Java Class Methods

A class's methods are what give its objects behavior. Fields hold what an object is; methods describe what it can do. Up to now the methods you've written have been static — utilities that belong to the class itself. This chapter is about instance methods, which belong to each object and operate on its fields.

The term "class methods" in Java is a little overloaded. Most authors use it to mean methods defined inside a class — which includes both kinds. A few use it strictly to mean static methods (because those literally belong to the class). We'll cover instance methods here and static methods get a chapter of their own at java-static.

An instance method

Add a method to the class body, without static, and it becomes part of every instance:

public class Circle {
  double radius;

  double area() {
    return Math.PI * radius * radius;
  }
}

Now every Circle you create can answer area():

Circle c = new Circle();
c.radius = 5;
System.out.println(c.area());   // 78.539...

The body reads radius without qualifying it — Java resolves the bare name as this.radius, the field of the object the method was called on.

this — the receiver

Every instance method has an invisible parameter called this, which refers to the object the method was called on:

Circle a = new Circle();   a.radius = 2;
Circle b = new Circle();   b.radius = 5;

a.area();    // inside area(), this == a, so this.radius == 2
b.area();    // inside area(), this == b, so this.radius == 5

The same compiled method runs in both cases; what changes is which object this refers to. The this keyword chapter goes into when you have to write this. explicitly and when you can omit it.

Methods that change state

A method can write to fields too. Such methods are often called mutators:

public class Counter {
  int count;

  void increment() {
    count++;            // mutates this.count
  }
  void reset() {
    count = 0;
  }
  int get() {
    return count;
  }
}

Counter c = new Counter();
c.increment(); c.increment(); c.increment();
System.out.println(c.get());   // 3

Mutation through methods is how objects evolve over time without outside code touching their fields directly.

Calling other methods from inside

Instance methods can call other methods on the same object, just by name:

public class Rectangle {
  double width, height;

  double area()      { return width * height; }
  double perimeter() { return 2 * (width + height); }

  String describe() {
    return "area=" + area() + ", perimeter=" + perimeter();
  }
}

The compiler turns area() and perimeter() inside describe into this.area() and this.perimeter(). They call the methods on whichever rectangle describe was called on.

Static vs instance — which to use

The rule of thumb: does the method depend on per-object state?

• If yes, it's an instance method (no static).
• If no, it's static.

public class Math2 {
  public static int squared(int n) {       // pure calculation — static
    return n * n;
  }
}

public class Counter {
  int count;
  public void increment() {                // reads/writes this.count — instance
    count++;
  }
}

A common smell is an instance method that ignores this entirely — that's a static method wearing a costume. The static chapter goes through the trade-offs.

Calling an instance method needs an instance

Because instance methods read this, you can't call one without an object:

Counter.increment();      // ERROR — increment is not static
new Counter().increment(); // fine — increment is called on a fresh Counter (which is then thrown away)

Counter c = new Counter();
c.increment();             // fine — increment is called on c

This is the most common compile error newcomers hit: a non-static method referenced from a static context (typically from main, which is itself static).

Overloading and overriding

Instance methods support both:

• Overloading: multiple methods with the same name but different parameter lists in the same class (covered in method overloading).
• Overriding: a subclass replacing the inherited version of a method (covered in method overriding).

You've already seen overloading in Part 5. Overriding is one of the big payoffs of inheritance and comes up shortly.

Method bodies are blocks

A method body is just an ordinary block of code — variables you declare are local to the call, control flow works the same as anywhere else, and you can return early. Nothing about "this is in a class" changes how the body itself reads. The only extra capability is that bare identifiers can now refer to the object's fields and other methods.

A worked example

What's next

So far you've created objects by writing new Dog(), then filling in fields line by line. That's verbose and error-prone — and easy to forget. The fix is a constructor, a special method that runs when the object is created. Continue to the constructors chapter.

Practice