Java Tutorial/Data Type/Autobox Unbox

Autoboxing and Auto-Unboxing

1. Related to Java"s type wrappers.
2. How values are moved into and out of a wrapper instance
3. Java primitive type wrappers: Boolean, Byte, Character, Double, Float, Long, Integer, Short.

Autoboxing/unboxing: an argument passed to a method or returned from a method

Autoboxing/unboxing takes place with method parameters and return values.

public class MainClass {
  static int m(Integer v) {
    return v ; // auto-unbox to int
  } 
  public static void main(String args[]) {
    Integer iOb = m(100);
    System.out.println(iOb);
  }
}

100

Autoboxing/Unboxing Boolean and Character Values

public class MainClass {
  public static void main(String args[]) {
    Boolean booleanObject = true;
    if (booleanObject){
      System.out.println("b is true");
    }
    
    Character ch = "x"; // box a char
    char ch2 = ch; // unbox a char
    System.out.println("ch2 is " + ch2);
  }
}

b is true
ch2 is x

Autoboxing/unboxing occurs inside expressions

public class MainClass {
  public static void main(String args[]) {
    Integer intObject, intObject2;
    int i;
    intObject = 100;
    System.out.println("Original value of iOb: " + intObject);
    ++intObject;
    System.out.println("After ++iOb: " + intObject);
    intObject2 = intObject + (intObject / 3);
    System.out.println("iOb2 after expression: " + intObject2);
    i = intObject + (intObject / 3);
    System.out.println("i after expression: " + i);
  }
}

Original value of iOb: 100
After ++iOb: 101
iOb2 after expression: 134
i after expression: 134

Autoboxing/unboxing takes place with method parameters and return values.

class AutoBox2 {
  static int m(Integer v) {
    return v; // auto-unbox to int
  }
  public static void main(String args[]) {
    Integer iOb = m(100);
    System.out.println(iOb);
  }
}

Auto-unboxing allows you to mix different types of numeric objects in an expression.

class AutoBox4 {
  public static void main(String args[]) {
    Integer iOb = 100;
    Double dOb = 98.6;
    dOb = dOb + iOb;
    System.out.println("dOb after expression: " + dOb);
  }
}

Auto-unboxing: mix different types of numeric objects in an expression.

public class MainClass {
  public static void main(String args[]) {
    Integer intObject = 100;;
    Double doubleObject = 98.6;

    doubleObject = doubleObject + intObject;
    System.out.println("dOb after expression: " + doubleObject);
  }
}

dOb after expression: 198.6

Boxing and Unboxing

1. Boxing refers to the conversion of a primitive to a corresponding wrapper instance, such as from an int to a java.lang.Integer.
2. Unboxing is the conversion of a wrapper instance to a primitive type, such as from Byte to byte.

public class MainClass{
  public static void main(String[] args){
     
     Integer number = new Integer (100);
     int [] ints = new int [2];
     ints [0] = number;
  }
}

Manually boxes the value 100 into an Integer

public class MainClass {
  public static void main(String args[]) {
    Integer iOb = new Integer(100);
    int i = iOb.intValue();
    System.out.println(i);
  }
}

100

The modern way to construct an Integer object that has the value 100

public class MainClass {
  public static void main(String args[]) {
    Integer iOb = 100; // autobox an int
    System.out.println(iOb);
  }
}

100

To unbox an object

Simply assign that object reference to a variable of its corresponding primitive type

public class MainClass {
  public static void main(String args[]) {
    Integer iOb = 100; // autobox an int
    int i = iOb; // auto-unbox
    System.out.println(i + " " + iOb); 
  }
}

Type conversion (JDK1.5 Autoboxing/Unboxing)

public class Main {
  public static void main(String... args) {
    Integer integer = 1; // int into Integer
    System.out.println(integer);
    int i = integer + 3; // mix Integer and ints
    System.out.println(i);
  }
}

Using an integer object to control a switch statement

public class MainClass {
  public static void main(String args[]) {
    Integer intObject = 2;
    switch(intObject) {
      case 1: System.out.println("one");
        break;
      case 2: System.out.println("two");
        break;
      default: System.out.println("error");
    }
  }
}

two