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;
}

I'm somewhat confused here. What is a sample input, and a sample output? — PearsonArtPhoto
– PearsonArtPhoto, Commented Nov 17, 2012 at 23:14

ShyJ · Accepted Answer · 2012-11-17 23:19:04Z

16

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;
    }

answered Nov 17, 2012 at 23:19

ShyJ

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4 Comments

Tríona Lounds Over a year ago

Thank you. Still a little confused though... Do I then call generalConcatAll 4 times and pass in each jobs array?

Tríona Lounds Over a year ago

Or do I call it once and run the for loops 4 times once for each of the jobs arrays?

ShyJ Over a year ago

No. The body of concatAll method gives you the anwer - you pass each job array as a seperate argument.

nkr Over a year ago

@TríonaLounds You might want to read about Varargs.

yoni · Accepted Answer · 2014-03-13 14:53:19Z

4

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;

}

answered Mar 13, 2014 at 14:53

yoni

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