3.5 Methods: How to Write Them

1. Exam Points

• Declare and call methods.
• Implement encapsulation (getter and setter methods)
• declare instance variables as private to hide them
• add accessor methods (getters) to obtain the value of instance variables.
  • Ex: public int getAge() { return age; }
  • has a return type and no parameter, so a value must be returned.
• add mutator methods (setters) to update the value of instance variables.
  • Ex: public void setAge(int a) { age = a; }
  • has a parameter and no return type

• Pass parameters
  • For a primitive type parameter, changes to the parameter have no effect on the corresponding argument.
  • For a reference type parameter, changes to the parameter will reflect on the corresponding argument.
  • For a String type parameter, changes to the parameter will not reflect on the argument. String is an immutable class.
• Note
  • If a method has a return type of non-void, a value must be returned.
  • If a method has a return type of void, do not return a value.
  • Only a method with a return type of non-void can be used in print() or println().

2. Knowledge Points

(1) Understand Methods

• If you want to reuse a code segment, make it a named method.
• To use a method, you should declare a method and then call it.
• Each part of a method:
  • method header: public returnType methodName(parameter list)
  • method body: {} and the code inside {}
• Example:
  
  • In this example:
    • method header: public int sum(int a, int b)
    • method body: {} and the code inside {}
    • method signature: sum(int a, int b)
    • a, b: parameters(形式参数)
    • 3, 4: arguments(实际参数)

(2) Write Methods

• A method with no parameter
• A method with parameters
• A method with a return type
  • Note: For a method with a return type of void, do not use it in the 3 ways above as a method with a return type.

(3) The return Keyword

• The return keyword is used to return the flow of control to the point where the method or constructor was called.
• Purpose of a return statement:
  • return a value
  • return back the flow of control
• Example:
  • order of statement execution: 1,2,3 -> 7,8 -> 4,5,6
  • output: 12ab034
• Any code that is sequentially after a return statement will never be executed.
  
• Executing a return statement inside a selection or iteration statement will halt the statement and exit the method or constructor.
  • In this example, the value of result would be 2.

• Note:
  • If a method has a return type of non-void, a value must be returned.
  • If a method has a return type of void, do not return a value.
  • Only a method with a return type of non-void can be used in print() or println().

(4) Accessor(访问器) and Mutator(更改器) methods

• An accessor method(getter) allows objects of other classes to obtain a copy of the value of instance variables or class variables. (read/get value).
• An accessor method:
  • has a name of getAttributeName
  • has a return type of the attriburte retrieved
  • has no parameters.
  • Example:
    
• A mutator method(setter) is a method that changes the values of the instance variables or class variables. (update/change value)
• A mutator method:
  • has a name of setAttributeName
  • has a return type of void
  • has a parameter of the same type as the attribute
  • Example:
    

(5) Pass Arguments to Parameters

• For a primitive type parameter, changes to the parameter have no effect on the corresponding argument.
  
• For a reference type parameter, changes to the parameter will reflect on the corresponding argument.
  
• Among reference types, String is an exception, because String is an immutable class.
  

3. Exercises

• Practice
• Solution