Exploration of the behaviour of nested try blocks

NestedTry.java

public class NestedTry {
    public static void main(String[] args) {
        try {
            try {
                Long one = Long.parseLong("1");
            } catch(NumberFormatException e) {
                System.out.println("First block!");
            }
            
            Long two = Long.parseLong("2");
        } catch(NumberFormatException nfe) {
            System.out.println("Second block!");
        }
    }
}

To understand how nested try blocks work in Java, the above code has a two level deep of nested try blocks. Both are simply parsing a string into a Long, and printing a message given the exception. The content of the message acts as a marker to easily show which catch block is being executed.

By changing the input to the parseLong method, we can force exceptions to be thrown in different areas, and observe the results. The table below outlines the results.

Test Number	Input to one	Input to two	Output
1	"1"	"2"
2	"swag"	"2"	"First block!"
3	"1"	"swag"	"Second block!"
4	"swag"	"swag"	"First block!" & "Second block!"

What is important to note is that in test number 2, the exception from the nested block, did not bubble up to the outer block.

In test number 4, we see two outputs, as the code continues to execute, after handling each exception in the respective catch blocks. If this is undesired, it is possible to use a return in the nested catch to end the function early.