BigDecimal decimal = new BigDecimal(123);
decimal = decimal.multiply(new BigDecimal(-1));
System.out.println(decimal);

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public BigDecimal negate()
Returns a BigDecimal whose value is (-this), and whose scale is this.scale().

public BigDecimal negate(MathContext mc)
Returns a BigDecimal whose value is (-this), with rounding according to the context settings.
Parameters: mc - the context to use.
Returns: -this, rounded as necessary.
Throws: ArithmeticException - if the result is inexact but the rounding mode is UNNECESSARY.