Compiling and running java programs from command prompt or terminal

After installing Java SDK, you can write a code in a basic text editor (like notepad, atom, sublime) and then compile and run Java programs from command prompt or terminal.

A java source file HelloWorld.java is compiled as

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javac HelloWorld.java

Once it is compiled, there will be java class file generated with name HelloWorld.class. What you execute is this class file, as,

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java HelloWorld

This is a good skill and the benefit of doing this over using an IDE like Eclipse include,

1. reducing reliance on auto-complete done by Eclipse.

2. not relying on Eclipse to tell you each time there is a syntactical error.

3. program will not compile, and therefore the class file will not be generated to execute if there is a syntactical error.

4. Helps with pen and paper design.

Let’s say, following are the contents of HelloWorld.java

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public class HelloWorld {
	public static void main(String[] args) {
		System.out.println(5+);
	}
}

When you compile the program, you’ll get the following message,

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I-2:intro gauravgupta$ javac HelloWorld.java
HelloWorld.java:3: error: illegal start of expression
System.out.println(5+);
^
1 error
I-2:intro gauravgupta$

You can see that it points to the line number (3) with syntactical errors so you can correct them. In this case, we change the 5+ to 5+3, and our program compiles, and executes, successfully.

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public class HelloWorld {
	public static void main(String[] args) {
		System.out.println(5+3);
	}
}