About Question enthuware.ocpjp.v21.2.3343 :

[n199a_]

Autoboxing

I encountered a very interesting question where it was required to identify examples of autoboxing:

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Long l = Long.valueOf(200)
Integer i = 10;
Integer getValue() { return 2; }
Long getValue() { return 2; }
System.out.println(2 + "");

Initially, I did not select the option

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System.out.println(2 + "");

, but the author gave the following explanation:

Whenever only one operand of the + operator is a String, the other operand is converted into a String using a string conversion. If the other operand is a numeric primitive type, then it is first converted to a reference type using the boxing conversion and then the boxed reference is used to produce a String. Thus, in this case, 2 will first be boxed into an Integer object and then the Integer object will be used to produce the String "2", which will then be concatenated with "". Hence, this is also a valid example where autoboxing occurs. Note that, no autoboxing occurs in System.out.println(2); because the println(int) method is invoked in this case.

This explanation means that

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2

is first boxed into an

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Integer

wrapper, and then the

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Integer

is converted into a

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String

.

To confirm or disprove this statement, we need to write code that contains all of the above cases, compile the class using the command:

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$ javac Test.java

Then disassemble it using the javap command (https://www.ibm.com/docs/de/i/7.3?topic=programs-javap):

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$ javap -c Test

Preparing the class with the answer options:

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public class Main {
    public static void main(String[] args) {
        Long l = Long.valueOf(200);
        Integer i = 10;
        System.out.println(2 + "string");

        // Not compile:
        // String s = (String) (1 + 2);
    }

    static Integer getInteger() {
        return 2;
    }

    static Long getLong() {
        // Not compile:
        // return 2;
        return 2L;
    }
}

Disassembler output:

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PS W:\testspace\untitled> javap -c W:\testspace\untitled\build\classes\java\main\l3\Main.class
Compiled from "Main.java"
public class l3.Main {
  public l3.Main();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc2_w        #7                  // long 200l
       3: invokestatic  #9                  // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
       6: astore_1
       7: bipush        10
       9: invokestatic  #15                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      12: astore_2
      13: getstatic     #20                 // Field java/lang/System.out:Ljava/io/PrintStream;
      16: ldc           #26                 // String 2string
      18: invokevirtual #28                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      21: return

  static java.lang.Integer getInteger();
    Code:
       0: iconst_2
       1: invokestatic  #15                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
       4: areturn

  static java.lang.Long getLong();
    Code:
       0: ldc2_w        #34                 // long 2l
       3: invokestatic  #9                  // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
       6: areturn
}
PS W:\testspace\untitled>

Your code and the corresponding bytecode demonstrate several important things about the use of primitive types, wrapper objects, and conversions in Java.

1. Initializing the Long variable:

-

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ldc2_w #7

loads the value

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200L

(type

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long

) from the constant pool.
-

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invokestatic #9

calls the static method

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Long.valueOf(long)

to create a

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Long

object.
-

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astore_1

stores the reference to the

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Long

object in a local variable.

2. Initializing the Integer variable:

-

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bipush 10

pushes the integer value

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10

onto the stack.
-

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invokestatic #15

calls the static method

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Integer.valueOf(int)

to create an

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Integer

object.
-

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astore_2

stores the reference to the

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Integer

object in a local variable.

3. String concatenation and printing:

-

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getstatic #20

loads the static field

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System.out

, which is an instance of

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PrintStream

.
-

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ldc #26

loads the string

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"2string"

from the string pool.
-

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invokevirtual #28

calls the method

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println(String)

to print the string.

4. Returning an Integer from a method:

-

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iconst_2

pushes the value

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2

onto the stack.
-

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invokestatic #15

calls

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Integer.valueOf(int)

to convert

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int

to

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Integer

.
-

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areturn

returns the object reference.

5. Returning a Long from a method:

-

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ldc2_w #34

loads the value

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2L

(type

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long

).
-

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invokestatic #9

calls

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Long.valueOf(long)

to convert

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long

to

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Long

.
-

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areturn

returns the object reference.

These examples show how Java uses

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valueOf

methods for autoboxing of primitive values into their corresponding wrapper classes.

It’s also clear that the concatenation of

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2 + "string"

is performed at compile time, so no actual boxing or runtime conversion happens there. The compiler directly inserts the

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"2string"

literal into the bytecode using

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ldc

, which means this is not a case of runtime autoboxing.

In other words, converting a number to a string or joining it with a string (concatenation) is not autoboxing, because

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String

is not a wrapper class for primitive types.

[admin]

What you have described is correct but you are overlooking the fact that it is a compiler optimization technique and is not guaranteed. Here is what section 15.18.1 of JLS 21 says:

If only one operand expression is of type String, then string conversion (§5.1.11) is performed on the other operand to produce a string at run time.

and further in the same section, it clearly says:

For primitive types, an implementation may also optimize away the creation of a wrapper object by converting directly from a primitive type to a string

String conversion is explained In section 5.1.11 and there it says:

A value x of primitive type T is first converted to a reference value as if by giving it as an argument to an appropriate class instance creation expression.

So, to summarize, theoretically, it is autoboxing but behind the scene the compiler my implement is differently. A JVM is free to not optimize it this way.