1.14 Calling Instance Methods

What you need to know

• Instance methods are called on an object: obj.method(args). The method operates on that object's instance variables.
• An accessor (getter) returns information about the object's state without changing it: getName(), length().
• A mutator (setter) changes the object's state; typically void: setAge(5), deposit(100).
• Method calls can be chained if each returns an object: s.substring(1).toUpperCase().
• The dot operator on a null reference throws NullPointerException. The dot operator on a primitive doesn't compile (int has no methods).
• Calling a method with the wrong number or types of arguments, or calling a method the class doesn't have, is a compile-time error.

Worked example

Account acct = new Account("Ram", 100.0);
acct.deposit(50.0);                 // mutator: changes balance
double b = acct.getBalance();       // accessor: 150.0
String who = acct.getOwner();       // accessor: "Ram"
acct.getBalance();                  // legal but pointless: value discarded

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• Instance methods are called on an object and have access to that object's instance variables. acct.deposit(50) changes that account's balance. Calling it on a different account changes a different balance.
• Accessor (getter): returns state, doesn't change it. Convention: getX(), or isX() for booleans. Mutator (setter): changes state, usually void. Convention: setX(value), or verbs like deposit, increment.
• A method that both changes state and returns something exists (ArrayList's remove does), but the exam's own classes mostly follow the accessor/mutator split cleanly.
• Method chaining works when a method returns an object: s.substring(2).toUpperCase().length(). Read left to right: each call operates on what the previous one returned.
• The dot operator requires a reference to an object. On a primitive (int x; x.foo()) it's a compile error. On null it's a run-time error. On a valid reference to a class without that method, compile error.
• Calling an accessor and ignoring the result is legal and useless: acct.getBalance(); on its own line does nothing observable.

Mistakes that cost points

• Calling a mutator and expecting a return value. double b = acct.deposit(50); where deposit is void — compile error.
• Calling a method on a primitive. int n = 5; n.toString() — compile error. Primitives have no methods.
• Not tracking state across calls. Two mutator calls in a row compound. Update the object's state in your table after each.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.