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, Nov 17 '12 at 23:14
How did this not get closed? [Double Duplicate](http://stackoverflow.com/questions/3750500/how-do-i-concatenate-static-string-arrays) — Edward J Beckett, Nov 17 '12 at 23:57

score 16 · Accepted Answer · answered Nov 17 '12 at 23:19

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 '12 at 23:19

ShyJ

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Thank you. Still a little confused though... Do I then call generalConcatAll 4 times and pass in each jobs array? – Tríona Lounds Nov 17 '12 at 23:23
Or do I call it once and run the for loops 4 times once for each of the jobs arrays? – Tríona Lounds Nov 17 '12 at 23:24
No. The body of `concatAll` method gives you the anwer - you pass each job array as a seperate argument. – ShyJ Nov 17 '12 at 23:24
@TríonaLounds You might want to read about [Varargs](http://docs.oracle.com/javase/1.5.0/docs/guide/language/varargs.html). – nkr Nov 17 '12 at 23:25

score 4 · Answer 2 · answered Mar 13 '14 at 14:53

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;

}

How to concatenate string arrays in Java

2 Answers2

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