Improve AppendableWriter performance

gson/src/main/java/com/google/gson/internal/Streams.java

@@ -93,12 +93,31 @@ private static final class AppendableWriter extends Writer {
       appendable.append(currentWrite, offset, offset + length);
     }
 
+    @Override public void flush() {}
+    @Override public void close() {}
+
+    // Override these methods for better performance
+    // They would otherwise unnecessarily create Strings or char arrays
+
     @Override public void write(int i) throws IOException {
       appendable.append((char) i);
     }
 
-    @Override public void flush() {}
-    @Override public void close() {}
+    @Override public void write(String str, int off, int len) throws IOException {
+      // Appendable.append turns null -> "null", which is not desired here
+      $Gson$Preconditions.checkNotNull(str);
+      appendable.append(str, off, off + len);
+    }
+
+    @Override public Writer append(CharSequence csq) throws IOException {
+      appendable.append(csq);
+      return this;
+    }
+
+    @Override public Writer append(CharSequence csq, int start, int end) throws IOException {
+      appendable.append(csq, start, end);
+      return this;
+    }
 
     /**
      * A mutable char sequence pointing at a single char[].

gson/src/test/java/com/google/gson/internal/StreamsTest.java

@@ -0,0 +1,68 @@
+package com.google.gson.internal;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
+
+import java.io.IOException;
+import java.io.Writer;
+import org.junit.Test;
+
+public class StreamsTest {
+  @Test
+  public void testWriterForAppendable() throws IOException {
+    StringBuilder stringBuilder = new StringBuilder();
+    Writer writer = Streams.writerForAppendable(stringBuilder);
+
+    writer.append('a');
+    writer.append('\u1234');
+    writer.append("test");
+    writer.append(null); // test custom null handling mandated by `append`
+    writer.append("abcdef", 2, 4);
+    writer.append(null, 1, 3); // test custom null handling mandated by `append`
+    writer.append(',');
+
+    writer.write('a');
+    writer.write('\u1234');
+    // Should only consider the 16 low-order bits
+    writer.write(0x4321_1234);
+    writer.append(',');
+
+    writer.write("chars".toCharArray());
+    try {
+      writer.write((char[]) null);
+      fail();
+    } catch (NullPointerException e) {
+    }
+
+    writer.write("chars".toCharArray(), 1, 2);
+    try {
+      writer.write((char[]) null, 1, 2);
+      fail();
+    } catch (NullPointerException e) {
+    }
+    writer.append(',');
+
+    writer.write("string");
+    try {
+      writer.write((String) null);
+      fail();
+    } catch (NullPointerException e) {
+    }
+
+    writer.write("string", 1, 2);
+    try {
+      writer.write((String) null, 1, 2);
+      fail();
+    } catch (NullPointerException e) {
+    }
+
+    String actualOutput = stringBuilder.toString();
+    assertEquals("a\u1234testnullcdul,a\u1234\u1234,charsha,stringtr", actualOutput);
+
+    writer.flush();
+    writer.close();
+
+    // flush() and close() calls should have had no effect
+    assertEquals(actualOutput, stringBuilder.toString());
+  }
+}