Packages and Library Classes

Hello students, welcome to Kwickprep. Do you need to write your own square root code in Java? No, it is already written for you. Today we learn packages, java dot lang, java dot util, import statements, and how to use library classes.

First, recall that a class is a blueprint that holds data and methods. A package is a group of related classes kept together under one name. It works like a folder on your computer that groups similar files. Two classes can share a name if they sit in different packages, so names do not clash. Java comes with many ready packages, and together they are called the Java class library.

Here are some built in packages. Java dot lang holds the core classes, such as String and Math. Java dot util holds useful tools, such as the Scanner class for input. Java dot io is for input and output, such as reading files. Java dot awt holds classes for windows and graphics. Package names are always written in small letters.

Java dot lang is special. It is imported automatically into every Java program, so you never write an import for it. It holds String, Math and System. It also holds the wrapper classes, like Integer, Double and Character, which wrap a primitive value inside an object. That is why System dot out dot println works without any import line.

Here is the Math class from java dot lang, with no import. Math dot sqrt of forty nine gives the square root, seven point zero. It prints seven point zero because sqrt always returns a double. Math dot max of four and nine gives the larger value, nine. Math dot abs of minus six gives six.

Java dot util is a toolbox package, and it is not imported automatically. Scanner reads input from the keyboard, with methods like next int. Arrays gives ready methods to sort and search arrays. ArrayList is a list that can grow and shrink. Random makes random numbers, like rolling a dice for a game.

To use a class from another package, we write an import statement. Import java dot util dot Scanner, semicolon, brings in only the Scanner class. Import java dot util dot star brings in all the classes of that package. Imports are written at the very top of the file, before the class. The star does not bring in sub packages, so java dot star does not import java dot util.

Here the import line brings in Scanner. We create a Scanner called sc that reads from System dot in, the keyboard. Next int reads a whole number. If the user types fifteen, the program prints sixteen. Without the import line, the compiler gives an error, cannot find symbol Scanner.

An import is only a shortcut. You can also write the full name of a class, called its fully qualified name. Here we write java dot util dot Scanner every time, with no import line. It works, but it is long, so programmers prefer import.

Now the real benefit of library classes. Here m holds three marks, seventy two, forty five and ninety. Instead of writing bubble sort ourselves, we call Arrays dot sort of m. It sorts the marks in ascending order. Arrays dot toString turns the array into text, stored in t. Pause and predict the output. It prints forty five, seventy two, ninety, inside square brackets.

Why should you use library classes instead of rewriting them? They are tested by millions of programmers, so they have fewer bugs. They save time, since one line replaces a whole loop. They are usually well optimised, so they often run faster. And other programmers instantly understand Arrays dot sort. In exams, still write your own logic when the question asks for it, like sorting without library methods.

You can also create your own package. The first line of the file is package, then the name, like school. The class file must sit in a folder with the same name. Other programs then use it with import school dot Student. If you write no package line, the class goes into the default package, which is what our small programs use.

Let us revise. A package is a group of related classes, like a folder. Java dot lang is imported automatically and gives String, Math and System. Java dot util needs an import and gives Scanner, Arrays and ArrayList. We import one class by name, or all classes with a star. Use library classes instead of rewriting them, unless the exam asks you to write the logic. Try Math and Arrays methods in a program of your own today.