Is Java pass-by-reference or pass-by-value?

Resolves Java's pass-by-value confusion by separating mutation through a copied reference from reassignment of the caller's variable.

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TL;DR

Java is always pass-by-value. A method receives a copy of the value stored in each argument variable. For a primitive, that copied value is the primitive value. For an object variable, the copied value is a reference to the same object. The method can therefore mutate that shared object through its copied reference, but reassigning the parameter to a different object does not change the caller's variable. The phrase pass-by-reference is inaccurate because the method cannot replace the caller's variable itself.

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Precise parameter semantics and the ability to explain why object mutation can be visible even though references are passed by value.

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Core answer
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This is a persistent source of confusion because the value being copied for an object variable is itself a reference.
Java is always pass-by-value. A method receives a copy of the value stored in each argument variable. For a primitive, that copied value is the primitive value. For an object variable, the copied value is a reference to the same object. The method can therefore mutate that shared object through its copied reference, but reassigning the parameter to a different object does not change the caller's variable. The phrase pass-by-reference is inaccurate because the method cannot replace the caller's variable itself.
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Stronger answer
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If update(order) calls order.confirm(), the caller can observe the changed order because both references identify the same object. If the method instead executes , only its local parameter now refers to the new object. The caller still holds its original reference. This distinction also explains why swapping two object parameters inside a method does not swap the caller's variables. APIs can reduce accidental shared mutation by using immutable values or returning a new result explicitly.

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Details

Saying primitives are passed by value but objects are passed by reference.
Using visible object mutation as proof of pass-by-reference.
Claiming a method can replace the caller's variable.
Confusing a copied reference with a copied object.

Is Java pass-by-reference or pass-by-value?

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