How a Java program is compiled and run

Every program you write in Java goes through two stages: first it is compiled, then it is run. Knowing this distinction earns points in two places: the fill-in-the-blank questions on the midterm (javac, bytecode, JVM) and the “what kind of error is this?” questions. There is also the difference between print and println, which is hidden inside almost every output question.

Golden rules:

1. .java → javac → .class → java → JVM. Memorize this chain.
2. The name of a public class = the file name. public class Hello can only live in Hello.java.
3. The program starts at the main method and lines run from top to bottom in order.
4. print stays on the line, println moves to the next line.
5. The compiler only finds syntax and type errors. It can never find a logic error.

From source code to a running program

The computer’s processor does not understand Java code directly. There are two steps in between.

Bytecode is not machine code. It is Java’s own intermediate language. That is why the same .class file runs on Windows, macOS and Linux: every system has a JVM (Java Virtual Machine) suited to that system, and it translates the bytecode on the spot. Java’s slogan “write once, run anywhere” comes from this.

Do not mix up the abbreviations:

TIP · running a single file directly. In newer JDKs the command java Hello.java compiles the file in the background and runs it right away. It works fine in practice, but in the exam write the chain as two separate commands: first javac Hello.java, then java Hello.

Anatomy of the first program

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

Merhaba, Java!

Line by line:

• public class Hello defines a class. In Java all code lives inside a class. By convention a class name starts with a capital letter. Because it is public, the file name has to be Hello.java.
• { and } open and close a block. Every opening curly brace must have a closing one.
• public static void main(String[] args) is the program’s entry point. When the JVM starts the program, it looks for this method. It is written exactly like this, word for word. Memorize it for now; static and void will make sense in week 5.
• System.out.println(...) writes what is inside the parentheses to the screen. This line is a statement and it has a ; at the end. In Java every statement ends with a semicolon.

Java is case-sensitive. If you write system.out.println, it does not compile; the compiler says package system does not exist. Main and main are different names.

print, println and escape sequences

System.out.print("Bir ");
System.out.print("iki ");
System.out.println("uc");
System.out.println("dort");

Bir iki uc
dort

print writes the text and leaves the cursor where it is. println writes the text and ends the line, so the next output starts on the next line. print adds no spaces; if you want spaces between words, you have to put them inside the text.

To put a special character inside a String, you use an escape sequence. All of them start with a backslash \:

System.out.println("Ad\tSoyad");
System.out.println("Satir1\nSatir2");
System.out.println("Hoca \"tamam\" dedi");
System.out.println("C:\\Users\\deniz");

Ad	Soyad
Satir1
Satir2
Hoca "tamam" dedi
C:\Users\deniz

PITFALL · single backslash. "C:\Users" does not compile: \U is not a valid escape sequence, and the compiler says illegal escape character. When you write a file path, double every backslash.

Comments

A comment is an explanation that the compiler ignores completely. There are two kinds:

// System.out.println("gizli");      single-line comment
System.out.println("gorunur"); /* System.out.println("bu da gizli"); */

gorunur

// turns everything up to the end of the line into a comment. Everything between /* ... */ is a comment, even if it spans several lines. In a “what is printed?” question, skip the println lines inside comments; treat them as if they were never written.

The three kinds of errors

The classic short exam question: “Is this error compile-time, runtime or logic?” The way to tell them apart is to look at when the error shows up.

Compile-time error

javac finds it while translating the code. The program is never produced, so it never runs. Syntax errors and type errors belong here.

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hi")
    }
}

Hello.java:3: error: ';' expected
        System.out.println("Hi")
                                ^
1 error

Other common examples: an undefined variable (cannot find symbol), a wrong file name (class Greet is public, should be declared in a file named Greet.java), assigning a double to an int (possible lossy conversion), an unclosed curly brace.

Runtime error

The code compiles and the program starts, but while running it crashes on a line that asks for something impossible. Java calls this an exception.

System.out.println("start");
System.out.println(10 / 0);
System.out.println("end");

start
Exception in thread "main" java.lang.ArithmeticException: / by zero
	at Div.main(Div.java:4)

The lines before the crash have run (start was printed). Nothing after the crashing line runs (no end). Other examples you will see later: going outside an array (ArrayIndexOutOfBoundsException), typing a letter instead of a number into a Scanner (InputMismatchException).

Logic error

The program compiles, runs and does not crash, but gives a wrong result. This is the sneakiest kind, because nobody warns you.

int x = 4, y = 6;
double avg = x + y / 2;   // intent: the average
System.out.println(avg);

7.0

The average should have been 5.0. The division was done before the addition: 4 + 6 / 2 = 4 + 3. The fix is (x + y) / 2.0. You can only find this kind of error by testing by hand (comparing the result with an example whose answer you know).

Worked example: reading an error message

javac’s message always has the same pattern: FileName.java:line: error: description, with the faulty line and a ^ marker underneath. The program below has two errors:

public class Typo {
    public static void main(String[] args) {
        int total = 5;
        System.out.println(totl);
        system.out.println("x");
    }
}

Typo.java:4: error: cannot find symbol
        System.out.println(totl);
                           ^
  symbol:   variable totl
  location: class Typo
Typo.java:5: error: package system does not exist
        system.out.println("x");
              ^
2 errors

Reading order: first go to the line number (4), look at the word the ^ marker points to (totl), then read the description (cannot find symbol: nothing with this name is defined). The fix is total. The second error is the lowercase system; because Java is case-sensitive, it took that for a package name. Messages are sometimes indirect, but the line number almost always points to the right place. If there are several errors, start with the topmost one: fixing the first error often makes the ones below it disappear too.

Common mistakes

System.out.print("Java");
System.out.print("Code");
System.out.println("!");

System.out.println("A");
System.out.print("B");
System.out.println();
System.out.println("C");

System.out.println("1\t2\n3");

System.out.println("She said \"hi\"");

System.out.println("C:\\new\\test");

System.out.print("x");
// System.out.print("y");
System.out.print("z"); /* System.out.print("w"); */
System.out.println();

System.out.println("Line 1");
System.out.print("Line 2\n");
System.out.print("Line 3");
System.out.println(" end");

int x = 4, y = 6;
double avg = x + y / 2;
System.out.println(avg);

System.out.println("start");
System.out.println(10 / 0);
System.out.println("end");