notes.md

Notes (September 26, 2022)

Topics

• Exception

General Notes

• Compiler finds all errors like type mismatch errors, etc during runtime.
• Segmentation core dump is a runtime error.
• Some common errors are core dump, index out of bound, etc these exceptions are handled by default handler.
• Interrupted exception is handled by the compiler. If I don't know the exception I can give an interrupted exception.
• Most common checked exception is Interrupted exception which is available in java.lang package.
• If we don't know the derived class name we can use the base class object reference. If we don't know the exact name of the checked exception we can just give throws Exception.
• All user-defined exception comes under unchecked exception.
• Unchecked exceptions are exceptions which may or may not arise.

Commands

• javap java.lang.Throwable displays the exceptions available in Java.
• javap java.lang.Thread displays the exceptions and whether it is interrupted (check) or not.

Exception

• Exception is Unexpected Runtime error.
• Segmentation core dump is a runtime error. We don't know which line causes the segmentation error as no information is provided and the program gets terminated. This can be remedied by using Checkpoint or error injection code which is useful for procedural language programs which use all of the settings. In Java, exception handling responds by providing a clear description of the stack trace of the error.
• Default exception handler is present in JVM (Java Virtual Machine).
• Custom exception has limited visibility in C++. But here in Java, most of the class exceptions are user-defined exceptions.
• Main class is Throwable
• Throwable has 2 subclasses namely Error and Exception.
• Throwable -> Error, Exception
• Some common examples of fatal error Stack overflow. JVM will display the error and the program will be terminated. This type of error happens in Sockets, Database handling, etc.

# Flowchart (Types)

Throwable -> Error, Exception

Exception -> RunTimeException

Keywords

• Some keywords that define exception handling in Java are try, catch, finally, throw, throws.

Built-in Exceptions

• Some exceptions are unchecked while some are checked.

• Some common unchecked exceptions are

1. ArithmeticException (Base class: Exception)
2. ArrayIndexOutOfBoundsException
3. NullPointerException

• Some common checked exception are

1. IOException
2. FileNotFoundException
3. ClassCastExceptions

Program 1a: Arithmetic Exception program but without initialization

class Test{
    public static void name(String args[]) {
        int a,b;
        b=0;
        a=a/b;
        System.out.println("a="+a);
    }
}

//Excepting divide by 0 arithmetic exception

Error:
Test.java:5: error: variable a might not have been initialized
        a=a/b;
          ^
1 error

Program 1b: Arithmetic Exception

class Test{
    static void test(){
        int a,b;
        b=0;a=10;
        a=a/b;
        System.out.println("a="+a);
    }
    public static void main(String args[]) 
    {
        test();
    }
}
//Excepting divide by 0 arithmetic exception

[user]$ javac Test.java
[user]$ java Test.java
Exception in thread "main" java.lang.ArithmeticException: / by zero
at Test.test(Test.java:5)
at Test.main(Test.java:10)

Program 2: IOException program

import java.io.DataInputStream;
class Test{
    public static void main(String args[])throws InterruptedException{
        DataInputStream din=new DataInputStream(System.in);
        int a=din.readInt();
        System.out.println(a);
    }
}

[user]$ javac Test.java
Test.java:5: error: unreported exception IOException; must be caught or declared to be thrown
        int a=din.readInt();
                         ^
1 error

Program 3: Check Exception Program

• Program will be excitted after Interrupted exception.

import java.io.DataInputStream;
import java.io.IOException;
class Test{
    public static void main(String args[])throws Exception{
        DataInputStream din=new DataInputStream(System.in);
        int a=din.readInt();
        Thread.sleep(100);
        System.out.println(a);
    }
}

• Output:

[user]$ javac Test.java
[user]$ java Test
123
825373450

Program 4: Exception

import java.io.DataInputStream;
import java.io.IOException;
class Test{
    static void test(){
        DataInputStream din=new DataInputStream(System.in);
        int a=din.readInt();
        Thread.sleep(100);
        System.out.println(a);
    }
    public static void main(String args[]){
        test();
    }
}

Test.java:6: error: unreported exception IOException; must be caught or declared to be thrown
        int a=din.readInt();
                         ^
Test.java:7: error: unreported exception InterruptedException; must be caught or declared to be thrown
        Thread.sleep(100);
                    ^
2 errors