4.7 Wrapper Classes

1. Exam Points

• If a method has a parameter of int or Integer, you can pass into an int value or an Integer value.
• If a method has a parameter of double or Double, you can pass into a double value or a Double value.
• Autoboxing is done automatically: int -> Integer, double -> Double
• Unboxing is done automatically: Integer -> int, Double -> double
• String to int or double:
  • Integer.parseInt(String x): String to int
  • Double.parseDouble(String x): String to double
• You can perform arithmetic operations on wrapper objects.

2. Knowledge Points

(1) Wrapper Classes

• You can not create objects of primitive data types (int, double, boolean).
  
• To convert primitive values to objects, Java provides wrapper classes.
• The wrapper classes are from the java.lang package.
• Wrapper classes for primitive data types:
  • int -> Integer
  • double -> Double
  • boolean -> Boolean

(2) Autoboxing and Unboxing

• Autoboxing (自动装箱) is the automatic conversion from primitive types to the corresponding object wrapper classes. (primitive types -> wrapper classes)

  • Ex. int to Integer, double to Double, boolean to Boolean.
  • Example:
    

    int a = 10; // primitive
    Integer b = a; // int -> Integer
    
    double d = 2.3;
    Double e = d; // double -> Double
    

  • The Java compiler applies autoboxing when a primitive value is:
    • passed as a parameter to a method that expects an object of the corresponding wrapper class.
      

    public static void printSomething(Integer x) {
                System.out.println(x);
    }
    
    public static void main(String[] args) {
        int a = 10;
        printSomething(a); // int(a) -> Integer(x);
    }
    

    • assigned to a variable of the corresponding wrapper class.
      

    int a = 10; // primitive
    Integer b = a; // int -> Integer
    

  • You can perform arithmetic operations on Wrapper objects:
    

    double x = 10.0;
    Double y = 20.0;
    Double z = 30.0;
    System.out.println(x + y + z); // 60.0 is printed
    

  • In summary:
    • use an Integer value like an int value, like in arithmetic operations.
    • if a parameter is of int type, you can pass an argument of either int or Integer.

• Unboxing (拆箱) is the automatic conversion from a wrapper class to its primitive type. (wrapper classes -> primitive types)

  • Ex. Integer to int, Double to double, Boolean to boolean
  • Example:
    

  Integer a = new Integer(10);
  int b = a;  //Integer -> int
  
  Double a = new Double(2.3);
  double b = a; //Double -> double
  

  • The Java compiler applies unboxing when a wrapper class object is:
    • passed as a parameter to a method that expects a value of the corresponding primitive type.

    public static void printSomething(int x) {
                System.out.println(x);
    }
    
    public static void main(String[] args) {
        Integer a = new Integer(10);
        printSomething(a); // Integer(a) -> int(x);
    }
    

    • assigned to a variable of the corresponding primitive type.

    Integer a = new Integer(10);
    int b = a;  //Integer -> int
    

• If a parameter is of Integer type, you can pass an argument of either int or Integer.

(3) Convert String Values

• static int parseInt(String s) : a method of the Integer class, and is used to convert a String value to an int value.
  • Example:
    

  String a = "123";
  String b = "abc";
  	
  // parseInt() is a static method, call it by the class name Integer
  int x = Integer.parseInt(a); // String -> int
  int y = Integer.parseInt(b); // error, abc can not be converted to an int value
  

• static double parseDouble(String s) : a method of the Double class, and is used to convert a String value to a double value.
  • Example:
    

  String a = "23.6";
  // parseDouble() is a static method, call it by the class name Double
  double x = Double.parseDouble(a);
  

3. Exercises

• Practice
• Solution