The 1st tutoring session of CSc2310Fall, 2012

Haidong Xue

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!");

}}

Hellow World!

Code:

Output:

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

• 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)

public class HelloWorld {public static void main(String[] args) {

System.out.println("Hellow World!");

}}

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

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 valuepublic class Variables {

public static void main(String[] args) {String name = "Haydon";boolean male = true;double height = 1.76; //metersSystem.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);

}}

Use variablespublic class Variables {

public static void main(String[] args) {String name = "Haydon";boolean male = true;double height = 1.76; //metersSystem.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);

}}

Hello World!I am: HaydonAm I a male: trueMy height is: 1.76

Code:

Output:

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

Use operatorspublic class Operators{

public static void main(String[] args) {double radius = 3.5;double area = 3.1415926 * radius * radius;System.out.println("Area: " + area);

}}

Area: 38.48450935

Code:

Output:

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

Use constantspublic 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?

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.

Use methods• Methods are members of classes• You can use the “.” operator to use the public

methodspublic 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)

}}

8.01.41421356237309512

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