Java中字符串类的垃圾回收

Question

In this code I have declared a Initialized a String variable and then printed its hashcode, then reinitialized it to another value and then invoked the Garbage Collector to clear the dereferenced objects.

But when I reinitialize the String variable to its original value and print the hashcode, the same hashcode is getting printed. How?

public class TestGarbage1 {

    public static void main(String args[]) {

        String m = "JAVA";   
        System.out.println(m.hashCode());         
        m = "java";
        System.gc();
        System.out.println(m.hashCode());
        m = "JAVA";
        System.out.println(m.hashCode());
    }
}

Answer 1

Hash code relates to object equality, not identity.

a.equals(b) implies a.hashCode() == b.hashCode()

(Provided the two methods have been implemented consistently)

Even if a gc were actually taking place here (and you weren't simply referencing strings in the constant pool), you wouldn't expect two string instances with the same sequence of chars not to be equal - hence, their hash codes will also be the same.

String a = new String("whatever");
String b = new String(a);
System.out.println(a == b); // false, they are not the same instance
System.out.println(a.equals(b)); // true, they represent the same string
System.out.println(a.hashCode() == b.hashCode()); // true, they represent the same string

Answer 2

I think you are misunderstanding something about how hashcodes work. Without going in to too much detail, in Java, hashcodes are used for many things. One example is used to find an item in a Hash datastructure like HashMap or HashSet .

A hash of the same value should always return the same hash. In this case, a hash of "JAVA" should never change because then it will break the agreement set forth in Java.

I think it's too complicated to go about how hashcodes for String are calculated. You can read more about it here . I can give you an example though.

Let's say you have a class Fruit and it has fields like shape, color and weight.

You must implement equals AND hashcode for this class. It is very important to do both because otherwise you are breaking the way Hashmap work. Let's say you make this for your hashCode() method.

@Override
public int hashCode() {
    int hash = 1;
    hash = hash * 17 + this.color;
    hash = hash * 31 + this.shape.hashCode();
    hash = hash * 31 + this.weight;
    return hash;
}

This will generate the same hash value EVERY TIME for the two Fruit instances that are equal. That is exactly what you would want.

Really quick, how would this be actually used in a HashMap ? Let's say you want to see if you have foo = new Fruit(); HashMap first calculates foo.hashCode() . It checks to see if there is anything in the bucket for that hashCode. If there is then it will use the equals() method until it returns true. It must do this because there might be hashcode collisions. And that's why it is important why equals and hashCode should be implemented together.