java - passing a double value by reference

Question

how can I pass a double value by reference in java?

example:

Double a = 3.0;
Double b = a;
System.out.println(\"a: \"+a+\" b: \"+b);
a = 5.0;
System.         


        

Answer 1

Java doesn't support pointers, so you can't point to a's memory directly (as in C / C++).

Java does support references, but references are only references to Objects. Native (built-in) types cannot be referenced. So when you executed (autoboxing converted the code for you to the following).

Double a = new Double(3.0);

That means that when you execute

Double b = a;

you gain a reference to a's Object. When you opt to change a (autoboxing will eventually convert your code above to this)

a = new Double(5.0);

Which won't impact b's reference to the previously created new Double(3.0). In other words, you can't impact b's reference by manipulating a directly (or there's no "action at a distance" in Java).

That said, there are other solutions

public class MutableDouble() {

   private double value;

   public MutableDouble(double value) {
     this.value = value;
   }

   public double getValue() {
     return this.value;
   }

   public void setValue(double value) {
     this.value = value;
   }
 }

 MutableDouble a = new MutableDouble(3.0);
 MutableDouble b = a;

 a.setValue(5.0);
 b.getValue(); // equals 5.0