将.indexOf（）与ArrayList一起使用<String>

Question

I am attempting to create a battleship game using Java. The actual game works, but I ran into a surprising (for me) issue along the way. The code below generates a random location from a grid of 49 values that a user has to guess from. As a debugging tool, I figured I would print out the index of the ArrayList's number value, but I got a -1 (found out that means a value not found). Why didn't it return a number, considering my math was right?

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;

public class GameHelper 
{
String alphabet = "abcdefg";
private int gridLength=7;
private int gridSize=49;
private int [] grid = new int [gridSize];
private int comCount=0;

public String getUserInput(String prompt)
{
    String inputLine=null;
    System.out.println(prompt);

    try
    {
        BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
        inputLine=is.readLine();
        if (inputLine.equals(null))
        {
            return null;
        }
    }

    catch(IOException e)
    {
        System.out.println("Buffered Reader Failure");
    }
    return inputLine;
}


public ArrayList <String> placeDotCom (int size)
{
    ArrayList <String> alphaCells = new ArrayList <String>();

    String temp=null;
    int [] coords=new int[size];
    int attempts=0;
    boolean success=false;
    int location=0;

    comCount++;
    int incr=1;
    if((comCount%2)==1)
    {
        incr=gridLength;
    }

    while (!success & attempts++ < 200)
    {
        location = (int)(Math.random()*gridSize);
        int x=0;
        success=true;
        while (success && x < size)
        {
            if (grid[location] ==0)
            {
                coords[x++]=location;
                location+=incr;
                if(location >= gridSize)
                {
                    success = false;
                }
                if(x>0 && (location % gridLength ==0))
                {
                    success = false;
                }
            }
                else
                {
                    System.out.println("used" + location);
                    success=false;
                }
            }
        }


        int x=0;
        int row=0;
        int column=0;

        while(x < size)
        {
            grid[coords[x]] = 1;
            row = (int)(coords[x]/gridLength);
            column = coords[x] % gridLength;
            temp = String.valueOf(alphabet.charAt(column));

            alphaCells.add(temp.concat(Integer.toString(row)));
            x++;
        }
        System.out.println("coord "+x+" = "+alphaCells.get(x-1));
        return alphaCells;
    }

} 

My attempted call:

for (DotCom dcset : dc)
{
ArrayList <String> newLocation = helper.placeDotCom(3);
dcset.setLocationCells(newLocation);
System.out.println(newLocation.indexOf(1));
}

I figured this was the only relevant code. If you would like me to post more, please tell me. This is my first time posting, and I'm not sure I asked this question correctly. Is this too vague?

Answer 1

From the documentation of ArrayList#indexOf(Object o)

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that Objects.equals(o, get(i)) , or -1 if there is no such index.

Notice that the method expects an Object as parameter (this has historical reasons, it stems from the dark ages where no generics existed in Java and all containers used Object ).

Now, you declared ArrayList <String> newLocation and call .indexOf(1) on this object. The passed int -value gets autoboxed into an Integer . Since your newLocation -list contains only String s, the passed Integer cannot be found in the list. Thus, it returns a -1 .

---

A remark on your code: As I wrote in my comment , inputLine.equals(null) will either return false (if inputLine is not null ) or throw a NullPointerException (if inputLine is null ). This is due to the fact that if the program tries to access an attribute or method on a null -reference, a NullPointerException will be thrown. Just write inputLine == null (in this case, == is correct). An alternative solution was proposed by Szychan in the comments : You can use Objects.isNull(inputLine) to check whether inputLine is null .