问题

I'm new to JAVA, currently learning Oracle tutorial generics section. I think there is a mistake there, and I want to make sure I'm not wrong. I'll appreciate your feedback.

I saw the following explanation at https://docs.oracle.com/javase/tutorial/java/generics/restrictions.html

Pair < Integer, Character > p = new Pair<>(8, 'a');

Note that the Java compiler autoboxes 8 to Integer.valueOf(8) and 'a' to Character('a'):

Pair < Integer, Character > p = new Pair<>(Integer.valueOf(8), new Character('a'));

I think this is a mistake and 'a' is actually auto-boxed to Character.valueOf('a').

I wrote the following program to check it. as I thought maybe generics have special boxing, I tried generic method, regular method and generic class:

class TestCharacter<T>{
public <T> void check( T first, T second){
    System.out.println("values: " + first + "  " + second);
    System.out.println(first.equals(second));
    System.out.println( "first == second is " + (first == second) );
    System.out.println( "first == second is " + (first == Character.valueOf('a')) );
}
}

public class TestAutoBoxingIssue{

public static <T> void check1( T first, T second){
    System.out.println("values: " + first + "  " + second);
    System.out.println(first.equals(second));
    System.out.println( "first == second is " + (first == second) );
    System.out.println( "first == second is " + (first == Character.valueOf('a')) );
}
public static void check2( Character first, Character second){
    System.out.println("values: " + first + "  " + second);
    System.out.println(first.equals(second));
    System.out.println( "first == second is " + (first == second) );
    System.out.println( "first == second is " + (first == Character.valueOf('a')) );
}
public static void main(String[] args){
    char first = 'a';
    char second = 'a';
    System.out.println("generic method usage: ");
    check1( first, second );
    System.out.println("=============");

    System.out.println("regular method usage: ");
    check2( first, second );
    System.out.println("=============");

    TestCharacter<Character> t = new TestCharacter<>();
    System.out.println("generic class usage: ");
    t.check(first, second );
    System.out.println("=============");
}

}

output is:

generic method usage: values: a a true first == second is true

first == second is true

regular method usage: values: a a true first == second is true

first == second is true

generic class usage: values: a a true first == second is true

first == second is true

So, I think this demonstrates that 'a' is autoboxed to Character.valueOf.

Am I missing something? Is this the right way to check it?

Thanks.

Eliyahu

回答1:

Yes, autoboxing is done with valueOf everywhere to take advantage of any caches. For example with Character:

public static Character valueOf(char c) {
    if (c <= 127) { // must cache
        return CharacterCache.cache[(int)c];
    }
    return new Character(c);
}

It's a documentation oversight.

来源：https://stackoverflow.com/questions/51634863/oracle-java-tutorial-possible-error-regarding-character-autoboxing-java-commen

标签

java

autoboxing

Oracle java tutorial - possible error regarding Character autoboxing java comment [duplicate]

问题

first == second is true

first == second is true

first == second is true

回答1: