Difference between using fully qualified name and import in Java

Question

Are there any differences using an "inline import" (a fully qualified name) and normal import in terms of performance, memory, compile-time, etc. in Java?

chooser.setCurrentDirectory(new java.io.File("."));

and

import java.io.File;
...
   chooser.setCurrentDirectory(new File("."));

Answer 1

The main thing you should focus in is readability. I find the second one more readable.

In rare cases, I prefer the second approach. Let's consider the following scenario: For some reason, I wrote a class and named it File. I typed File file = new File(...) and my IDE auto-imported the java.io.File for me. But I don't want that kind of object, I want my File class. So instead of importing the correct class, I prefer inline-import it, just that other users won't get confused with the Java's File class.

Regarding the performance, they're exactly the same, and here's the proof -

This is the bytecode generated for the first snippet:

public class java8.tests.General {
  public java8.tests.General();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class javax/swing/JFileChooser
       3: dup
       4: invokespecial #3                  // Method javax/swing/JFileChooser."<init>":()V
       7: astore_1
       8: aload_1
       9: new           #4                  // class java/io/File
      12: dup
      13: ldc           #5                  // String .
      15: invokespecial #6                  // Method java/io/File."<init>":(Ljava/lang/String;)V
      18: invokevirtual #7                  // Method javax/swing/JFileChooser.setCurrentDirectory:(Ljava/io/File;)V
      21: return
}

This is the bytecode for the second:

public class java8.tests.General {
  public java8.tests.General();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class javax/swing/JFileChooser
       3: dup
       4: invokespecial #3                  // Method javax/swing/JFileChooser."<init>":()V
       7: astore_1
       8: aload_1
       9: new           #4                  // class java/io/File
      12: dup
      13: ldc           #5                  // String .
      15: invokespecial #6                  // Method java/io/File."<init>":(Ljava/lang/String;)V
      18: invokevirtual #7                  // Method javax/swing/JFileChooser.setCurrentDirectory:(Ljava/io/File;)V
      21: return
}

Answer 2

No both are exactly same in the terms of performance, memory, compile-time. The only difference between them is that normal import save your typing efforts and it's more readable that' it.