i++) {
     Pending p = pending.get(i);
     if (a.length() + pn.getPm_str().length() <= 160) {
         a+=pn.getPm_str();
         if(i != pending.size()-1){
             a+=",";
         }
     } else {
         resultingStrings.add(a);
          a = pn.getPm_str();
     }
 }
 resultingStrings.add(a);

Answer 4

Use StringBuilder. Once the string is ready, delete the last character, cast it to String and you have your answer.

List<Pending> pending = db.getAllPending();
 List<String> resultingStrings = new ArrayList<String>();
 StringBuilder a = new StringBuilder;
 for (Pending pn : pending) 
     if (a.length() + pn.getPm_str().length() <= 160) {
         a.append(pn.getPm_str()).append(",");
     }
     else {
         a.deleteCharAt(a.length());
         resultingStrings.add(a);
         a = new StringBuilder();
         a.append(pn.getPm_str());
     }
 a.deleteCharAt(a.length());
 resultingStrings.add(a.toString());

Answer 5

Use StringBuilder (for efficiency), and only insert a comma when necessary.

List<Pending> pending = db.getAllPending();
List<String> resultingStrings = new ArrayList<>();
StringBuilder sb = new StringBuilder();
for (Pending pn : pending) {
    final String str = pn.getPm_str();
    if (sb.length() > 0) {
        sb.append(',');
    }
    if (sb.length() + str.length() <= 160) {
        sb.append(str);
    } else {
        resultingStrings.add(sb.toString());
        sb.setLength(0);
        sb.append(str);
    }
}
resultingStrings.add(sb.toString());

Answer 6

Why do not you add directly to resultingString List? Since List stores the strings separated by comma you do not need to do it by yourself. And you can count the length of the strings stored using resultingString.toString().length() .

List<Pending> pending = db.getAllPending();
List<String> resultingStrings = new ArrayList<String>();
for (Pending pn : pending) {
     // 162 considering the starting and closing brackets
     if ((resultingStrings.toString().length()+pn.getPm_str())<= 162) {
         resultingStrings.add(pn.getPm_str());
      }
  }