Of course! Let's break down the concept of double long in Java. It's a great question because it touches on a common point of confusion for developers coming from other languages or with a C/C++ background.

The short and direct answer is:

double long is not a valid data type in Java.

Java does not have a long long type like C or C++. However, you are likely asking about the combination of long and double, which are both valid and important types in Java. Let's explore them individually and then see how they interact.

The long Data Type

The long data type in Java is used to store a 64-bit signed integer.

• Size: 64 bits (8 bytes)
• Range: -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 (which is -2⁶³ to 2⁶³ - 1)
• Use Case: When you need to store whole numbers that are too large for the standard int type (which is 32 bits).

Example:

long populationOfEarth = 7_900_000_000L; // Note the 'L' suffix!
long distanceToSunInKm = 149_597_870L;
// Without the 'L', Java assumes it's an int, which can't hold this value.
// long bigNumber = 9_223_372_036_854_775_807; // COMPILE ERROR: integer number too large
long correctBigNumber = 9_223_372_036_854_775_807L; // Correct with 'L'

Key Point: You must append an L (or l) to a literal long value if it's outside the range of an int. It's a best practice to use uppercase L to avoid confusion with the number 1.

The double Data Type

The double data type in Java is used to store a 64-bit floating-point number.

• Size: 64 bits (8 bytes)
• Range: Very large range, approximately ±1.8 x 10³⁰⁸ (with limited precision)
• Precision: About 15-17 significant decimal digits.
• Use Case: For storing numbers with decimal points (fractional numbers) or very large whole numbers where precision isn't the absolute top priority.

Example:

double pi = 3.14159;
double price = 19.99;
double veryLargeNumber = 1.23e20; // Scientific notation for 1.23 * 10^20

Why double long Doesn't Exist: The long vs. double Conflict

Now, let's address the core of your question. You can't combine them into double long because they represent fundamentally different concepts and Java has already made its choice.

The Conflict:

• Precision: If you made a long double, what would it be? A long that is precise? A double with a wider range? Java's designers decided that the 64-bit double already provided an enormous range for most scientific and engineering applications, and for situations requiring absolute precision for very large numbers, the long integer type was sufficient.
• Simplicity: Java was designed to be simpler and more robust than C/C++. By not having long long, unsigned int, etc., the language avoids a lot of common pitfalls related to type conversion and overflow.

The Correct Way: long to double Conversion

Even though double long doesn't exist, you will often need to convert a long value to a double. This is possible and very common.

When you convert a long to a double, you are trading the exactness of the integer for the huge range of the floating-point type.

long myLong = 123456789012345L;
System.out.println("Original long value: " + myLong);
// Convert the long to a double
double myDouble = (double) myLong; // Explicit cast
// double myDouble = myLong; // Implicit cast also works
System.out.println("Converted double value: " + myDouble);
System.out.println("Is the double value exact? " + (myDouble == myLong)); // true, for this number

Output:

Original long value: 123456789012345
Converted double value: 1.23456789012345E14
Is the double value exact? true

Important Caveat: Precision Loss For most long values, the conversion to double is exact because a double has 53 bits of mantissa (precision), which is more than enough to represent a 64-bit long's 63 bits of magnitude.

However, if your long is larger than 2⁵³ (9,007,199,254,740,992), you will start to lose precision. The double type can no longer store every single integer value within its range.

long largeLong = 9_223_372_036_854_775_807L; // Max value for a long
double largeDouble = (double) largeLong;
System.out.println("Original long: " + largeLong);
System.out.println("Converted double: " + largeDouble);
System.out.println("Are they equal? " + (largeDouble == largeLong)); // false!

Output:

Original long: 9223372036854775807
Converted double: 9.223372036854776E18
Are they equal? false

Notice how the last few digits of the double are ...776 instead of the original ...807. This is precision loss.

What about BigDecimal? (For Ultimate Precision)

If you need to handle very large numbers with both a large range and perfect decimal precision (e.g., for financial calculations), you should use the java.math.BigDecimal class. It is not a primitive type like long or double, but an object that can represent numbers of arbitrary scale and precision.

import java.math.BigDecimal;
// A number too big for long and requiring decimal precision
BigDecimal veryPreciseNumber = new BigDecimal("123456789012345.987654321");
System.out.println("BigDecimal value: " + veryPreciseNumber);
// This number is stored exactly, no precision loss.

Summary