Java: What is faster? Compare two object references or compare two primitives?

Question

So what is the logic to say the the default equals(Object obj)

public boolean equals(Object obj){
  this == obj; 
}

or:

int a = 1;
int b = 2;
a == b;

Could be another type with another value, I just don't find any documentation in what the cost of each one is.

Adding a reason for this:

I was reading this question and I was thinking, if there is an optimization for comparing for example a value inside the object (like an int) and compare them instead of the reference

Guessing... checking referential equality is less overhead than value equality — OneCricketeer, Apr 12 '18 at 03:16
Yeah... I had the same thought but... I really can't be able to have some facts on it, just frustrating — Damian Lattenero, Apr 12 '18 at 03:17
I would imagine integer would be faster as you are comparing a 32bit integer vs a 64 bit address — Mitch, Apr 12 '18 at 03:21
I'm wondering - what difference answer will make? if you have to compare integers - compare integers, if you have references - compare references, you cannot replace one comparison with another, so it does not matter what is faster, you have to use what you have — Iłya Bursov, Apr 12 '18 at 03:25
@IlyaBursov because of this question I read [https://stackoverflow.com/questions/49786873/comparator-interfaces-equals-method-why-it-is-always-safe-not-to-override-obje#comment86589421_49786873] — Damian Lattenero, Apr 12 '18 at 03:27
Though the answer is totally JVM-dependent, it's likely the JIT-compiled code for both will be similar: one instruction to compare and another to branch. It will be an extremely rare use case where any difference that does occur is important. — Gene, Apr 12 '18 at 03:27
@shmosel It is relevant in term of possible efficiency (at least that's the whole point of my research) — Damian Lattenero, Apr 12 '18 at 03:31
@z5h that's a good point, I could, but I'm looking an explaination of why, and not just the fact. — Damian Lattenero, Apr 12 '18 at 03:35

score 1 · Accepted Answer · answered Apr 12 '18 at 04:14

Wrote a little class (feel free to run in your editor of choice)..

public class HelloWorld {

    public static void main(String []args) {
        HelloWorld app = new HelloWorld();

        int n = 100000;

        long startTime = System.nanoTime();
        app.doEqualsNull(n);
        long endTime = System.nanoTime();
        System.out.println("doEqualsNull (ns): " + (endTime - startTime)) ; 

        startTime = System.nanoTime();
        app.doEqualsThis(n);
        endTime = System.nanoTime();
        System.out.println("doEqualsThis (ns): " + (endTime - startTime)) ;

        startTime = System.nanoTime();
        app.doEqualsSameInt(n);
        endTime = System.nanoTime();
        System.out.println("doEqualsSameInt (ns): " + (endTime - startTime)) ;

        startTime = System.nanoTime();
        app.doEqualsDifferentInt(n);
        endTime = System.nanoTime();
        System.out.println("doEqualsDifferentInt (ns): " + (endTime - startTime)) ;
    }

    public void doEqualsNull(int n) {
        for (int i = 0 ; i < n ; i++) {
            this.equals(null);
        }
    }

    public void doEqualsThis(int n) {
        for (int i = 0 ; i < n ; i++) {
            this.equals(this);
        }
    }

    public boolean equals(Object obj){
        return this == obj; 
    }

    public void doEqualsSameInt(int n) {
        for (int i = 0 ; i < n ; i++) {
            this.equalsInt(5);
        }
    }

    public void doEqualsDifferentInt(int n) {
        for (int i = 0 ; i < n ; i++) {
            this.equalsInt(50);
        }
    }

    public boolean equalsInt(int other){
        return 5 == other; 
    }
}

Output:

doEqualsNull (ns): 3660106
doEqualsThis (ns): 3237291
doEqualsSameInt (ns): 2887368
doEqualsDifferentInt (ns): 2992786

Integer equality seems to take slightly shorter time (it's really really small difference) -- but again, I am guessing some branch prediction did kick into my super scientific calculations. :)

How many times did you run this? In which Java version? https://stackoverflow.com/q/504103/2308683 — OneCricketeer, Apr 12 '18 at 13:07

Java: What is faster? Compare two object references or compare two primitives?

1 Answers1