Hierarchy of Java exception class
The Throwable class is the root class of Java exception hierarchy, it contains two child classes.
=> Exception
=> Error
Note: we have so many different types of exceptions and errors, not only shown in the diagram.
Exception:
Exceptions are occurring by the programmer during runtime and it is recoverable.
E.g: ArithmeticException, it will occur when we divide any number by zero
There are mainly two types of exceptions:
checked exceptions and unchecked exceptions.
Checked Exception:
Checked exception classes which extend Throwable class except RuntimeException and Error.
Checked exceptions are checked by the compiler during compilation for smooth execution of the program at runtime.
e.g: SQLException, FileNotFoundException, ClassNotFoundException, InterruptedException etc.
Example:
package com.exceptions;
import java.io.PrintWriter;
public class FileNotFoundTest {
public static void main(String[] args) {
PrintWriter pw = new PrintWriter("abc.txt");
pw.println("hello world");
pw.close();
}
}
Output:
Exception in thread "main" java.lang.Error: Unresolved compilation problem:
Unhandled exception type FileNotFoundException
at com.exceptions.FileNotFoundTest.main(FileNotFoundTest.java:5)
Checked exceptions don’t occur during compile time but checked by the compiler during compilation and notify to the developer that there could be an exception occurs at runtime.
Unchecked Exception:
Unchecked exception classes which extend RuntimeException class and unchecked exceptions checked at runtime.
e.g: ClassCastException, ArithmeticException, NullPointerException, NumberFormat Exception, ArrayIndexOutOfBoundException and etc.
Note: checked exceptions and unchecked exceptions are occurring during runtime only, not at compile time.
Example:
package com.exceptions;
public class ArithmeticExceptionTest {
public static void main(String[] args) {
int i=10/0;
System.out.println(i);
}
}
OutPut:
Exception in thread "main" java.lang.ArithmeticException: / by zero
at com.exceptions.ArithmeticExceptionTest.main(ArithmeticExceptionTest.java:4)
Error:
Errors occur by the system resources and it is not recoverable by the programmer.
e.g: StackOverFlowError, OutOfMemoryError, NoClassDefFoundError and etc.
Example: OutOfMemoryError, it will occur whenever system doesn’t contain sufficient memory to run our program.
Example:
public class StackOverflow {
void method1(){
method2(); //call method2
}
void method2(){
method1(); //call method1
}
public static void main(String[] args) {
StackOverflow obj=new StackOverflow();
obj.method1();
}
}
Output
Exception in thread "main" java.lang.StackOverflowError
at practise.StackOverflow.method1(StackOverflow.java:6)
at practise.StackOverflow.method2(StackOverflow.java:10)
at practise.StackOverflow.method1(StackOverflow.java:6)
at practise.StackOverflow.method2(StackOverflow.java:10)
at practise.StackOverflow.method1(StackOverflow.java:6)
at practise.StackOverflow.method2(StackOverflow.java:10)
at practise.StackOverflow.method1(StackOverflow.java:6)
at practise.StackOverflow.method2(StackOverflow.java:10)
