Practice›Java Fundamentals›Module 4. Classes & Objects›4.3 Instance methods

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4.3 Instance methods — passing objects to methods

Lesson

Что метод может изменить, а что нет

Java always passes a copy. For a number that means the method works on its own copy, and the original is untouched. For an object the reference is copied — and a copy of the address still leads to the same object. So the method can change the object's fields, and the caller sees it.

Worked example

static void bump(int n) { n = n + 100; }        // original unchanged
static void rename(Dog d) { d.name = "changed"; } // field DOES change
static void replace(Dog d) { d = new Dog("new"); } // caller sees nothing

Compare the last two. rename reaches through the reference and edits the object — visible outside. replace points the local copy at a different object — the caller's variable still points at the old one.

Rule of thumb: a method can change what the object contains, but cannot change which object the caller's variable points at.

▶ Try it: run this, then change something and run it again.

class Dog {
    String name;

    Dog(String name) {
        this.name = name;
    }
}

public class Main {

    static void bump(int n) {
        n = n + 100;
    }

    static void rename(Dog d) {
        d.name = "changed";
    }

    static void replace(Dog d) {
        d = new Dog("new object");
    }

    public static void main(String[] args) {
        int n = 5;
        bump(n);
        System.out.println(n);
        Dog d = new Dog("Rex");
        rename(d);
        System.out.println(d.name);
        replace(d);
        System.out.println(d.name);
    }
}

A method changes a field of an object it was given. The caller...

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A method assigns a brand-new object to its parameter. The caller...

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Type exactly what this program prints.

class Dog {
    String name;

    Dog(String name) {
        this.name = name;
    }
}

public class Main {

    static void rename(Dog d) {
        d.name = "Bo";
    }

    public static void main(String[] a){
        Dog d = new Dog("Rex");
        rename(d);
        System.out.println(d.name);
    }
}

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Why is a number unchanged but an object field changed?

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True or false?

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