Arrays subtle behaviour in Java

Question

import java.util.Arrays;
class B {

    int i;

    B(int i) {
    this.i = i;
    }

     public String toString() {
          return "i = " + this.i;
       }
}

 public class MainClass{

        public static void main(String[] args) {
            B [] x = new B[2];
            x[0] = new B(90);
            x[1] = new B(100);
            B obj = new B(10);
            System.out.println(obj);
                    System.out.println(x);//toString method of class B is not called here.

    }
}

//When i printed obj the toString method of B class was called but when I tried to print x it was not called.Can anybody explain why!!!

score 1 · Accepted Answer · answered Apr 17 '14 at 17:12

1

Actually, the toString method of the Array class was called, and Array does not override the Object toString() - so you get a class name and (essentially) a reference address. What you wanted was probably Arrays.toString(Object[]) - like so,

// System.out.println(x); // <-- calls toString of java.lang.Array
System.out.println(Arrays.toString(x));

answered Apr 17 '14 at 17:12

Elliott Frisch

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I know after Arrays.toString(x) correct info will be printed, but why does not it called toString() method of class B when I only printed x. – Aamir Apr 17 '14 at 17:14
@Aamir Edited to answer you more directly. – Elliott Frisch Apr 17 '14 at 17:15
thanks now I got it!! – Aamir Apr 17 '14 at 17:17

score 0 · Answer 2 · edited May 23 '17 at 12:06

0

Java does not automatically invoke toString on the members of an array. As mentioned here, you want Arrays.toString(x).

edited May 23 '17 at 12:06

Community

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answered Apr 17 '14 at 17:12

merlin2011

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score 0 · Answer 3 · answered Apr 17 '14 at 17:12

0

It's because you're printing the array itself, not the individual elements of the array. Arrays are objects, too, but they don't override toString() from Object. Try

System.out.println(Arrays.toString(x));

answered Apr 17 '14 at 17:12

rgettman

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Careful! Java arrays are objects in a general sort of way, but they are not _Objects_ as defined by the Java language spec. – Solomon Slow Apr 17 '14 at 17:20
@jameslarge What would `x instanceof Object` yield? `true`. – rgettman Apr 17 '14 at 17:23
@jameslarge Why do you think so? – Sotirios Delimanolis Apr 17 '14 at 17:30
@jameslarge Yes they are. – Boann Apr 17 '14 at 17:33
Oops! I stand corrected. I was thinking about how the langage treats array references somewhat differently from the way it treats other object references, and I probably had some old C++ thoughts rattling around in my head as well. – Solomon Slow Apr 17 '14 at 17:57

Arrays subtle behaviour in Java

3 Answers3