What is the meaning of double tilde (~~) in Java?

java

bitwise-operations

gwt

javascript-interop

byAnton Shumikhin·Jan 24, 2025

Whereas the double tilde ~~ in Java doesn't have a contextual essence, in saying that, it does play the role of a bitwise hack which truncates a floating number into an integer. With the first ~, it flips all the bits and then does a reverse flip with the second ~. Interestingly, while it removes the decimal part, it leaves the integer part untouched.

Quick example:

double value = 8.9;
// If "value" was a cake, we're taking away the cherry (decimal part). Sorry, cherry lovers!
int intValue = ~~value; // intValue now equals 8

Note: This operation is only effective within the integer range and for positive numbers.

Understanding Truncation and Conversion Practices

Dive into Truncation in Bitwise Context

The ~~ operator sequence in good ol' Java doesn't technically have a particular function, unlike in JavaScript where it's a vital player. Utilised alone, the tilde ~ stands as a bitwise complement operator and applying it twice cancels its effect. But there's a twist - dealing with numbers above the 32-bit integer range using ~~ may lead to a surprise party of unexpected results due to overflow behavior.

Exploring Type Conversion Nuances

Now let's bring Google Web Toolkit (GWT) into the picture, where Java transitions into JavaScript. Within this context, the ~~ pattern does what any decent relay racer does - it ensures a smooth and predictable handover during the conversion and truncation from a floating-point to an integer.

Handling Special Cases and Their Effects

For the standard Java, ~~ appears redundant, but don't get fooled! There's a unique case with Integer.MAX_VALUE. In JavaScript, ~~ likes to show off a bit and yields -1 when applied to 2^31 - 1 (Integer.MAX_VALUE), thanks to 32-bit integer wrapping. So keep this in mind while wandering around in the adventurous land of GWT and Java to JavaScript crossovers.

Effect on GWT and Cross-Language Compatibility

The Cross-Compilation Adventure with GWT

As Google Web Toolkit (GWT) transforms Java into JavaScript, it sneakily employs ~~ like a magic trick to maintain consistency in number handling across both languages. It keeps in check the differences between Java's school-tight discipline in typing and JavaScript's laissez-faire approach towards 32-bit signed integers during bitwise operations.

Unraveling JavaScript-Encoding Habits

Here's a fun JavaScript fact for you: JavaScript wields bitwise operators like a wand to magically transform any numeric operand into a 32-bit integer, through a procedure known as sign propagation. This helps keep hash codes and other calculations behaving in a consistent manner even after the numbers have been truncated.

Recognizing the Role of `~~` in JAVA

When it comes to pure Java code, the application of ~~ can be an alien encounter. It's often seen in legacy systems, or for ensuring seamless interop between Java and JavaScript in performance-intensive applications.