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The %= (remainder assignment) operator is a compound assignment operator that divides the left-hand operand by the right-hand operand and assigns the resulting remainder back to the left-hand operand. In Java, this operator performs a remainder operation rather than a strict mathematical modulo, meaning the sign of the result always matches the sign of the dividend (the left-hand operand).

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Syntax and Evaluation

variable %= expression;
At compile time, the expression E1 %= E2 is evaluated as E1 = (T) ((E1) % (E2)), where T is the declared data type of E1. The Java Language Specification guarantees that the left-hand operand E1 is evaluated exactly once.

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Implicit Type Casting

A critical mechanical feature of the %= operator is its built-in implicit narrowing conversion. If the evaluation of E1 % E2 results in a wider data type than T, the compiler automatically casts the result back to T without requiring explicit cast syntax. 
byte a = 10;
a %= 3.5; 

// The above is internally processed as:
// a = (byte) (10 % 3.5);
// Result: a = 3

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Operand Behavior and Edge Cases

The behavior of the %= operator changes depending on whether the operands are integral or floating-point types:
  • Integer Operands: If both operands are integers, the operation truncates toward zero. If the right-hand operand evaluates to 0, the JVM throws an ArithmeticException at runtime.
  • Floating-Point Operands: If either operand is a float or double, the operation explicitly does not compute the IEEE 754 remainder operation. According to the Java Language Specification (JLS §15.17.3), Java’s floating-point remainder uses a truncating division so that it behaves analogously to the integer remainder operator, rather than the rounding division required by IEEE 754. Dividing by 0.0 does not throw an exception; instead, the variable is assigned NaN (Not-a-Number).
  • Sign Preservation: The sign of the divisor (right-hand operand) is ignored. The assigned remainder will always carry the sign of the original variable.
int x = -10;
x %= 3;   // x becomes -1 (Sign matches the dividend)

int y = 10;
y %= -3;  // y becomes 1 (Divisor sign is ignored)

double z = 5.0;
z %= 0.0; // z becomes NaN (No ArithmeticException)

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Operator Precedence

The %= operator has the same low precedence as other assignment operators (=, +=, *=, etc.). It evaluates from right to left. If the right-hand side is a complex expression, that entire expression is evaluated before the remainder operation occurs. 
int val = 20;
val %= 2 + 3; 

// Evaluated as val = val % (2 + 3), NOT (val % 2) + 3
// Result: val = 0
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