Constructors

A new plant should get its name and height the moment it's created. That's the job of a constructor: code in the class that runs when an object is built with new, and gives the instance variables their first values.

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

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

    public Plant(String name, int height) {
        this.name = name;
        this.height = height;
        System.out.println("Planted a " + this.name + ", " + this.height + " cm");
    }
}

Start
Planted a fern, 30 cm
Planted a cactus, 12 cm
End

(Constructors don't usually print anything. This one does, so that you can see when it runs: once for each new.)

• A constructor is written like a method, but it has the class's name and no return type, not even void: public Plant(String name, int height).
• Its parameters work like a method's. new Plant("fern", 30) gives "fern" to name and 30 to height, in that order.
• this means the object being built. this.name is the object's instance variable name, while name on its own is the parameter. So this.name = name; copies the parameter's value into the object. A parameter with the same name as the instance variable it fills is the usual way to write a constructor; module 18 looks at this more closely.
• The parameters disappear when the constructor ends, but the values stored in the instance variables stay with the object for as long as it exists.

Not every instance variable needs a parameter. The Lamp in step 1 is built with only its place, and its constructor starts every lamp off with this.on = false;.

When the values don't fit

A class without any constructor gets the default constructor from step 2, which takes no values, so it's built with empty parentheses. Once Plant has a constructor of its own that takes a name and a height, Java no longer adds the default one, and every new Plant must give both values, just as a method call must fit the method's parameters:

public class Main {
    public static void main(String[] args) {
        Plant basil = new Plant();
    }
}

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

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

Main.java:3: error: constructor Plant in class Plant cannot be applied to given types;
        Plant basil = new Plant();
                      ^
  required: String,int
  found:    no arguments
  reason: actual and formal argument lists differ in length
1 error

required lists the constructor's parameters, and found what the new gave. With the values the wrong way round, as in new Plant(30, "basil"), javac says incompatible types: int cannot be converted to String.