1.9 Method Signatures

1. Exam Points

• Declare and call instance methods.
• Pass arguments(实际参数) into parameters(形式参数).
• The arguments must match the parameters of a method.
• Call instance methods(non-static) using an instance name.
• One instance method can call another method from the same class directly.
• Store the value returned from a method in a variable of the right type.
• A method must a have return type (either void or any type).
• Only constructors can have no return type.
• If a method has a non-void return type, a value must be returned.

2. Knowledge Points

(1) Methods(方法)

• A method is a named block of code enclosed in braces {} that only runs when it is called.
• Declaring a method is the process of procedural abstraction.
• Procedural abstraction allows a programmer to use a method by knowing what the method does even if they do not know how the method was written.

(2) Declare Methods (声明方法)

• Syntax:

  public [static] returnType methodName(parameter list){ //method header
      // method body: code to execute in the method
  }
  

  • In AP CSA, just declare a method as public
  • static is optional, it is used when you define a class method.
  • methodName is the name of the method, use a meaningful name.
  • parameter(参数) list can contain one or multiple parameters, you should specify the data type and name of each parameter.

• A method signature for a method with parameters consists of the method name and the ordered list of parameter types.

  • Example: public void add(int x, int y){}
  • Method signature: add(int x, int y)

• Examples:

  • A method with no parameter
    • declare a method: public void welcome(){}
    • call a method: welcome();
  • A method with parameters
    • declare a method:
      • public void add(int x, int y){}
      • public void play(String name){}
    • call a method:
      • add(10,20);
      • play("LOL");
  • A method with a return type
    • declare a method: public int square(int x){ return x*x; }
    • call a method: int x = square(4);

• About the return statement:

  • A method's return value is a specific value returned by the method.
  • A return value is obtained through a return statement within a method. Ex. rerurn 3;
  • A method can have more than one return statement, but only one return statement is executed, the first matching return is executed.
  • A method's return value must be of a specific type, the value returned must match the declared return type of a method.
  • For methods without a return value, the "void" keyword should be used to declare the method as "untyped" (or "empty type").
  • A return statement also returns the execution back to where the method is called.
  • Statements after a return statement are not reachable.
  • Example 1:
    
  • Example 2:
    
  • Example 3:
    

(3) Parameters and Arguments

• A parameter (形式参数) is a variable declared in the header of a method or constructor and can be used inside the body of the method.
• An argument (实际参数) is a value that is passed into a method when the method is called.
• The arguments passed to a method must be compatible in number and order with the types identified in the parameter list of the method signature.
• Example:
  • declare a method: public void sum(int x, int y){}
  • call a method: sum(10,20);
  • In this example:
    • x, y are parameters
    • 10, 20 are arguments

(4) Method Overloading(方法重载)

• Methods are said to be overloaded when there are multiple methods with the same name but different signatures (i.e. different parameter list).

(5) Constructors(构造函数)

• A constructor is a special method inside a class.
• A class contains constructors that are called to create objects.
• Example:
  • Student s1 = new Student();
  • Student s2 = new Student("xiaoming");
• A constructor has the following features:
  • has the same name as the class in which it is declared.
  • has no return type.
  • is called when an object is created using the new keyword.
  • can be overloaded, meaning there can be multiple constructors with different signatures inside a class.
  • purpose of a constructor: to initialize instance variables or do other initialization work.
• Example:
  

3. Exercises

• Practice
• Solution