programming languages - What is the purpose of the unsigned right shift operator ">>>" in Java?

Question

I understand what the unsigned right shift operator ">>>" in Java does, but why do we need it, and why do we not need a corresponding unsigned left shift operator?

Answer 1

The >>> operator lets you treat int and long as 32- and 64-bit unsigned integral types, which are missing from the Java language.

This is useful when you shift something that does not represent a numeric value. For example, you could represent a black and white bit map image using 32-bit ints, where each int encodes 32 pixels on the screen. If you need to scroll the image to the right, you would prefer the bits on the left of an int to become zeros, so that you could easily put the bits from the adjacent ints:

 int shiftBy = 3;
 int[] imageRow = ...
 int shiftCarry = 0;
 // The last shiftBy bits are set to 1, the remaining ones are zero
 int mask = (1 << shiftBy)-1;
 for (int i = 0 ; i != imageRow.length ; i++) {
     // Cut out the shiftBits bits on the right
     int nextCarry = imageRow & mask;
     // Do the shift, and move in the carry into the freed upper bits
     imageRow[i] = (imageRow[i] >>> shiftBy) | (carry << (32-shiftBy));
     // Prepare the carry for the next iteration of the loop
     carry = nextCarry;
 }

The code above does not pay attention to the content of the upper three bits, because >>> operator makes them

There is no corresponding << operator because left-shift operations on signed and unsigned data types are identical.