A COBOL program cannot modify the contents of a String passed to it, but it can change a StringBuffer. To make handling strings easier, javatypes.cpy defines the type MF-JSTRING. This data type has the following structure:

01 mf-jstring is typedef.
    03 len                  jint.
    03 capacity             jint.
    03 ptr2string           pointer.

The len field defines the length of the string. The capacity field defines the size of the buffer - this field is always 0 for a String. The ptr2string field is a pointer to the start of the actual string. In the case of a StringBuffer, you can either modify the existing buffer, or allocate a new one and set ptr2string to the start of the new buffer.

The COBOL program below concatenates a String and StringBuffer from Java, and returns the result in the StringBuffer.

 program-id. StringAdd.  
 thread-local-storage section.
 copy "javatypes.cpy".
 01 lsNewPtr                 pointer.
 01 lsSize         jint. *> type defined in javatypes.cpy
 01 i              jint.
 01 lsStatus       jint.
 01 lsBigBuffer              pic x(1024).
 linkage section.
 01 lnkJString               mf-jstring.
 01 lnkJStringBuffer         mf-jstring.
 01 lnkString                pic x(256).
 01 lnkStringbuffer          pic x(256).
 procedure division using lnkJString lnkJStringBuffer.
     set address of lnkStringBuffer to
                              ptr2string of lnkJStringBuffer
     set address of lnkString to ptr2string of lnkJString
*>   Check that lnkJStringBuffer is a Java StringBuffer
     if capacity of lnkJStringBuffer > 0
         add len of lnkJString to len of lnkJStringBuffer
                                   giving lsSize
*>   Check we don't overflow concatenation buffer
         if lsSize < 1024
             move len of lnkJString to i
             move lnkString(1:i) to lsBigBuffer
             add 1 to i
             move lnkStringBuffer to lsBigBuffer(i:lsSize)
             set ptr2string of lnkJStringBuffer
                                 to address of lsBigBuffer
             move lsSize to len of lnkJStringBuffer

The Java class StringsToCobol passes "fred" and "ginger" to StringAdd and then displays the result.

import mfcobol.*;
public class StringsToCobol {
  public static void main(String[] args) throws Exception
      StringBuffer sb1 = new StringBuffer("ginger") ;
      Object theParams[] = {"fred" ,
                             sb1} ;
       RuntimeSystem.cobcall_int("stringadd", theParams) ;
       System.out.println(sb1) ;