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From: Knute Johnson <nospam@knutejohnson.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: How are multiple Java files compiled together?
Date: Sun, 27 May 2012 16:34:42 -0700
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On 5/27/2012 3:48 PM, Jason Kim wrote:
> I am new to Java.
>
> Here are two files.
>
> VolcanoApplication.java
> class VolcanoApplication {
> 	public static void main(String[] arguments) {
> 		VolcanoRobot dante = new VolcanoRobot();
> 		dante.status = "exploring";
> 		dante.speed = 2;
> 		dante.temperature = 510;
> 		
> 		dante.showAttributes();
> 		System.out.println("Increasing speed to 3.");
> 		dante.speed = 3;
> 		dante.showAttributes();
> 		System.out.println("Changing temperature to 670");
> 		dante.temperature = 670;
> 		dante.showAttributes();
> 		System.out.println("Checking the temperature.");
> 		dante.checkTemperatur();
> 		dante.showAttributes();
> 	}
> }
>
> VolcanoRobot.java
> class VolcanoRobot {
> 	String status;
> 	int speed;
> 	float temperature;
> 	
> 	void checkTemperatur() {
> 		if (temperature>  50) {
> 			status = "returning home";
> 			speed = 5;
> 		}
> 	}
> 	
> 	void showAttributes() {
> 		System.out.println("Status: " + status);
> 		System.out.println("Speed: " + speed);
> 		System.out.println("Temperature: " + temperature);
> 	}
> }
>
> I compiled the program by doing
> $ javac VolcanoApplication.java
>
> But how does Java know that VolcanoRobot.java should also be in the compilation?

If it didn't know, think about what a pain that would be every time you 
had to compile a program with a lot of files.  You reference 
VolcanoRobot in VolcanoApplication, the compiler looks for that that 
class in the classpath and compiles it if necessary.

That feature can be the source of a very tricky problem and that is if 
there is a VolcanoRobot.class file in the classpath the compiler won't 
compile VolcanoRobot.java even if it has changed since the 
VolcanoRobot.class file was created.

-- 

Knute Johnson