1.14 - Calling Instance Methods

Instance methods are called on actual objects of a class, using the dot operator with the object’s name.


Dog myDog = new Dog("Rex");
myDog.bark(); // calling an instance method on the object

You’ve already seen this with strings. length is an instance method, so you call it on a specific String object:


String school = "Dove";
int len = school.length(); // 4

Each object has its own attribute values, so calling the same method on two different objects can give different results. Say you’re given this class:


public class Counter {
    private int count;

    public Counter() {        // starts count at 0
        count = 0;
    }

    public void increment() { // adds 1 to count
        count++;
    }

    public int getCount() {   // returns count
        return count;
    }
}


Counter clicks = new Counter();
Counter taps = new Counter();
clicks.increment();
clicks.increment();
taps.increment();
System.out.println(clicks.getCount()); // 2
System.out.println(taps.getCount());   // 1

increment is a void method, so it’s called on its own line. getCount is non-void, so its return value gets used, here by printing it.

Compare this to class methods: an instance method needs an object in front of the dot (clicks.increment()), while a class method uses the class name (Math.sqrt(9.0)). Calling an instance method with just the class name, like Counter.increment(), won’t compile.

Careful: calling a method on a null reference throws a NullPointerException, since there’s no actual object there to call the method on.


Dog noDog = null;
noDog.bark(); // NullPointerException, nothing to call bark() on

This is a run-time error, not a compile error. The code compiles fine because noDog is a Dog variable, but when it runs there’s no object to call bark on, so the program crashes.

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