第一次使用构造函数

Question

It compiles just fine... but throws a " java.lang.NullPointerException " error on when I try to enter the index (as an integer). I thought I already gave index an int type so I'm not sure why this happens.

I'm new to java so if you guys have any pointers on something else I need to look into or try those tips, would be appreciated as well.

import java.util.Scanner;

class LineEditor
{
  public static void main (String [ ] args)
  {
    //variables
    String myLine;
    String str;
    int index;

    Scanner scan = new Scanner(System.in);

    //creates original myLine
    myLine = new String ("Computer Science");
    System.out.println ("The original string of text is: " + myLine);

    //variable inputs
    System.out.println("Enter a string to alter myLine: ");
    str = scan.next();
    System.out.println("Enter an index for the string to be inserted at: ");
    index = scan.nextInt();

    Insert insert = new Insert(str, index);

    System.out.println ("The altered string is: " + insert.strIntoMyLine());
  }
}

class Insert
{
  String str;
  int index;
  String myLine;

  public Insert (String s, int i)
  {
      str = s;
      index = i;
  }

  String strIntoMyLine()
   {
      String part1;
      String part2;
      part1 = myLine.substring (0, index);
      part2 = myLine.substring (index);
      return (part1 + str + part2);
    }
}

Answer 1

It seems that there is nothing present in myLine .

Replace Insert insert = new Insert(str, index); with Insert insert = new Insert(str, index,myLine); .

and public Insert (String s, int i) with public Insert (String s, int I, String myLine)

Also

this.myLine = myLine; in the constructor

Answer 2

your constructor should be

public Insert(String s, int i, String r) {
    str = s;
    num = i;
    myLine = r;
}

and pass value like this "Insert insert = new Insert(str, index,myLine);"