Question

When the Java compiler autoboxes a primitive to the wrapper class, what code does it generate behind the scenes? I imagine it calls:

• The valueOf() method on the wrapper
• The wrapper’s constructor
• Some other magic?

Answer 1

You can use the javap tool to see for yourself. Compile the following code:

public class AutoboxingTest {     public static void main(String []args)     {         Integer a = 3;         int b = a;     } } 

To compile and disassemble:

javac AutoboxingTest.java javap -c AutoboxingTest 

The output is:

Compiled from 'AutoboxingTest.java' public class AutoboxingTest extends java.lang.Object{ public AutoboxingTest();   Code:    0:   aload_0    1:   invokespecial   #1; //Method java/lang/Object.'<init>':()V    4:   return  public static void main(java.lang.String[]);   Code:    0:   iconst_3    1:   invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;    4:   astore_1    5:   aload_1    6:   invokevirtual   #3; //Method java/lang/Integer.intValue:()I    9:   istore_2    10:  return  } 

Thus, as you can see, autoboxing invokes the static method Integer.valueOf(), and autounboxing invokes intValue() on the given Integer object. There’s nothing else, really – it’s just syntactic sugar.