Which would make a Class file bigger? import java.awt.*, or a bunch or single import statements? [duplicate]

Question

Okay, so if I had a project that used:

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Color;
import java.awt.Polygon;

Would it make the Class file smaller to use:

import java.awt.*

I'm thinking against it because I'm importing a lot of things I don't need. I'm thinking for it cause it makes the file have a lot less characters.

Answer 1

No difference. These are a compile-time construct only.

Answer 2

No, there is no difference at all.

According to The Java Virtual Machine Specifications, Second Edition, Chapter 5: Loading, Linking and Initializing says the following:

The Java virtual machine dynamically loads (§2.17.2), links (§2.17.3), and initializes (§2.17.4) classes and interfaces. Loading is the process of finding the binary representation of a class or interface type with a particular name and creating a class or interface from that binary representation. Linking is the process of taking a class or interface and combining it into the runtime state of the Java virtual machine so that it can be executed.

At compile time, there are no linking of classes, therefore, using wildcards for importing makes no difference. Other classes are not included together into the resulting class file.

In fact, if you look at the bytecode of the class file (via javap or such disassembler), you won't find any import statements, so having more or less numbers of import statements in your source won't affect the size of the class file.

Here's a simple experiment: Try writing a program, and compiling with imports using wildcards, and another one with explicit imports. The resulting class file should be the same size.

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Using explicit import statements on specific classes are perhaps less readable (and troublesome, if one doesn't use an IDE like Eclipse which will write it for you), but will allow you to deal with overlaps of class names in two packages.

For example, there is a List class in both the java.util and java.awt packages. By importing both packages, there will be a conflict for the class named List:

import java.util.*;
import java.awt.*;

// ... snip ... //

List l;    // which "List" are we talking about?

By only importing the specific classes you need, these conflicts could be somewhat avoided:

import java.util.Hashmap;
import java.awt.List;

// .. snip ... //
List l;    // Now we know for sure it's java.awt.List

Of course, if you had to use both java.util.List and java.awt.List then you're out of luck; you'll need to explicitly use their fully-qualified class names.

Answer 3

The Java import statement does not add any size to the class file. It allows the compiler to deduce what names refer to at compile time.

The only difference the line

import java.util.*;

makes is that you can write:

Set<String> set;

rather than:

java.util.Set<String> set;