Concatenation with arraylist

Question

This is my code. I know how to use the concat operator, but not with arrays or arraylist.

import java.util.*;
class stringProject {
    static String concat(String str1, String str2) {
        return str1.concat(str2);
    }

    public static void main(String[] args) 
    {
      ArrayList<String> projectStrings = new ArrayList<String>();
      projectStrings.add("The ");
      projectStrings.add("quick ");
      projectStrings.add("brown ");
      projectStrings.add("fox ");
      projectStrings.add("jumped ");
      projectStrings.add("over ");
      projectStrings.add("the ");
      projectStrings.add("lazy ");
      projectStrings.add("dog.");
    }
    System.out.println(projectStrings.get(1).concat(projectStrings.get(0)));
    }
}

And my errors are:

File: C:\Users\Daniel\stringProject.java  [line: 23]
Error: Syntax error on token "println", Identifier expected after this token
File: C:\Users\Daniel\stringProject.java  [line: 23]
Error: Syntax error on tokens, AnnotationName expected instead
File: C:\Users\Daniel\stringProject.java  [line: 23]
Error: Syntax error on token ")", delete this token
File: C:\Users\Daniel\stringProject.java  [line: 23]
Error: Syntax error, insert "Type VariableDeclaratorId" to complete FormalParameter
File: C:\Users\Daniel\stringProject.java  [line: 23]
Error: Syntax error, insert ")" to complete MethodDeclaration
File: C:\Users\Daniel\stringProject.java  [line: 25]
Error: Syntax error on token "}", delete this token

Answer 1

The main problems in your current code:

1. You have to define you're working with a list of Strings. You can do this by adding the String generic in your ArrayList declaration, otherwise you're declaring a raw ArrayList, thus all the elements will be treated as Objects:

   ArrayList<String> projectStrings = new ArrayList<String>();
   

   or the Java 7 way, using the diamond operator:

   ArrayList<String> projectStrings = new ArrayList<>();
   

2. You haven't defined a concat method in your code. Probably you want to use String#concat:

   System.out.println(projectStrings.get(1).concat(projectStrings(0)));
   

Another way to concat Strings is by using the + symbol:

System.out.println(projectStrings.get(1) + projectStrings(0));

Just in case you want your current code to work, define a concat method outside the main method:

class stringProject {
    //this method should be static in order to be used in other static methods like main
    static String concat(String str1, String str2) {
        //naive implementation
        return str1.concat(str2);
    }

    public static void main(String[] args) {
        //your current code here...
        //since you have defined a concat method, you can use it with no problems
        System.out.println(concat(projectStrings.get(1), projectStrings.get(0)));
    }
}

Another advice: you don't need to add import java.lang.*, this is added by default by the compiler, so you can remove this line.

---

After your code edit, you closed the main method just before printing your output. This is how your code looks now:

public static void main(String[] args) 
{ //this is where your main method starts
  ArrayList<String> projectStrings = new ArrayList<String>();
  projectStrings.add("The ");
  projectStrings.add("quick ");
  projectStrings.add("brown ");
  projectStrings.add("fox ");
  projectStrings.add("jumped ");
  projectStrings.add("over ");
  projectStrings.add("the ");
  projectStrings.add("lazy ");
  projectStrings.add("dog.");
} //this is where your main method ends
//this line is outside any method, thus you get those errors
System.out.println(projectStrings.get(1).concat(projectStrings.get(0)));
} //this is where your stringProject class definition ends

Solution: just move that System.out.println(...) line inside your main method:

public static void main(String[] args) 
{ //this is where your main method starts
  ArrayList<String> projectStrings = new ArrayList<String>();
  projectStrings.add("The ");
  projectStrings.add("quick ");
  projectStrings.add("brown ");
  projectStrings.add("fox ");
  projectStrings.add("jumped ");
  projectStrings.add("over ");
  projectStrings.add("the ");
  projectStrings.add("lazy ");
  projectStrings.add("dog.");
  //easy fix
  System.out.println(projectStrings.get(1).concat(projectStrings.get(0)));
} //this is where your main method ends
} //this is where your stringProject class definition ends

Answer 2

All you need is simply

ArrayList<String> projectStrings = new ArrayList<>();
//populate as needed
System.out.println(projectStrings.get(0) + projectStrings.get(1));

if you want to use ArrayList.

Note my use of generics. The ArrayList you defined is a collection of Object. Mine is a collection of String. Doing it your way costs you compile-time guarantees and also makes some of the other answers unworkable.

Answer 3

I think you just need to understand how ArrayList's work. You are probably trying to do this:

ArrayList<String> projectStrings = new ArrayList<String>();

...

System.out.println(projectStrings.get(1).concat(projectStrings.get(0)));