Numeric Literals in Java

Java’s numeric literals are not unusual compared to other languages. You add a suffix like 2.4F to force a float, and you add L to make an integer literal a long.

public class TestClass {
    public void someMethod() {
        // int: -2147483648 ~ 2147483647

        // compile error: integer number too large
        long value1 = 2147483648;

        // okay
        long value2 = 2147483648L;
    }
}

The int range is -2147483648 ~ 2147483647. Anything beyond that overflows. For reference, long is -9223372036854775808 ~ 9223372036854775807.

In the code above, value1 is a long, but the literal is still an int. That is why it fails. Add the L suffix to make it a long literal.

Large Numbers Are Hard to Read

Since Java 7, numeric literals can include underscores _ to improve readability. You can group digits like a thousands separator.

public void someMethod() {
    // much easier to read
    long valueWithUnderscore = 2_147_483_648L;
}

However, underscores are only allowed between digits. Here are a few examples:

public void someMethod() {
    long value1 = 2_147_483_648L; // 2147483648
    System.out.println(value1);

    int value2 = 3_2; // 32
    System.out.println(value2);

    int value3 = 2____4; // 24
    System.out.println(value3);

    long value4 = 2_222_L; // error: only between digits

    float value5 = _24F; // error: treated as a variable name
}

If an underscore appears at the beginning, the compiler can read it as an identifier. And remember: this feature exists only since Java 7.

Oracle Java Docs: Underscores in Numeric Literals