FrançaisGet started

Compile-time, run-time and logic errors

The three families of errors in a Java program: what the compiler refuses, what crashes at run time and what runs but gives a wrong result, including how to read a real javac message.

  • 25 cards
  • 5 topics
  • Free
Study this deck

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

  1. Concepts
  2. Questions
  3. Sort and link
  4. Scenarios
  5. Understand

A few cards from the deck

  • Compile-time error: what is it?

    Show the answer

    An error the compiler detects before anything runs (syntax, type, unknown symbol, missing return); as long as it remains, the program cannot be launched.

    javac gives the file, the line and the message, for instance B.java:6: error: ';' expected. It gets fixed at the keyboard, before any user ever meets it.

  • javac prints A.java:8: error: missing return statement, on the closing brace line of signe. What is the cause?

    ```java
    static int signe(int n) {
    if (n > 0) {
    return 1;
    } else if (n < 0) {
    return -1;
    }
    }
    `

    • If n is 0, no return is reached
    • A method can hold only one return, placed on its last line
    • An else if must always be followed by an empty else, even if unneeded
    • Both return statements are inside blocks, so invisible to the method
    Show the answer

    If n is 0, no return is reached

    The compiler examines every execution path: for n == 0, neither if nor else if applies, and the method ends without a value. javac reports the end of the method, not the faulty if; adding else { return 0; } settles the case.

  • To produce the .class files, you run the … command; to run the compiled program, you run the … command.

    Show the answer

    To produce the .class files, you run the javac command; to run the compiled program, you run the java command.

    The two steps are separate: a compile-time error prevents the second, a run-time error only shows up in the second.

  • Syntax error: what is it?

    Show the answer

    Code that breaks the grammar of the language: a forgotten semicolon, an unclosed parenthesis or brace, a misspelled keyword.

    It is a compile-time error: nothing runs. int x = 3 without a semicolon gives error: ';' expected, with an arrow right after the 3.

  • The code int sum = 7, count = 2; double avg = sum / count; prints 3.0 instead of 3.5. With no error message. Which family is this flaw?

    • Compile-time: javac should have refused to store an int in a double without a cast
    • Run-time: the virtual machine silently swallows an exception during the division
    • Logic: 7 / 2 is an integer division, computed before the conversion to double
    • Warning: javac reports that the result of a double is always approximate
    Show the answer

    Logic: 7 / 2 is an integer division, computed before the conversion to double

    Storing an int in a double is allowed, with no loss. The flaw is upstream: both operands are integers, so the quotient is 3. Converting one operand is enough: (double) sum / count.

Study this deck