How to concatenate string arrays in Java

Question

I'm wondering how I can concatenate 4 string arrays in Java.

There is a question about this already. How can I concatenate two arrays in Java?

But I tried to replicate it but it does not work for me.

This is what my code looks like:

Calling the method:

concatAll(jobs1, jobs2, jobs3, jobs4);

The method itself:

public String[] concatAll(String[] jobsA, String[] jobsB, String[] jobsC, String[] jobsD) {
 int totalLength = jobsA.length;
 for (String[] array : jobsD) {
 totalLength += array.length;
 }
 String[] result = Arrays.copyOf(jobsA, totalLength);
 int offset = jobsA.length;
 for (String[] array : jobsD) {
 System.arraycopy(array, 0, result, offset, array.length);
 offset += array.length;
 }
 return result;
}

• PearsonArtPhoto: I'm somewhat confused here. What is a sample input, and a sample output?
• Edward J Beckett: How did this not get closed? Double Duplicate

Answer 1

Putting aside things like checking if an array is null, you can create a general method for it and use it in your specific case, like this:

 public String[] concatAll(String[] jobsA, String[] jobsB, String[] jobsC, String[] jobsD) 
 {
 return generalConcatAll (jobsA, jobsB, jobsC, jobsD);
 }
 public String[] generalConcatAll(String[]... jobs) {
 int len = 0;
 for (final String[] job : jobs) {
 len += job.length;
 }
 final String[] result = new String[len];
 int currentPos = 0;
 for (final String[] job : jobs) {
 System.arraycopy(job, 0, result, currentPos, job.length);
 currentPos += job.length;
 }
 return result;
 }

• Tríona Lounds: Thank you. Still a little confused though... Do I then call generalConcatAll 4 times and pass in each jobs array?
• Tríona Lounds: Or do I call it once and run the for loops 4 times once for each of the jobs arrays?
• ShyJ: No. The body of concatAll method gives you the anwer - you pass each job array as a seperate argument.
• nkr: @TríonaLounds You might want to read about Varargs.

Answer 2

This is a bit more concise, and handles all null cases correctly using Apache Commons Lang library. ArrayUtils.addAll(T[], T...)

public String[] generalConcatAll(String[]...arrays) {
 String[] result = null;
 for(String[] array : arrays) {
 result = ArrayUtils.addAll(result, array);
 }
 return result;
}