[Java] Add support for shared strings on FlatBufferBuilder. (#6012)

Added a method FlatBufferBuilder::createSharedString that
enable string sharing for building messages on java.

The shared pool will only contains strings inserted by
this methods.

tests/JavaTest.java

@@ -23,7 +23,9 @@ import java.io.*;
 import java.math.BigInteger;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
+import java.nio.CharBuffer;
 import java.nio.channels.FileChannel;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
@@ -101,6 +103,8 @@ class JavaTest {
 
         TestVectorOfBytes();
 
+        TestSharedStringPool();
+
         System.out.println("FlatBuffers test: completed successfully");
     }
 
@@ -1216,6 +1220,15 @@ class JavaTest {
         TestEq(monsterObject8.inventoryLength(), 2048);
     }
 
+    static void TestSharedStringPool() {
+        FlatBufferBuilder fb = new FlatBufferBuilder(1);
+        String testString = "My string";
+        int offset = fb.createSharedString(testString);
+        for (int i=0; i< 10; i++) {
+            TestEq(offset, fb.createSharedString(testString));
+        }
+    }
+
     static <T> void TestEq(T a, T b) {
         if (!a.equals(b)) {
             System.out.println("" + a.getClass().getName() + " " + b.getClass().getName());