2.13 Passing and returning objects

Learning Objectives

•Explain what it means that Java passes object references "by value".
•Predict whether a method call can observably modify a caller's object.
•Write a method that returns an object.

Introduction

Java passes all arguments by value — but for objects, the "value" being copied is the reference, not the object's data. This means a method can modify the fields of an object passed to it (both the caller and the method see the same object), but reassigning the parameter itself inside the method does not affect the caller's reference. Methods can also return objects, handing a reference back to the caller.

Key Concepts

Passing an object argument copies the reference — the method and caller point to the same object.

Mutating fields through that reference inside the method is visible to the caller after the call returns.

Reassigning the parameter itself inside the method only changes the local copy of the reference, not the caller's variable.

Methods declare a return type and use `return` to hand an object back to the caller.

Java Syntax

Code Examples

`mutate` changes a field on the shared object, so the caller sees the update. `reassign` only points the local `s` parameter at a new object — `ama` in `main` is untouched.

Common Mistakes

⚠Assuming reassigning a parameter inside a method (`s = new Student();`) changes what the caller's variable points to — it doesn't.
⚠Confusing "pass by reference" (which Java does not do) with "pass the value of a reference" (which is what actually happens).

Summary

Object arguments are passed by copying their reference: field mutations through that reference are visible to the caller, but reassigning the parameter itself is not.

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