diff --git a/runtime/Go/antlr/v4/file_stream.go b/runtime/Go/antlr/v4/file_stream.go
index 7f072e25ba..5f65f809be 100644
--- a/runtime/Go/antlr/v4/file_stream.go
+++ b/runtime/Go/antlr/v4/file_stream.go
@@ -5,8 +5,7 @@
 package antlr
 
 import (
-	"bytes"
-	"io"
+	"bufio"
 	"os"
 )
 
@@ -14,38 +13,53 @@ import (
 //  when you construct the object.
 
 type FileStream struct {
-	*InputStream
-
+	InputStream
 	filename string
 }
 
 //goland:noinspection GoUnusedExportedFunction
 func NewFileStream(fileName string) (*FileStream, error) {
 
-	buf := bytes.NewBuffer(nil)
-
 	f, err := os.Open(fileName)
 	if err != nil {
 		return nil, err
 	}
+
 	defer func(f *os.File) {
 		errF := f.Close()
 		if errF != nil {
 		}
 	}(f)
-	_, err = io.Copy(buf, f)
+
+	reader := bufio.NewReader(f)
+	fInfo, err := f.Stat()
 	if err != nil {
 		return nil, err
 	}
 
-	fs := new(FileStream)
+	fs := &FileStream{
+		InputStream: InputStream{
+			index: 0,
+			name:  fileName,
+		},
+		filename: fileName,
+	}
 
-	fs.filename = fileName
-	s := buf.String()
-	fs.InputStream = NewInputStream(s)
+	// Pre-build the buffer and read runes efficiently
+	//
+	fs.data = make([]rune, 0, fInfo.Size())
+	for {
+		r, _, err := reader.ReadRune()
+		if err != nil {
+			break
+		}
+		fs.data = append(fs.data, r)
+	}
+	fs.size = len(fs.data) // Size in runes
 
+	// All done.
+	//
 	return fs, nil
-
 }
 
 func (f *FileStream) GetSourceName() string {
diff --git a/runtime/Go/antlr/v4/input_stream.go b/runtime/Go/antlr/v4/input_stream.go
index c02bbc76d3..b737fe85fb 100644
--- a/runtime/Go/antlr/v4/input_stream.go
+++ b/runtime/Go/antlr/v4/input_stream.go
@@ -4,6 +4,11 @@
 
 package antlr
 
+import (
+	"bufio"
+	"io"
+)
+
 type InputStream struct {
 	name  string
 	index int
@@ -11,16 +16,43 @@ type InputStream struct {
 	size  int
 }
 
-// NewInputStream creates a new input stream from the given string
-func NewInputStream(data string) *InputStream {
+// NewIoStream creates a new input stream from the given io.Reader reader.
+// Note that the reader is read completely into memory and so it must actually
+// have a stopping point - you cannot pass in a reader on an open-ended source such
+// as a socket for instance.
+func NewIoStream(reader io.Reader) *InputStream {
 
-	is := new(InputStream)
+	rReader := bufio.NewReader(reader)
 
-	is.name = "<empty>"
-	is.index = 0
-	is.data = []rune(data)
+	is := &InputStream{
+		name:  "<empty>",
+		index: 0,
+	}
+
+	// Pre-build the buffer and read runes reasonably efficiently given that
+	// we don't exactly know how big the input is.
+	//
+	is.data = make([]rune, 0, 512)
+	for {
+		r, _, err := rReader.ReadRune()
+		if err != nil {
+			break
+		}
+		is.data = append(is.data, r)
+	}
 	is.size = len(is.data) // number of runes
+	return is
+}
+
+// NewInputStream creates a new input stream from the given string
+func NewInputStream(data string) *InputStream {
 
+	is := &InputStream{
+		name:  "<empty>",
+		index: 0,
+		data:  []rune(data), // This is actually the most efficient way
+	}
+	is.size = len(is.data) // number of runes, but we could also use len(data), which is efficient too
 	return is
 }