Intro, Hello World Java

Introduction

What is 61B about?

• Writing codes that runs efficiently. (Algorithms and data structures.)

• Writing code efficiently.

First Java Program

Hello World

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

• All codes must be in a class.

• Curly braces and semi-colons.

• Variables have declared types and must be declared before use.

• Functions must have a return type or declared as void function.

• The compiler ensures type consistency.

Compilation

The common way to run a Java program is to run it through two programs: javac and java.

$ javac HelloWorld.java
$ java HelloWorld
Hello World!

Defining Functions

Since all Java code is part of a class, we must define functions so that they belong to some class. Functions that are part of a class are commonly called "methods".

For instance, here's a piece of code in Java which defining a function that could compare two integers and return the larger one.

public class LargerDemo {
    public static int larger(int x, int y) {
        if (x > y) {
            return x;
        }
        return y;
    }

    public static void main(String[] args) {
        System.out.println(larger(8, 10));
    }
}

Code Style

In this course, we will aim to make our code readable. Here are some tips to achieve that goal:

• Consistent style

• Comments (use // for line comments and /* and */ for block comments)

• Avoidance of repetitive code

• Descriptive naming (variables, functions, classes)

Moreover, we could use Javadoc to automatically generate documents.