Imports

To use a class defined in another package we use its fully-qualified class name:

package com.example.utilities.app;

public class ParserDriver {
  public static void main(String[] args) {
    com.example.utilities.parsing.Parser parser;
    parser = new com.example.utilities.parsing.Parser();
    parser.parse();
  }
}

This gets tedious when we use a lot of classes from other packages.

import brings class names into our current program's namespace.

package com.example.utilities.app;

import com.example.utilities.parsing.Parser;

public class ParserDriver2 {
  public static void main(String[] args) {
    Parser parser;
    parser = new Parser();
    parser.parse();
  }
}

• The special package java.lang, containing classes like System and String, is automatically imported in every Java program.

Note that we need the entire package name, even if we are in a sibling or parent package.

package com.example;                         // We're in a parent package

// import utilities.parsing.Parser;                  // WILL NOT COMPILE
import com.example.utilities.parsing.Parser; // Correct.

public class MainApp {
  public static void main(String[] args) {
    Parser p = new Parser();
    p.parse();
  }
}

We can import individual class names one at a time, or include all names for an entire package using the wildcard *.

package com.example.utilities.app;

import com.example.utilities.parsing.*;

public class ParserDriver3 {
  public static void main(String[] args) {
    Parser parser = new Parser();
    parser.parse();
  }
}

• Import only makes the other package's names part of our current namespace, it doesn't add actual code to our program.

• There's no memory or code-size penalty for using *.

• Importing a package does not import any sub-packages.

If two imported packages define a class with the same name, we still have to fully-qualify that class in our code.

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