How Numeric literal with underscore works in java and why it was added as part of jdk 1.7

This is used to group the digits in your numeric (say for example for credit card etc)

From Oracle Website:

In Java SE 7 and later, any number of underscore characters (_) can appear anywhere between digits in a numerical literal. This feature enables you, for example, to separate groups of digits in numeric literals, which can improve the readability of your code.

For instance, if your code contains numbers with many digits, you can use an underscore character to separate digits in groups of three, similar to how you would use a punctuation mark like a comma, or a space, as a separator.

To conclude, it's just for a sake of readability.

See Underscores in Numeric Literals:

In Java SE 7 and later, any number of underscore characters (_) can appear anywhere between digits in a numerical literal. This feature enables you, for example, to separate groups of digits in numeric literals, which can improve the readability of your code.

Try this:

int num = 111_222;
System.out.println(num); //Prints 111222

This feature was added due to the fact that long numbers can be hard to read sometimes, so instead of counting how many "zeros" a number has to figure out if it's a million or one hundred thousand, you can do:

int myNum = 1_000_000;

Now it's easy to see that there is two groups of 3 zeros, and clearly the number is million. Compare it with:

int myNum = 1000000;

Admit.. here you had to count each zero..

JDK 7 _ for numeric literals feature is only for the sake of readability. As per docs:

In Java SE 7 and later, any number of underscore characters (_) can appear anywhere between digits in a numerical literal. This feature enables you, for example, to separate groups of digits in numeric literals, which can improve the readability of your code.