Example of ==, equals and hashcode in java

问题:

Given this:

String s1= new String("abc");
String s2= new String("abc");
String s3 ="abc";
System.out.println(s1==s3);
System.out.println(s1==s2);
System.out.println(s1.equals(s2));
System.out.println(s1.equals(s3));
System.out.println(s1.hashCode());
System.out.println(s2.hashCode());
System.out.println(s3.hashCode());

Output is:

false
false
true
true
96354
96354
96354

Here == is giving false for each object but the hashcode for each String object is same. Why is it so?

回答1:

== does compare real equality of objects (I mean - both references point to the same object), not their content, whereas .equal compares content (at least for String).

String a = new String("aa");
String b = new String("aa"); 

a and b are pointing to different objects.

Notice also that if objects are equal then their hashchodes must be the same, but if hashcodes are the same, it doesn't mean that objects are equal.

回答2:

The equals contract says that if o1.equals(o2), then o1.hashCode() == o2.hashCode(). It doesn't specify anything about the hash codes of unequal objects. You could have a method like

public int hashCode()
{
    return 42;
}

and it'd fulfill the contract. It's just expected that the hash code be related to the value of the object, in order to make hash tables work more efficiently.

Now, as for why your == doesn't work, two objects will always be compared by reference. That is, if o1 == o2, then o1 and o2 are the exact same object. That's rarely what you want; you usually want to see if o1.equals(o2) instead.

回答3:

When you use ==, you are comparing if two variables hold reference to the same Object. In other words s1 == s2 is like asking: are the s1 and s2 variables referring to the same String object? And that's not true, even when both String objects have the same "abc" value.

When you use equals(), you are comparing the value of both objects. Both objects may not be the same, but their value (in this case "abc") is the same, so it returns true.

How do you define whether an object is equal to another? That's up to you. In this case the String object already defines this for you, but for example if you define a Person object, how do you know if a person P1 is equal to P2? You do that by overriding equals() and hashCode().

回答4:

== tells you whether the two variable references point at the same object in memory, nothing more. equals() and hashCode() both look at the contents of the object and each uses its own algorithm for calculation.