Automatic packing and unpacking

1. Packing

• Boxing: Basic Type--reference data type

int num = 20;integer number = num;  Automatic Boxing

The last statement above corresponds to the following: integer number = new integer (num);

• But here's the problem.

• Double d1=1.0;double D2=new Double (1.0);

  Double D2 = new double (d1) and integer number = new integer (num); Is it a concept?

• The Integer wrapper class implements the constant pool technique, so num 's is obtained from the constant pool
  

• a double does not implement constant pool technology, and instead of looking for data from a constant pool, it is a new object from the heap. That is, 1.0 is in the heap, not in a constant pool.

2. Unpacking

• Unboxing: Reference data Type-Basic type

Integer number = new integer, int num = number;  Automatic unpacking

The last statement above is equivalent to: int num = Number.intvalue ();

3. Example 1 3.1 Code

Package com.ilaoda.day0905;/** * Automatic packing  unpacking * @author Administrator * */public class Test3 {public static void main (Strin G[] args {integer in1 = new Integer, Integer in2 = new Integer, integer in3 = 100;integer in4 = 100; System.out.println ("in1 = = in2:" + (in1 = = in2));  FalseSystem.out.println ("in1 = = in3:" + (in1 = = in3));  FalseSystem.out.println ("in3 = = In4:" + (in3 = = in4));  True  automatically boxed, the value is within the byte range, so pointing to the same area/** * Why the comparison with the above in3, Int4 is true. But in5 = =  in6 will appear false? */integer In5 = 128;integer in6 = 128; System.out.println ("In5 = = in6:" + (In5 = = in6));  False}}

3.2 Answers

in1 = = In2:falsein1 = = In3:falsein3 = = In4:truein5 = In6:false

3.3 Explanation

Here's a little detail to note:
For an auto-boxing operation, if the value is assigned in the byte range, that is, -128~127, they point to the same heap space area. if the range of byte is exceeded, then each creates its own object, which is a new space to be re-created.

4. Example 24.1 Code

Package Com.ilaoda.day0905;public class Test4 {public      static void Main (string[] args) {              objpooltest ();      } Public        static void Objpooltest () {          int i = +;          int i0 = +;          Integer i1 = +;          Integer i2 = +;          Integer i3 = 0;          Integer i4 = new Integer (+);          Integer i5 = new Integer (+);          Integer I6 = new Integer (0);          Double d1=1.0;          Double d2=1.0;                    System.out.println ("I=i0:" + (i = = I0));         System.out.println ("I1=i2:" + (I1 = = I2));         System.out.println ("I1=i2+i3:" + (I1 = = i2 + i3));          System.out.println ("I4=i5:" + (I4 = = i5));         System.out.println ("I4=i5+i6:" + (I4 = = i5 + I6));         System.out.println ("D1=D2:" + (D1==D2));                  System.out.println ();       }  }  

4.2 Answers

I=i0:truei1=i2:truei1=i2+i3:truei4=i5:falsei4=i5+i6:trued1=d2:false

4.3 explanation

Automatic packing and unpacking