1. Programming in Java¶

Note

• Java programs are made up of class and method definitions.
• A method is a named sequence of statements.
• A statement is a line of code that performs a basic action.

In the hello world program below , this line is a print statement that displays a message on the screen:

System.out.println("Hello, World!");

System.out.println displays results on the screen; the name println stands for “print line”.  Like most statements, the print statement ends with a semicolon ;.

Java is “case-sensitive”, which means that uppercase and lowercase are not the same. In this example, System has to begin with an uppercase letter; system and SYSTEM won’t work.

This program defines one method named main:

public static void main(String[] args)

The name and format of main is special. Java starts execution in the main method (public static void main(String[] args)). The body of the main method is all the code between the first { and the last } of the main method. When the program runs, it starts at the first statement in main and ends when it finishes the last statement. Every class in Java can have a main method.

Later in the course, we will see programs that define more than one method.

Note

A  class is a collection of related methods. The program below defines a class named Hello. You can give a class any name you like, but it is conventional to start with a capital letter. The name of the class has to match the name of the file it is in, so this class has to be in a file named Hello.java.

Read the code and run the program below.

Note

Java uses curly braces { and } to group blocks of related code together. In Hello.java, the outermost braces contain the class definition, and the inner braces contain the method definition.

The line that begins with two slashes (//) is a comment, which is a bit of English text that explains the code. When Java sees //, it ignores everything from there until the end of the line. Comments have no effect on the execution of the program, but they make it easier for other programmers (and your future self) to understand what you meant to do.

Note

To define a class in Java use public class followed by a ClassName. Then the body of the class is enclosed in a starting { and ending } as shown below.

public class ClassName
{
}

Note

In Java every open curly brace { must have a matched close curly brace }. These are used to start and end both class definitions and method definitions.

Note

Java is a high-level language. Other high-level languages you may have heard of include Python, C and C++, PHP, Ruby, and JavaScript.

Before they can run, programs in high-level languages have to be translated into a low-level language, also called machine language. This translation takes some time, which is a small disadvantage of high-level languages. But high-level languages have two major advantages:

• It is much easier to program in a high-level language. Programs take less time to write, they are shorter and easier to read, and they are more likely to be correct.
• High-level languages are portable, meaning they can run on different kinds of computers with few or no modifications. Low-level programs can only run on one kind of computer, and have to be rewritten to run on another.

Note

There are two kinds of programs translate high-level languages into low-level languages: interpreters and compilers. Compiled programs often run faster than interpreted programs.

• An interpreter reads a high-level program and executes it, meaning that it does what the program says to do.
• A compiler reads the entire program and translates it completely before the program starts running. The high-level program is called the source code, and the translated program is called the object code or the executable.

Java is both compiled and interpreted. Instead of translating programs directly into machine language, the Java compiler generates byte code. Similar to object code, byte code is easy and fast to interpret. But it is also portable, so it is possible to compile a Java program on one machine, transfer the byte code to another machine, and run the byte code on the other machine.

Note

Steps of the development process

The Java compiler is a program named javac. It translates .java files into .class files that store the resulting byte code. The Java interpreter is a program named java, which is short for “Java Virtual Machine” (JVM).

The process of compiling and running a Java program.

The programmer writes source code in the file Hello.java and uses javac to compile it. If there are no errors, the compiler saves the byte code in the file Hello.class. To run the program, the programmer uses java to interpret the byte code. The result of the program is then displayed on the screen.

Although it might seem complicated, these steps are automated for you in most program development environments . In this course, we will use tools (including this lab) where the Java code is actually being sent to a server to compile and run (as long as you have an internet connection) and the output will be shown in the browser.