Copy Constructor in Java

intermediate· OOP · Constructors

Problem

A copy constructor takes another object of the same class and copies its field values into a brand-new object — Java doesn't provide one automatically the way some other languages do, so it has to be written by hand.

Create a Car class with a constructor that builds a new Car from an existing one's field values, independent of the original.

Input

new Car(original), then modify the copy's year

Output

Tesla Model 3 (2022)
Tesla Model 3 (2023)

Java Program

Java

class Car {
    String model;
    int year;

    Car(String model, int year) {
        this.model = model;
        this.year = year;
    }

    // Copy constructor: builds a new object from an existing one's field values
    Car(Car other) {
        this.model = other.model;
        this.year = other.year;
    }

    void display() {
        System.out.println(model + " (" + year + ")");
    }
}

public class CopyConstructorDemo {
    public static void main(String[] args) {
        Car original = new Car("Tesla Model 3", 2022);
        Car copy = new Car(original);
        copy.year = 2023; // changing the copy doesn't touch the original

        original.display();
        copy.display();
    }
}

▶ Run Code

Output

Tesla Model 3 (2022)
Tesla Model 3 (2023)

Core Logic

Reading another object's fields into a fresh object's fields, instead of sharing a reference to the original, produces a genuinely separate copy.

How It Works

1Car(Car other) takes an existing Car object instead of raw values.
2this.model = other.model; and this.year = other.year; copy each field's current value into the new object.
3new Car(original) builds copy as a distinct object holding the same values original had at that moment.
4Changing copy.year afterward only affects copy's own field — it doesn't reach back and change original.year, since they're two separate objects.

After copy.year = 2023;, original still prints "Tesla Model 3 (2022)" while copy prints "Tesla Model 3 (2023)", confirming they're independent.

💡

Key Point: Because String and int fields are copied by value here, the two objects share no state at all after construction — mutating one can never leak into the other.

Key Concepts

copy constructorobject independencefield copying

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