Java Left Shift Assignment

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Syntax

lhs <<= rhs;

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Mechanics

1. Single Evaluation: The lhs expression is evaluated exactly once.
2. Binary Translation: The evaluated lhs is treated as a two’s complement binary integer.
3. Bit Shifting: The bits of lhs are shifted to the left by the number of positions specified by rhs.
4. Zero-Padding: The vacated bit positions on the right (least significant bits) are filled with zeros (0).
5. Truncation: Bits shifted beyond the bounds of the data type’s memory size (most significant bits) are discarded.
6. Assignment: The final bit sequence is assigned back to the memory location of lhs.

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Single Evaluation and Implicit Type Casting

int[] arr = {1, 2, 3};
int i = 0;

// i++ is evaluated only once. 
// arr[0] is shifted left by 2 (1 << 2 = 4). i becomes 1.
arr[i++] <<= 2; 

// If this were strictly expanded to arr[i++] = (int) (arr[i++] << 2), 
// i++ would be evaluated twice, mutating the index incorrectly.

byte b = 10;
// b = b << 2; // Compilation error: int cannot be converted to byte
b <<= 2;       // Compiles successfully. Implicitly casts the int result back to byte.

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Shift Distance Masking

• For int (and smaller types promoted to int): The shift distance is masked by 0x1F (31). Only the lower 5 bits of rhs are used. Therefore, a <<= 33 is mechanically identical to a <<= 1.
• For long: The shift distance is masked by 0x3F (63). Only the lower 6 bits of rhs are used. Therefore, a <<= 65 is mechanically identical to a <<= 1.

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Code Visualization

int val = 5; 
// Binary representation of 5 (32-bit int):
// 00000000 00000000 00000000 00000101

val <<= 3; 
// Shifts all bits 3 positions to the left.
// Vacated positions on the right are filled with 000.
// 00000000 00000000 00000000 00101000

System.out.println(val); // Outputs 40

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Sign Bit Behavior

int val = 1073741824; 
// Binary: 01000000 00000000 00000000 00000000 (Positive)

val <<= 1;
// Binary: 10000000 00000000 00000000 00000000 (Negative)
// The MSB is now 1.

System.out.println(val); // Outputs -2147483648