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Introduction to Java Programming: Compiling, Running, and Using Variables

This tutorial covers the basics of compiling and running a Java program via the command line, using the classic "Hello World" example. You will learn to use Java's syntax for declaring variables, operators, constants, and methods. The tutorial emphasizes the significance of compiling with the JDK, case sensitivity in filenames, and provides guidance on variable types, initialization, and method utilization. Engage with practical coding examples and outputs to solidify your understanding of Java programming fundamentals.

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Introduction to Java Programming: Compiling, Running, and Using Variables

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  1. The 1st tutoring session of CSc2310Fall, 2012 HaidongXue

  2. How to compile and run a java program (in command-line)? Code: public class HelloWorld { public static void main(String[] args) { System.out.println("Hellow World!"); } } Output: Hellow World!

  3. How to compile and run a java program (in command-line)? public class HelloWorld{ public static void main(String[] args) { System.out.println("Hellow World!"); } } • Install JDK (not JRE) • Compile: javac HelloWorld.java (You will see the bytecode file: HelloWorld.class) • Run: java HelloWorld (not HelloWorld.class here) (All the file names are case-sensitive)

  4. How to compile and run a java program (in command-line)?

  5. Use variables • Each variable has a type • Use a type keyword to declare a variable • In the most recent grammar, you have to initialize the variable with certain value public class Variables { public static void main(String[] args) { String name = "Haydon"; booleanmale = true; double height = 1.76; //meters System.out.println("Hello World!"); System.out.println("I am: " + name); System.out.println("Am I a male: " + male); System.out.println("My height is: " + height); } }

  6. Use variables Code: public class Variables { public static void main(String[] args) { String name = "Haydon"; booleanmale = true; double height = 1.76; //meters System.out.println("Hello World!"); System.out.println("I am: " + name); System.out.println("Am I a male: " + male); System.out.println("My height is: " + height); } } Output: Hello World! I am: Haydon Am I a male: true My height is: 1.76

  7. Use operators • There are many operators, their usage is very similar to their counterparts in mathematics. • E.g.: + Addition - Subtraction * Multiplication / Division • There are also many other operators like: modulus(%), increment(++) and so on

  8. Use operators Code: public class Operators{ public static void main(String[] args) { double radius = 3.5; double area = 3.1415926 * radius * radius; System.out.println("Area: " + area); } } Output: Area: 38.48450935

  9. Use constants • Like a variable, but the value cannot be changed. (You can only initialize it) • It is very useful to replace your “magic numbers” with constants to increase the readability of your code

  10. Use constants public class Operators{ public static void main(String[] args) { double radius = 3.5; double area = 3.1415926 * radius * radius; System.out.println("Area: " + area); } } public class Constants { public static void main(String[] args) { final double PI = 3.1415926; double radius = 3.5; double area = PI * radius * radius; System.out.println("Area: " + area); } } Which one is better?

  11. Use constants • What will happen, if you use “3.2415” as PI at multiple places, and suddenly find out that it should be “3.1415”? • When you read a 2000 line program, what if you see 8.314? • To avoid those pains, coders use constants.

  12. Use methods • Methods are members of classes • You can use the “.” operator to use the public methods public class Methods { public static void main(String[] args) { double value = 2; System.out.println(Math.pow(value, 3)); // System.out.println(Math.sqrt(value)); // System.out.println(Math.round(value)); // round(value) } } 8.0 1.4142135623730951 2

  13. If you have more questions, I will be here till 8:30pm

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