How do I fix this java.lang.OutOfMemoryError: Java heap space?

Question

I'm getting a java.lang.OutOfMemoryError: Java heap space. I'm not sure what I'm doing wrong. Here is my code:

StringBuffer finalString = new StringBuffer();

    try {
        BufferedReader br = new BufferedReader(new FileReader("collegeData 2.txt"));
        StringBuffer returnFile = new StringBuffer();
        returnFile.append(br.readLine() + br.readLine());
        ArrayList<String> allData = new ArrayList<String>();

        boolean completedFirstSection = false;
        int count = 2;
        String addString = "";  

        String nextLine;

        while ((nextLine=br.readLine())!=null) {
            if(count < 990) {
                addString += nextLine;
                count++;
            } else {
                String sub = nextLine.substring(16);
                sub = sub.substring(0, sub.length());

                addString = sub + ":{" + nextLine + "," + addString.substring(0, addString.length()-2) + br.readLine();
                br.readLine();
                allData.add(addString);
                count = 2;
            }
        }
        allData.add("}}]");
        System.out.println("here");
        System.out.println(returnFile.toString());
        finalString = returnFile;
        br.close();
    } catch (IOException e) {
        e.printStackTrace();
    }

collegeData 2.txt is 44.2 mb and it is a json file. Here is my error message.

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.base/jdk.internal.misc.Unsafe.allocateUninitializedArray(Unsafe.java:1250)
at java.base/java.lang.invoke.StringConcatFactory$MethodHandleInlineCopyStrategy.newArray(StringConcatFactory.java:1605)
at java.base/java.lang.invoke.DirectMethodHandle$Holder.invokeStatic(DirectMethodHandle$Holder)
at java.base/java.lang.invoke.LambdaForm$BMH/754666084.reinvoke(LambdaForm$BMH)
at java.base/java.lang.invoke.LambdaForm$MH/801197928.linkToTargetMethod(LambdaForm$MH)
at RunnerCombined.main(RunnerCombined.java:31)

RunnerCombined.java:31 is 'addString += nextLine' above.

Thank you in advance for your help!

Not sure if this the cause of the error or not but you should really use a StringBuilder, or StringBuffer if synchronization is a concern, instead of a String for addString. Strings are immutable so every appending requires allocating a new object and copying. — twain249
– twain249, Commented Jan 25, 2018 at 2:38
Oh, that's a good point! Let me give that a shot, should I create one string buffer and then clear that string buffer each time? — Xcelerate Reviews
– Xcelerate Reviews, Commented Jan 25, 2018 at 2:48

Xcelerate Reviews · Accepted Answer · 2018-01-25 03:01:55Z

Thank you to twain249! After changing my code to this, my program runs quickly and it's perfect!

StringBuffer finalString = new StringBuffer();

    try {
        BufferedReader br = new BufferedReader(new FileReader("collegeData 2.txt"));
        StringBuffer returnFile = new StringBuffer();
        returnFile.append(br.readLine() + "\n" + br.readLine() + "\n");

        boolean completedFirstSection = false;
        int count = 2;
        StringBuffer addString = new StringBuffer();

        String nextLine;

        while ((nextLine=br.readLine())!=null) {
            if(count < 990) {
                addString.append(nextLine + "\n");
                count++;
            } else {
                String sub = nextLine.substring(16);
                sub = sub.substring(0, sub.length());

                // addString = sub + ":{" + nextLine + "," + addString.substring(0, addString.length()-2) + br.readLine();
                returnFile.append(sub + ":{" + nextLine + "," + "\n");
                returnFile.append(addString.substring(0, addString.length()-2) + "\n");
                returnFile.append(br.readLine() + "\n");
                addString.setLength(0);
                count = 2;
                br.readLine();
            }
        }
        returnFile.append("}}]");
        finalString = returnFile;
        br.close();
    } catch (IOException e) {
        e.printStackTrace();
    }

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