Do you understand automatic packing and unpacking? What is the principle?

Automatic packing and unpacking understand ？ What is the principle ？

What is an automatic box ？

• Packing ： Wrap basic types with their corresponding reference types ;
• Unpacking ： Convert wrapper type to base data type ;

give an example ：

Integer i = 10;  // Packing 
int n = i;   // Unpacking 

Bytecode file

  L1

    LINENUMBER 8 L1

    ALOAD 0

    BIPUSH 10

    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;

    PUTFIELD AutoBoxTest.i : Ljava/lang/Integer;

   L2

    LINENUMBER 9 L2

    ALOAD 0

    ALOAD 0

    GETFIELD AutoBoxTest.i : Ljava/lang/Integer;

    INVOKEVIRTUAL java/lang/Integer.intValue ()I

    PUTFIELD AutoBoxTest.n : I

    RETURN

From bytecode , We found that boxing is actually calling The wrapper class valueOf() Method , Unpacking is actually calling xxxValue() Method .

therefore ,

• Integer i = 10 Equivalent to Integer i = Integer.valueOf(10)
• int n = i Equivalent to int n = i.intValue();

Be careful ： If the box is disassembled frequently , It will also seriously affect the performance of the system . We should try to avoid unnecessary disassembly and assembly of the box

private static long sum() {
    
    //  You should use  long  instead of  Long
    Long sum = 0L;
    for (long i = 0; i <= Integer.MAX_VALUE; i++)
        sum += i;
    return sum;
}

Reference resources