Java 8+

Use String.join():

String str = String.join(",", arr);

Note that arr can also be any Iterable (such as a list), not just an array.

If you have a Stream, you can use the joining collector:

Stream.of("a", "b", "c")
      .collect(Collectors.joining(","))

Legacy (Java 7 and earlier)

StringBuilder builder = new StringBuilder();
for(String s : arr) {
    builder.append(s);
}
String str = builder.toString();

Alternatively, if you just want a "debug-style" dump of an array:

String str = Arrays.toString(arr);

Note that if you're really legacy (Java 1.4 and earlier) you'll need to replace StringBuilder there with StringBuffer.

Android

Use TextUtils.join():

String str = TextUtils.join(",", arr);

General notes

You can modify all the above examples depending on what characters, if any, you want in between strings.

DON'T use a string and just append to it with += in a loop like some of the answers show here. This sends the GC through the roof because you're creating and throwing away as many string objects as you have items in your array. For small arrays you might not really notice the difference, but for large ones it can be orders of magnitude slower.

List<String> list = ..;
String[] array = list.toArray(new String[0]);

For example:

List<String> list = new ArrayList<String>();
//add some stuff
list.add("android");
list.add("apple");
String[] stringArray = list.toArray(new String[0]);

The toArray() method without passing any argument returns Object[]. So you have to pass an array as an argument, which will be filled with the data from the list, and returned. You can pass an empty array as well, but you can also pass an array with the desired size.

Important update: Originally the code above used new String[list.size()]. However, this blogpost reveals that due to JVM optimizations, using new String[0] is better now.

Another alternative to System.arraycopy:

String[] stringArray = Arrays.copyOf(objectArray, objectArray.length, String[].class);

Use like this.

List<String> stockList = new ArrayList<String>();
stockList.add("stock1");
stockList.add("stock2");

String[] stockArr = new String[stockList.size()];
stockArr = stockList.toArray(stockArr);

for(String s : stockArr)
    System.out.println(s);

If the individual strings might themselves include a comma followed by a space, then this would not be feasible. Multiple String[] arrays could map to the same flat String, and an inverse function would not exist.

However, you note in a comment that your strings cannot include the comma separator. You can split the flat string back into the original substrings, using ", " (comma,space) as a separator.

APIs that support this include the standard Java String.split() method, and Guava's Splitter class.

Here's an example with Java's split() method:

public static void main(String[] args) {
    String[] strs = new String[] { "Foo", "Bar", "Baz" };

    String joined = Arrays.toString( strs );

    String joinedMinusBrackets = joined.substring( 1, joined.length() - 1);

    // String.split()
    String[] resplit = joinedMinusBrackets.split( ", "); 

    for ( String s : resplit ) {
        System.out.println( s );
    }

}

And here's an example with Guava's Splitter class:

public static void main(String[] args) {
    String[] strs = new String[] { "Foo", "Bar", "Baz" };

    String joined = Arrays.toString( strs );

    String joinedMinusBrackets = joined.substring( 1, joined.length() - 1);

    // Guava Splitter class
    List<String> resplitList = Splitter.on( ", " ).splitToList( joinedMinusBrackets );

    for ( String s : resplitList ) {
        System.out.println( s );
    }
}

What you want is the Arrays.toString(int[]) method:

import java.util.Arrays;

int[] array = new int[lnr.getLineNumber() + 1];
int i = 0;

..      

System.out.println(Arrays.toString(array));

There is a static Arrays.toString helper method for every different primitive java type; the one for int[] says this:

public static String toString(int[] a)

Returns a string representation of the contents of the specified array. The string representation consists of a list of the array's elements, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (a comma followed by a space). Elements are converted to strings as by String.valueOf(int). Returns "null" if a is null.