Created: 2023-09-06 18:05
Status: #concept
Subject: Programming
Tags: Java Java Class java.lang Error Handling

Java Exception

They are a Throwable class that have many subclasses which denote different error types in the Java runtime.

Handling Errors with try-catch

When an exception is thrown inside the try { ... } block, the catch(Exception e) block is executed.

  • there are useful methods like e.getMessage(), e.printStackTrace(), etc.

Scanner reader = new Scanner(System.in);

System.out.print("Give a number: ");
int readNumber = -1;

try {
    readNumber = Integer.parseInt(reader.nextLine());
} catch (Exception e) {
    System.out.println("User input was not a number.");
}

Give a number: no!
User input was not a number.

Shifting Responsibility

We can make methods throws Exception which signals the programmer to handle the method call's exception when used or handle it somewhere else.

public List<String> readLines(String fileName) throws Exception {
    ArrayList<String> lines =  new ArrayList<>();
    Files.lines(Paths.get(fileName)).forEach(line -> lines.add(line));
    return lines;
}

Throwing Custom Errors

Similar to JavaScript, we can throw new ExceptionType(); to force an exception.

public class Program {

    public static void main(String[] args) throws Exception {
        throw new NumberFormatException(); // Program throws an exception
    }
}

Common ___Exception Types

  1. NumberFormatException is thrown if the string it has been given cannot be parsed into an integer.
int readNumber = Integer.parseInt("pog");
// **Exception in thread "..." java.lang.NumberFormatException: For input string: "dinosaur"**
  1. IOException & FileNotFoundException is thrown when we try to java.nio.file.Paths .get("nonexistent.txt") and it doesn't exist.
ArrayList<String> lines =  new ArrayList<>();

// create a Scanner object for reading files
try (Scanner reader = new Scanner(new File("file.txt"))) {

    // read all lines from a file
    while (reader.hasNextLine()) {
        lines.add(reader.nextLine());
    }
} catch (Exception e) {
    System.out.println("Error: " + e.getMessage());
}

// do something with the lines

  1. IllegalArgumentException(String msg) can be manually thrown with a message, typically used to validate bad function arguments.

    • we do not need to manually handle this because it stops the program automatically.

  2. ArrayIndexOutOfBoundsException when we try to index an array outside its bounds.

  3. InterruptedException occurs when we use Thread.sleep(ms) in a Java Thread or Runnable's public void run() method.

    • it means the Thread stopped early or the class method ThreadInstance.interrupt() was called.

References