Step 1 of 8

An object as a parameter

So far, the methods of your classes have taken numbers and Strings as parameters. A parameter can also be an object of one of your own classes, and then two objects can work together. Here a printer prints documents, and each document knows its own title and number of pages:

Main.java

public class Main {
    public static void main(String[] args) {
        Printer printer = new Printer(10);
        Document letter = new Document("Letter", 2);
        Document report = new Document("Report", 12);
        printer.print(letter);
        printer.print(report);
        printer.print(new Document("Poster", 1));
        System.out.println(printer);
    }
}

Printer.java

public class Printer {
    private int paper;
    private int printed;

    public Printer(int paper) {
        this.paper = paper;
        this.printed = 0;
    }

    public void print(Document document) {
        if (document.getPages() <= this.paper) {
            this.paper = this.paper - document.getPages();
            this.printed++;
            System.out.println("Printed " + document.getTitle());
        } else {
            System.out.println("Not enough paper for " + document.getTitle());
        }
    }

    public String toString() {
        return this.printed + " documents printed, " + this.paper + " sheets left";
    }
}

Document.java

public class Document {
    private String title;
    private int pages;

    public Document(String title, int pages) {
        this.title = title;
        this.pages = pages;
    }

    public String getTitle() {
        return this.title;
    }

    public int getPages() {
        return this.pages;
    }
}

Output

Printed Letter
Not enough paper for Report
Printed Poster
2 documents printed, 7 sheets left

The parameter's type is the name of the class: public void print(Document document). The method takes a Document and nothing else: printer.print("Letter") doesn't compile.
Inside print, document is the Document it was given. The printer uses it through its public methods, getTitle() and getPages(), just as main would. Writing document.pages in Printer doesn't compile, because pages is private to the Document class.
The call can give an object that's in a variable, as in printer.print(letter), or a new one made right in the call: printer.print(new Document("Poster", 1)).
this.paper is the printer's own variable, and document.getPages() asks the document. Each object keeps its own data, and the method brings the two together.

The object itself, not a copy

As module 22 showed, a parameter of a class type holds a reference to an object, just like any other variable of that type: printer.print(letter) gives the method a reference to the very object in letter, not a copy of it. So when a method changes the object it was given, the change is still there after the method ends:

Main.java

public class Main {
    public static void main(String[] args) {
        Phone phone = new Phone(15);
        Charger charger = new Charger(2);
        System.out.println(phone);
        charger.charge(phone, 30);
        System.out.println(phone);
        charger.charge(phone, 60);
        System.out.println(phone);
    }
}

Charger.java

public class Charger {
    private int percentPerMinute;

    public Charger(int percentPerMinute) {
        this.percentPerMinute = percentPerMinute;
    }

    public void charge(Phone phone, int minutes) {
        phone.addBattery(minutes * this.percentPerMinute);
    }
}

Phone.java

public class Phone {
    private int battery;

    public Phone(int battery) {
        this.battery = battery;
    }

    public void addBattery(int percent) {
        this.battery = this.battery + percent;
        if (this.battery > 100) {
            this.battery = 100;
        }
    }

    public String toString() {
        return "Battery " + this.battery + "%";
    }
}

Output

Battery 15%
Battery 75%
Battery 100%

charge changes the phone through the phone's own method, addBattery, and main sees the new level in phone: the variable in main and the parameter refer to the same Phone. The charger can't set battery directly. Only Phone's own code can, and it makes sure the battery never goes over 100%.

An object as a parameter

The object itself, not a copy

Your task

Challenge 2

Challenge 3