Checked Exceptions:

Exceptions that are checked during the compile time are called the checked exception. Basically compiler check for the exception to verify that a method or class that is throwing the exception have code that have given the code contingency. A way of handling with try-catch.

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File file = new File("/path/of/a/non/existing/file);
FileInputStream fileInputStream = new FileInputStream(file);

For example, here when instantiating File object if the file is not present then it throws FileNotFoundException and its super class is Exception class which is checked at compile time.

Unchecked Exceptions:

Exceptions that are not checked during compile time are called unchecked exceptions. These exceptions occurs in run time.Furthermore, we don’t have to declare unchecked exceptions in a method with the throws keyword. Some common unchecked exceptions in Java are NullPointerException, ArrayIndexOutOfBoundsException and IllegalArgumentException.

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private static void divideByZero() {
    int numerator = 1;
    int denominator = 0;
    int result = numerator / denominator;
}

For example, here when result is calculated it casues ArithmaticException which is a runtime exception.

Java Exception Hierarchy

When to use Checked and Unchecked:

From Oracle Java Documentation provides guidance on when to use checked exceptions and unchecked exceptions:

“If a client can reasonably be expected to recover from an exception, make it a checked exception. If a client cannot do anything to recover from the exception, make it an unchecked exception.”