为什么“好”一词不显示？

Question

When I run this code, why doesn't the word "good" display? The value stored in position 0 of Array1 and Array2 is 1 for both.

public class ArrayStuff {

    public static void main(String[] args) {

        double value = 1;

        Double[] Array1 = new Double[10];
        Array1 = new Double[] {1.0};

        Double[] Array2 = new Double[10];

        Array2[0] = value;

        int i = 0;

        System.out.println(Array1[0]);
        System.out.println(Array2[i]);

        if (Array1[0] == Array2[i])
        {

            System.out.println("good");

        }
     }

}

Answer 1

In Java, when one asks

Double d1 = ...;
Double d2 = ...;
if (d1 == d2) {
    System.out.println("Equal");
} else {
    System.out.println("Not Equal");
}

the JRE does not unbox the Double objects, so it checks whether the two Double s are the same exact object. Here, they aren't. Note the difference between these two blocks of code:

Integer i1 = new Integer(1);
Integer i2 = new Integer(1);
if (i1 == i2) {
    System.out.println("Equal");
} else {
    System.out.println("Not Equal");
}

and

Integer i1 = Integer.valueOf(1);
Integer i2 = Integer.valueOf(1);
if (i1 == i2) {
    System.out.println("Equal");
} else {
    System.out.println("Not Equal");
}

The second version reuses the Integer object, so the first prints Not Equal and the second prints Equal .

Answer 2

You're learning that computers cannot represent floating point numbers to exact precision. The solution is to use code that finds a floating point solution that is "close enough" or else one that uses decimal numbers.

Answer 3

Because finite precision numbers have their limits especially with regard to comparing them for exact equality.

You should approach the problem by checking if the difference between to values is less than a certain threshold:

public final double EPSILON = 0.001;

if (Math.abs(Array1[0] - Array2[i]) < EPSILON)
 ...

Answer 4

does your first variable has to be a double? I mean with an int you solve that problem.

This raises a question: does this line actually create the array with 10 slots and assigns the first value: 1.0 to the 0 slot in the array or fails to create the array correctly?

 Double[] Array1 = new Double[10];
    Array1 = new Double[] {1.0};  

If not then that would be why, that code below is not working for you. Not showing you the "good" word.

   if (Array1[0] == Array2[i])
    {

        System.out.println("good");

    }

... However, maybe if you change to 1.0 as value in the first variable... but its not the same to compare doubles.. its not recommended.

Double Value = 1.0;