Reading Compiler Errors

You’re going to spend a significant portion of your life as a programmer reading error messages. That’s not pessimism, it’s just the job. The faster you can decode what went wrong, the faster you get back to writing code that actually works.

Java’s compiler is stricter than most. It refuses to produce a program until it’s satisfied everything is correct. That sounds annoying, and sometimes it is, but it also means Java catches entire categories of mistakes before your code ever runs. The trick is learning to read what the compiler is telling you, because the messages aren’t always obvious at first.

There are two kinds of errors: ones the compiler catches before running your program (compile-time errors), and ones that only appear while your program is running (runtime errors). We’ll focus mostly on compile-time errors here, since those are what you’ll hit first.

The anatomy of a compiler error

When the Java compiler finds a problem, it tells you three things:

What file the error is in
What line the problem is on
What it thinks went wrong

Here’s a simple broken program:

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

Terminal

The compiler output looks something like this:

Greeting.java:3: error: ';' expected        System.out.println("Hello, world!")                                           ^1 error

Break that down:

Greeting.java — the file with the problem
:3 — the problem is on line 3
error: ';' expected — what the compiler thinks is wrong
The little ^ points to where in the line it got confused

In this case the fix is obvious: add the semicolon. But notice something worth remembering: the compiler points to where it got confused, not necessarily where you made the mistake. Those are often the same place, but not always.

The three most common beginner errors

Missing semicolon

Java requires a semicolon at the end of every statement. Forget one and the compiler complains, usually on the line with the missing semicolon or the line right after it.

public class Example {    public static void main(String[] args) {        System.out.println("Line one")   // missing semicolon here        System.out.println("Line two");    }}

Terminal

Error message:

Example.java:3: error: ';' expected        System.out.println("Line one")   // missing semicolon here                                      ^1 error

The fix: add the semicolon at the end of line 3.

Mismatched braces

Every opening brace { needs a matching closing brace }. Forget one and the compiler often reports an error at the end of the file, which feels confusing if the actual missing brace is somewhere in the middle.

public class Example {    public static void main(String[] args) {        System.out.println("Hello");    // missing closing brace for the class

Terminal

Error message:

Example.java:3: error: reached end of file while parsing        System.out.println("Hello");                                    ^1 error

"Reached end of file while parsing" is almost always a missing }. Count your braces if you see this one.

A good habit: when you type an opening brace, type the closing brace immediately before writing anything inside it. Most modern editors do this for you automatically.

Wrong class name

Java requires the class name to match the filename exactly, including capitalisation. If your file is named HelloWorld.java, the class inside must be named HelloWorld, not helloworld, not Helloworld, not hello_world.

public class helloworld {   // wrong: lowercase    public static void main(String[] args) {        System.out.println("Hello");    }}

Terminal

Error message:

HelloWorld.java:1: error: class helloworld is public, should be declared in a file named helloworld.javapublic class helloworld {   // wrong: lowercase       ^1 error

The fix: rename the class to match the file, or rename the file to match the class.

When the error points to the wrong place

The compiler reads your code from top to bottom. If something early throws it off, it only notices something is wrong when it hits a point where the code stops making sense, which may be lines later, or not until it runs out of file entirely.

The mismatched braces example above is a good illustration. The missing } might be somewhere in the middle of the file, but the compiler doesn’t notice until it reaches the end, reporting “reached end of file while parsing.” The real problem is several lines earlier.

The rule of thumb: if the flagged line looks fine, check the lines above it.

Runtime errors: a quick preview

Compile-time errors happen before your program runs. Runtime errors are different. They slip past the compiler just fine, but crash the program while it’s actually running.

Click Run on this program. It will compile successfully, then immediately crash:

public class Example {    public static void main(String[] args) {        System.out.println(1 / 0);    }}

Terminal

You’ll see something like:

Exception in thread "main" java.lang.ArithmeticException: / by zero    at Example.main(Example.java:3)

Don’t worry about understanding what ArithmeticException means yet. The important parts for now are the short description (/ by zero) and the line number (Example.java:3). That’s where you start looking.

You’ll encounter runtime errors regularly as you progress. There’s a full lesson on reading them properly later in the course. For now, just know that when your program crashes, Java tells you what went wrong and where.

Checkpoint

1 / 3

Your program compiles, but the moment it runs it stops with an error message. What does this tell you?

2 / 3

You compile this program and the error message says “reached end of file while parsing.” Nothing looks wrong to you. What should you check first?

public class Example {
    public static void main(String[] args) {
        System.out.println("Hello");
        System.out.println("Goodbye");

3 / 3

You name your file MyProgram.java but declare public class myprogram inside it. What happens?

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Reading Compiler Errors

The anatomy of a compiler error

When the Java compiler finds a problem, it tells you three things:

What file the error is in
What line the problem is on
What it thinks went wrong

Here’s a simple broken program:

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

Terminal

The compiler output looks something like this:

Greeting.java:3: error: ';' expected        System.out.println("Hello, world!")                                           ^1 error

Break that down:

Greeting.java — the file with the problem
:3 — the problem is on line 3
error: ';' expected — what the compiler thinks is wrong
The little ^ points to where in the line it got confused

The three most common beginner errors

Missing semicolon

Java requires a semicolon at the end of every statement. Forget one and the compiler complains, usually on the line with the missing semicolon or the line right after it.

public class Example {    public static void main(String[] args) {        System.out.println("Line one")   // missing semicolon here        System.out.println("Line two");    }}

Terminal

Error message:

Example.java:3: error: ';' expected        System.out.println("Line one")   // missing semicolon here                                      ^1 error

The fix: add the semicolon at the end of line 3.

Mismatched braces

public class Example {    public static void main(String[] args) {        System.out.println("Hello");    // missing closing brace for the class

Terminal

Error message:

Example.java:3: error: reached end of file while parsing        System.out.println("Hello");                                    ^1 error

"Reached end of file while parsing" is almost always a missing }. Count your braces if you see this one.

A good habit: when you type an opening brace, type the closing brace immediately before writing anything inside it. Most modern editors do this for you automatically.

Wrong class name

public class helloworld {   // wrong: lowercase    public static void main(String[] args) {        System.out.println("Hello");    }}

Terminal

Error message:

HelloWorld.java:1: error: class helloworld is public, should be declared in a file named helloworld.javapublic class helloworld {   // wrong: lowercase       ^1 error

The fix: rename the class to match the file, or rename the file to match the class.

When the error points to the wrong place

The rule of thumb: if the flagged line looks fine, check the lines above it.

Runtime errors: a quick preview

Compile-time errors happen before your program runs. Runtime errors are different. They slip past the compiler just fine, but crash the program while it’s actually running.

Click Run on this program. It will compile successfully, then immediately crash:

public class Example {    public static void main(String[] args) {        System.out.println(1 / 0);    }}

Terminal

You’ll see something like:

Exception in thread "main" java.lang.ArithmeticException: / by zero    at Example.main(Example.java:3)

Checkpoint

1 / 3

Your program compiles, but the moment it runs it stops with an error message. What does this tell you?

2 / 3

You compile this program and the error message says “reached end of file while parsing.” Nothing looks wrong to you. What should you check first?

public class Example {
    public static void main(String[] args) {
        System.out.println("Hello");
        System.out.println("Goodbye");

3 / 3

You name your file MyProgram.java but declare public class myprogram inside it. What happens?