将循环结果另存为字符串

Question

    String input[] = request.getParameterValues("checkbox");

        for(int i=0;i<input.length;i++) {
            if (i==input.length-1) {
                System.out.print(input[i]+" ");
            } else {
                    System.out.print(input[i]+", ");
            }
        }

The result is that i print to console something like "CustomerId, FirstName, LastName, Phone". I want to save the result of the for loop as a string variable instead, so then i can do

     String query = "select "+result above+" from table";

How to do it?

Answer 1

You keep appending into a StringBuilder and then convert it to String

StringBuilder sb = new StringBuilder();
for(int i=0;i<input.length;i++) {
  if (i==input.length-1) {
     System.out.print(input[i]+" ");
     sb = sb.concat(input[i] + ",");
  } else {
     System.out.print(input[i]+", ");
     sb = sb.concat(input[i] + ",");
  }
}

Then,

String query = "select "+sb.toString()+" from table";

Answer 2

Use a StringBuilder to concatenate strings together. (it is technically possible to use String objects, and concatenate them together using the + operator, but that has a lot of downsides...)

    String input[] = request.getParameterValues("checkbox");

    StringBuilder sb = new StringBuilder(); // create empty StringBuilder instance
    for(int i=0;i<input.length;i++) {
        sb.append(input[i]);     //append element
        if (i==input.length-1) {
            sb.append(" ");      //append space
        } else {
            sb.append(", ");     //append comma
        }
    }

    String result = sb.toString();
    Systemout.println(result);

Or you could build ( see Builder pattern ) the whole query using a StringBuilder object, with methods for each part ( addFields(StringBuilder sb) , addFromPart(StringBuilder sb) , addWhereClause(StringBuilder sb) ), and voila, you have the insides of a small data access framework...

public abstract class MyQueryBuilder {

    protected abstract void addFields(StringBuilder sb);
    protected abstract void addFromPart(StringBuilder sb);
    protected abstract void addWhereClause(StringBuilder sb);

    public final String getQuery() {
        StringBuilder sb = new StringBuilder();
        sb.append("SELECT ");

        addFields(sb); //this adds the fields to be selected
        sb.append(" FROM ");

        addFromPart(sb); //this adds the tables in the FROM clause

        addWhereClause(sb); //this adds the where clause
        //...etc
        return sb.toString();
    }
}

Answer 3

You can do this using a StringBuilder instead of printing the values out.

String input[] = request.getParameterValues("checkbox");

    StringBuilder builder = new StringBuilder();

    for(int i=0;i<input.length;i++) {
        if (i==input.length-1) {
            builder.append(input[i]+" ");
        } else {
                builder.append(input[i]+", ");
        }
    }

Regards, kayz