Methods of an object

A Plant can be built now, but it can't do anything. What an object can do is given by its methods, written in the class after the constructor. Here's one that prints the plant's details:

public class Main {
    public static void main(String[] args) {
        Plant fern = new Plant("fern", 30);
        Plant cactus = new Plant("cactus", 12);
        fern.printInfo();
        cactus.printInfo();
        fern.printInfo();
    }
}

public class Plant {
    private String name;
    private int height;

    public Plant(String name, int height) {
        this.name = name;
        this.height = height;
    }

    public void printInfo() {
        System.out.println(this.name + ", " + this.height + " cm");
    }
}

fern, 30 cm
cactus, 12 cm
fern, 30 cm

• The method has no static. The methods in modules 10 and 11 were static: they belong to the class as a whole, and you call them on their own, as in printBanner();. A method without static belongs to each object, and you call it on an object: fern.printInfo();.
• Inside the method, this is the object the method was called on. In fern.printInfo(), this.name is "fern"; in cactus.printInfo(), it's "cactus". The method is written once, and every object runs it with its own variables.
• The method is public, so Main can call it, while the variables stay private. void means it returns nothing, as before.

A call needs an object in front of the dot. printInfo(); alone in main looks for a method of Main, and Plant.printInfo(); doesn't say which plant's info to print, so neither compiles.

Parameters and conditions

A method of an object can take parameters, like any method, and use them together with the object's own variables. printFits tells whether the plant fits under a shelf of a given height:

public class Main {
    public static void main(String[] args) {
        Plant fern = new Plant("fern", 30);
        Plant cactus = new Plant("cactus", 12);
        fern.printFits(25);
        cactus.printFits(25);
    }
}

public class Plant {
    private String name;
    private int height;

    public Plant(String name, int height) {
        this.name = name;
        this.height = height;
    }

    public void printInfo() {
        System.out.println(this.name + ", " + this.height + " cm");
    }

    public void printFits(int shelfHeight) {
        if (this.height < shelfHeight) {
            System.out.println("The " + this.name + " fits under the shelf");
        } else {
            System.out.println("The " + this.name + " is too tall for the shelf");
        }
    }
}

The fern is too tall for the shelf
The cactus fits under the shelf

shelfHeight comes from the call, and this.height from the object. A method of an object can hold anything a static method can: variables, conditions, loops and calls.