[SPARK-17331][CORE][MLLIB] Avoid allocating 0-length arrays

## What changes were proposed in this pull request?

Avoid allocating some 0-length arrays, esp. in UTF8String, and by using Array.empty in Scala over Array[T]()

## How was this patch tested?

Jenkins

Author: Sean Owen <sowen@cloudera.com>

Closes #14895 from srowen/SPARK-17331.

1 files changed, 4 insertions, 4 deletions

diff --git a/common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java b/common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java
index 54a5456924..dc03d893a5 100644
--- a/common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java
+++ b/common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java
@@ -470,7 +470,7 @@ public final class UTF8String implements Comparable<UTF8String>, Externalizable,
     while (e >= 0 && getByte(e) <= 0x20 && getByte(e) >= 0x00) e--;
     if (s > e) {
       // empty string
-      return UTF8String.fromBytes(new byte[0]);
+      return EMPTY_UTF8;
     } else {
       return copyUTF8String(s, e);
     }
@@ -482,7 +482,7 @@ public final class UTF8String implements Comparable<UTF8String>, Externalizable,
     while (s < this.numBytes && getByte(s) <= 0x20 && getByte(s) >= 0x00) s++;
     if (s == this.numBytes) {
       // empty string
-      return UTF8String.fromBytes(new byte[0]);
+      return EMPTY_UTF8;
     } else {
       return copyUTF8String(s, this.numBytes - 1);
     }
@@ -495,7 +495,7 @@ public final class UTF8String implements Comparable<UTF8String>, Externalizable,
 
     if (e < 0) {
       // empty string
-      return UTF8String.fromBytes(new byte[0]);
+      return EMPTY_UTF8;
     } else {
       return copyUTF8String(0, e);
     }
@@ -761,7 +761,7 @@ public final class UTF8String implements Comparable<UTF8String>, Externalizable,
 
     if (numInputs == 0) {
       // Return an empty string if there is no input, or all the inputs are null.
-      return fromBytes(new byte[0]);
+      return EMPTY_UTF8;
     }
 
     // Allocate a new byte array, and copy the inputs one by one into it.