In Java, what is the difference between the following declarations?

Considering Java. How are these 2 different and why?

public void languageChecks() {
    Integer a = 5;
    Integer b = new Integer(5);

  开发者_如何学Go  change(a); // a doesn't get incremented. value is 5
    change(b); // b does. value is now 6
}

public void change(Integer a) {
    a++;
}

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The only difference is that

Integer b = new Integer(5);

guarantees a new object is created. The first will use an instance from a cache (see Integer.valueOf()).

Both are immutable and the references to both are passed by value (as is everything in Java). So change() has no effect on either.

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I'd always been taught a++ was just shorthand for a = a + 1 in which case a local variable is created named a and immediately thrown away when the method returns. There're no methods on Integer that change the value (it's immutable), and likewise no operations on primitive ints that change their value.

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Neither call to change() affects the values passed in, because of auto-boxing/unboxing.

public void change(Integer a) {
    // This unboxes 'a' into an int, increments it and throws it away
    a++;
}

The above code seems to imply that a++ changes the value of a, since it's an object, not a primitive. However, ++ is not overloaded by Integer, so it unboxes it to be able to apply the ++operator on its int. To me the compiler shouldn't allow this.