Calling Instance Methods

Calling on an object

• An instance method 实例方法 is called on a specific object, using dot notation.
• word.length() calls length() on the object word.
• The object before the dot is the one the method acts on.
• Unlike a class method, you need a real object to call it.

The object is the context

• Inside an instance method, the code works with that object's state.
• "cat".length() returns 3; "hello".length() returns 5 — same method, different objects.
• The method reads (or changes) the state of the object it was called on.
• Two objects of one class can give different results from the same call.

One class, many objects: each instance has its own field values

Arguments and return values

• Instance methods can take arguments and return a value, just like class methods.
• "hello".substring(1, 3) passes 1 and 3, returns "el".
• Capture the returned value: String part = "hello".substring(1, 3);
• The dot's object plus the arguments together decide the result.

Instance vs. class, side by side

• Class method: Math.sqrt(16) — dot after a class name.
• Instance method: word.length() — dot after an object.
• If it needs a specific object's data, it's an instance method.
• If it's a general utility with no object, it's a class (static) method.