All about imports in Java

In any Java source file first line after package declarations are import statements.  These statements enable us to use different classes from outside package. Compiler always looks at the import statements to find class definition.

Instead of import statements we can write fully qualified name. For ex:

java.io. File file = new java.io.File(filepath);

This is a perfectly valid statement. Only problem is it looks littlie complicated to read.  Good codes are not only run well but also easy to read.

So we will stick to

File file = new File(filepath);

To do so we have to write import statement at the beginning right after the package declaration.

package mej.java.example;
import java.io.File;

If we use more than one classes or interface of java.io packages we need to write multiple import statements.

package mej.java.example;
import java.io.File;
import java. FileInputStream;

or we can use wild characters like

java,io.*;

At first impression it looks simple, there are few side effects with this. It takes down the performance of compiler, as compiler has to look whole package for match.  But it doesn’t affect runtime performance of code in anyway. Many time developers misunderstood it.  There can be cases if we use multiple wild character imports. And if both the package contains class with same name, compiler will not able to resolve. And it also reduces the readability of code. It’s difficult to read which class from which package is being used. So the best way is to use import individually for each class or interface.

Another type of imports are static imports. To use it I can say something like

import static java.lang.Match;

We can directly write  ceil(4.83) instead of Math.ceil(4.38).  By static imports we can directly use the methods or members instead of referring through their class name. This is widely used in junit test cases to refer Assert class methods.

In this case also too much use of static imports decrease code maintainability.  

Please let me know your thoughts about java imports.