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Exceptions are events that occur during the execution of programs that disrupt the normal flow of instructions. They can be caused by various reasons, such as invalid input, file not found, or divisions by zero. In Java, exceptions are classified into two types: checked and unchecked exceptions.

Checked exceptions are exceptional conditions that a well-written program should anticipate and recover from. They are also called compile-time exceptions because the Java compiler checks the code during compilation and reports an error if a checked exception is not handled or declared using the "throws" keyword. Examples of checked exceptions include FileNotFoundException, IOException, and ClassNotFoundException.

Unchecked exceptions are usually caused by programming errors, such as null pointer dereferencing, array index out of bounds, or illegal arguments. These exceptions are called runtime exceptions because they are not checked at compile-time; instead, they are thrown during the execution of the program. Examples of unchecked exceptions include NullPointerException, ArrayIndexOutOfBoundsException, and IllegalArgumentException.

In Java programming, you are required to use the throws clause in a method header in the following situations: 1. When your method can potentially throw a checked exception, you should use the "throws" keyword followed by the exception type in the method header. This indicates that the method might throw the specified exception type and informs the calling code to handle or declare the exception. Example: ```java public void readFile(String filePath) throws FileNotFoundException { // code to read the file } ``` 2. If your method overrides a superclass method that throws a checked exception, you must either handle the exception within the method using a "try-catch" block, or declare the checked exception in the method header with the "throws" keyword. Example: ```java @Override public void readFile(String filePath) throws FileNotFoundException { // code to read the file } ``` In the case of unchecked exceptions, it is not mandatory to use the "throws" clause in the method header, as they are not checked during compilation. However, you may still choose to include the "throws" clause for unchecked exceptions to indicate potential issues that could arise during runtime.