1 files changed, 28 insertions, 7 deletions
diff --git a/src/main/java/com/squareup/javapoet/ClassName.java b/src/main/java/com/squareup/javapoet/ClassName.java
index 99c4ed2..4bef49d 100644
--- a/src/main/java/com/squareup/javapoet/ClassName.java
+++ b/src/main/java/com/squareup/javapoet/ClassName.java
@@ -20,6 +20,7 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import javax.lang.model.element.Element;
 import javax.lang.model.element.PackageElement;
 import javax.lang.model.element.TypeElement;
@@ -32,6 +33,9 @@ import static com.squareup.javapoet.Util.checkNotNull;
 public final class ClassName extends TypeName implements Comparable<ClassName> {
   public static final ClassName OBJECT = ClassName.get(Object.class);
 
+  /** The name representing the default Java package. */
+  private static final String NO_PACKAGE = "";
+
   /** The package name of this class, or "" if this is in the default package. */
   final String packageName;
 
@@ -41,6 +45,8 @@ public final class ClassName extends TypeName implements Comparable<ClassName> {
   /** This class name, like "Entry" for java.util.Map.Entry. */
   final String simpleName;
 
+  private List<String> simpleNames;
+
   /** The full class name like "java.util.Map.Entry". */
   final String canonicalName;
 
@@ -51,7 +57,7 @@ public final class ClassName extends TypeName implements Comparable<ClassName> {
   private ClassName(String packageName, ClassName enclosingClassName, String simpleName,
       List<AnnotationSpec> annotations) {
     super(annotations);
-    this.packageName = packageName;
+    this.packageName = Objects.requireNonNull(packageName, "packageName == null");
     this.enclosingClassName = enclosingClassName;
     this.simpleName = simpleName;
     this.canonicalName = enclosingClassName != null
@@ -108,11 +114,18 @@ public final class ClassName extends TypeName implements Comparable<ClassName> {
   }
 
   public List<String> simpleNames() {
-    List<String> simpleNames = new ArrayList<>();
-    if (enclosingClassName != null) {
-      simpleNames.addAll(enclosingClassName().simpleNames());
+    if (simpleNames != null) {
+      return simpleNames;
+    }
+
+    if (enclosingClassName == null) {
+      simpleNames = Collections.singletonList(simpleName);
+    } else {
+      List<String> mutableNames = new ArrayList<>();
+      mutableNames.addAll(enclosingClassName().simpleNames());
+      mutableNames.add(simpleName);
+      simpleNames = Collections.unmodifiableList(mutableNames);
     }
-    simpleNames.add(simpleName);
     return simpleNames;
   }
 
@@ -138,6 +151,14 @@ public final class ClassName extends TypeName implements Comparable<ClassName> {
     return simpleName;
   }
 
+  /**
+   * Returns the full class name of this class.
+   * Like {@code "java.util.Map.Entry"} for {@link Map.Entry}.
+   * */
+  public String canonicalName() {
+    return canonicalName;
+  }
+
   public static ClassName get(Class<?> clazz) {
     checkNotNull(clazz, "clazz == null");
     checkArgument(!clazz.isPrimitive(), "primitive types cannot be represented as a ClassName");
@@ -155,7 +176,7 @@ public final class ClassName extends TypeName implements Comparable<ClassName> {
     if (clazz.getEnclosingClass() == null) {
       // Avoid unreliable Class.getPackage(). https://github.com/square/javapoet/issues/295
       int lastDot = clazz.getName().lastIndexOf('.');
-      String packageName = (lastDot != -1) ? clazz.getName().substring(0, lastDot) : null;
+      String packageName = (lastDot != -1) ? clazz.getName().substring(0, lastDot) : NO_PACKAGE;
       return new ClassName(packageName, null, name);
     }
 
@@ -177,7 +198,7 @@ public final class ClassName extends TypeName implements Comparable<ClassName> {
       p = classNameString.indexOf('.', p) + 1;
       checkArgument(p != 0, "couldn't make a guess for %s", classNameString);
     }
-    String packageName = p == 0 ? "" : classNameString.substring(0, p - 1);
+    String packageName = p == 0 ? NO_PACKAGE : classNameString.substring(0, p - 1);
 
     // Add class names like "Map" and "Entry".
     ClassName className = null;