1.14 Calling Instance Methods

1. Calling instance methods with the dot operator

Instance methods are called on objects of the class. The dot operator is used with the object name to call an instance method 1.14.A.1. The method acts on that one object.

This continues the topic 1.13 example: p refers to a Plant named "Fern" with height 30, and q refers to a Plant named "Mint" with height 0.

Plant p = new Plant("Fern", 30);
Plant q = new Plant("Mint");
p.grow(5);                         // void: a statement on its own
System.out.println(p.getHeight()); // 35
System.out.println(q.getName() + " " + q.getHeight()); // Mint 0

Output:

35
Mint 0

p.grow(5) changed only the object p refers to. q still has height 0.

An instance method needs an object, so calling it with the class name does not compile:

// does not compile
Plant.grow(5);

Real view:

error: non-static method grow(int) cannot be referenced from a static context

2. NullPointerException

A method call on a null reference results in a NullPointerException 1.14.A.2. The code compiles, but the program stops when the call runs.

Plant r = null;                    // r refers to no object
System.out.println("Checking r");
System.out.println(r.getHeight());
System.out.println("Done");

Output:

Checking r
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "Plant.getHeight()" because "r" is null

Done is never printed: the exception interrupts the normal flow of the program (topic 1.1). The exact wording after the colon can differ between Java versions; the exception name is what matters.

Watch out: printing a null variable, as in System.out.println(r);, is fine and prints null. Only calling a method on it throws the exception.

3. Aliases: two names, one object

After Plant b = a;, both variables refer to the SAME object. Calling an instance method through either name changes that one object.

Plant a = new Plant("Basil", 10);
Plant b = a;
b.grow(5);
a.grow(3);
System.out.println(a.getHeight() + " " + b.getHeight());

Output:

18 18

10 + 5 + 3 is 18, and both names report it, because there is only one Plant. (Unit 3 studies aliasing in more depth.)

4. Looking ahead to Unit 3: inside a non-void method

Inside a class, a method’s statements can use the object’s attributes directly, and a return statement sends one value back to the caller and ends the method. The value must match the return type in the header. This is what the free-response practice for this unit asks you to write.

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

    public Plant(String n, int h)
    {
        name = n;
        height = h;
    }

    // inside the Plant class
    public String getInitial()
    {
        return name.substring(0, 1);
    }
}

// in main
Plant p = new Plant("Fern", 30);
String first = p.getInitial();
System.out.println(first);

Output:

F

If p refers to a Plant named "Fern", p.getInitial() returns "F". A method returns a value; it does not print it. The caller decides what to do with the value.