Exception Handling

When developing programs, countless errors occur. Of course, these errors are Java's way of preventing programs from malfunctioning. However, sometimes we want to ignore these errors, and other times we want to handle them appropriately when they occur. To address this, Java uses try catch statements to handle errors.

When do exceptions occur? #

• Let's try to open a non-existent file.

BufferedReader br = new BufferedReader(new FileReader("나없는파일"));
br.readLine();
br.close();

• Executing the code above will result in the following error.

Exception in thread "main" java.io.FileNotFoundException: 나없는파일 (지정된 파일을 찾을 수 없습니다)
    at java.io.FileInputStream.open(Native Method)
    at java.io.FileInputStream.<init>(Unknown Source)
    at java.io.FileInputStream.<init>(Unknown Source)
    at java.io.FileReader.<init>(Unknown Source)
    ...

Attempting to open a non-existent file will raise an exception named FileNotFoundException.

• Now, let's look at the case of dividing a number by zero.

int c = 4 / 0;

Executing the code above will result in the following error.

Exception in thread "main" java.lang.ArithmeticException: / by zero
    at Test.main(Test.java:14)

Dividing 4 by 0 will raise an ArithmeticException.

Finally, let's look at one more error. The following error occurs very frequently.

int[] a = {1, 2, 3};
System.out.println(a[3]);

The error message is as follows.

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
    at Test.main(Test.java:17)

Since a[3] is the 4th value in array a, it's a value that cannot be obtained from array a. Therefore, an ArrayIndexOutOfBoundsException occurred.

When such an exception occurs, Java stops the program and displays an error message.

Handling Exceptions #

• The following is the basic structure of a try catch statement for exception handling.

try{
  ...
}catch(예외1){
  ...
} catch(예외 2){

}

• If no exception occurs in the statements within the try block, the catch blocks will not be executed.
• If an exception occurs while executing a statement within the try block, the corresponding catch block for that exception will be executed.
• To handle an exception that occurs when dividing a number by zero, you can do the following.

int c;
try {
    c = 4 / 0;
} catch(ArithmeticException e) {
    c = -1;  // 예외가 발생하여 이 문장이 수행된다.
}

This handles the exception by assigning -1 to c if an ArithmeticException occurs. In ArithmeticException e, e refers to an object of the ArithmeticException class, which is an error object. Through this error object, methods of the corresponding exception class can be called.

finally #

• If an exception occurs during program execution, the program either stops or a catch block is executed due to exception handling.
• But what if there's a part that must be executed regardless of which exception occurs?

Let's look at the following example.

public class Sample {
    public void shouldBeRun() {
        System.out.println("ok thanks.");
    }

    public static void main(String[] args) {
        Sample sample = new Sample();
        int c;
        try {
            c = 4 / 0;
            sample.shouldBeRun();  // 이 코드는 실행되지 않는다.
        } catch (ArithmeticException e) {
            c = -1;
        }
    }
}

• The finally block is always executed, regardless of whether an exception occurred during the execution of the try block.
• Executing the code above will cause the sample.shouldBeRun() method to be executed, printing the message "ok.thanks".