Calling Instance Methods
Introduced| English |
|---|
| instance method/ˈɪnstəns ˈmeθəd/ |
Calling on an object
- An instance method 实例方法 is called on a specific object, using dot notation.
word.length()callslength()on the objectword.- 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()returns3;"hello".length()returns5— 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.
Arguments and return values
- Instance methods can take arguments and return a value, just like class methods.
"hello".substring(1, 3)passes1and3, 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.
An instance method needs an object before the dot: word.length(), not String.length(). The object before the dot supplies the state the method acts on, so the same call gives different results on different objects. Class methods (Math.sqrt) use a class name instead — knowing which is which tells you whether you need an object first.
Same method, different objects:
String a = "cat", b = "tiger";a.length()→3(acts on"cat").b.length()→5(acts on"tiger") — the object before the dot decides.
An instance method is called on an object with dot notation (word.length()); the object before the dot supplies the state the method works with. It can take arguments and return a value like any method. This contrasts with a class (static) method, called on a class name (Math.sqrt(16)).
An instance method acts on its object
length() reads the state of the object before the dot.
Which is a correct call to the instance method length on a String word?
Instance methods are called on an object: object.method().
What does "tiger".length() return?
"tiger" has 5 characters.
Math.sqrt(16) is a class method and word.length() is an instance method. The difference is...
Class name before the dot → class method; object → instance method.
The same instance-method call can give different results on different objects.
The object before the dot supplies the state.
An instance method is called on a specific ___ using dot notation (one word).
object.method() — the object supplies the context.