Reading compiler errors

Written Explanation

Anatomy of a message

Here is a real message, produced by JDK 21 for a program whose println statement lacks its semicolon:

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

It has five parts:

1. The file and line: Two.java:3, so look at line 3 of Two.java.
2. The description: error: ';' expected, a short sentence on what the compiler needed.
3. The source line: a copy of the line, so you do not need to count.
4. The caret ^: points to where the compiler noticed the problem.
5. The count: 1 error, how many problems it reported in this run.

The exact words, spacing, and even the line number can differ between JDK versions, so use the messages in this lesson as typical, not word-for-word promises. What does not change is the habit of reading the five parts.

The fix loop

1. Read the first error. Later errors can be side effects of the first, as one wrong bracket can confuse everything after it.
2. Go to the line it names, and look at the line above too. A missing semicolon is noticed only when the next token arrives.
3. Fix one thing.
4. Recompile. Repeat until javac is silent.

A silent javac is success. It also means that a program can compile and still be wrong, and some errors only appear when you run it.

Why only fix one thing at a time? The compiler works in stages. If it finds a grammar problem such as a missing semicolon, it often reports only that and stops before checking names and types. After you fix it, a new error, hidden before, can appear. This is normal, not a sign that you made things worse. Here is an example with two mistakes:

public class F5 {
    public static void main(String[] args) {
        System.out.println("Hi")
        System.out.printn("B");
    }
}

The first compile reports only ';' expected at line 3. Fix it, compile again, and the compiler reports cannot find symbol for printn on line 4. Two mistakes, two rounds.

The five common messages

These are the ones you will meet in the first weeks. The messages below were produced by JDK 21.

A sixth common mistake, a wrong main line, behaves differently and gets its own section below. First, each row in a little more detail.

1. ';' expected. The compiler needs a semicolon where the caret points, which is right after the last character of the statement. Add the semicolon. If the caret points at an odd place, such as after a ), look at the whole statement for a missing or extra parenthesis.

2. cannot find symbol / package ... does not exist. Both mean that Java met a name it does not know. Typical causes: a typo (Sytem, printn), a wrong capital (system, System.Out), or a missing pair of quotes around text (println(Hello)). The lines after the first one tell you more. For printn, JDK 21 adds symbol: method printn(String), and for System.Out it says symbol: variable Out with location: class System. For system.out.println("Hello"); JDK 21 says package system does not exist, because Java reads system.out as a package and a class. Both messages point you at the same fix: check the spelling and case of the name under the caret.

3. reached end of file while parsing. You opened a block and did not close it. The caret sits at the last line of the file, so you must look up the file to find the open block. Count braces, as in Lesson 2.

4. class, interface, enum, or record expected. At the top level of a file, Java expects the start of a class (or one of the other type kinds listed). It sees something else. Two causes: an extra } that closes the class too early so the next } is homeless, or a misspelled class keyword. In JDK 21, public Class F7 { (capital C) produced this message plus extra errors about "unnamed classes" being a preview feature. The first error is the one that matters.

5. unclosed string literal. A string literal must close on the same line. Add the closing ". Also check for a stray " inside the text; use \" for a quote that is part of the text.

Not every error comes from javac

A wrong main line is a special case. Consider:

public class B4 {
    public static void main(String args) {
        System.out.println("Hello");
    }
}

javac B4.java is silent. The program is valid Java, because a method called main that takes a String is legal. The problem appears when you run java B4: the launcher looks for public static void main(String[] args) and reports that it could not find it. JDK 21 prints:

Error: Main method not found in class B4, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

So the rule from BEG-007 applies: first ask which command printed the message. javac messages are compile-time errors. This one came from java, so it is a launch error, and the message even gives you the line to copy. The same message appears if you write Main instead of main, or leave out static.