Update java.sql.Timestamp

Question

I want to update java.sql.Timestamp in other method. But it is not updated in main method. Why? Is that passed by value or reference?

package test; import java.sql.Timestamp; import java.util.Date; public class Test { public static void main(String[] args) throws InterruptedException { Timestamp a = null; Date now = new Date(); Timestamp b = new Timestamp(now.getTime()); update(a, b); System.out.println(a); // returns "null", why? } /** * Sets the newer timestamp to the old if it is possible. * @param a an old timestamp * @param b a new timestamp */ private static void update(Timestamp a, Timestamp b) { if(b == null) { //nothing } else if(a == null) { a = b; } else if(a.after(b)) { a = b; } } }

possible duplicate of Is Java "pass-by-reference" or "pass-by-value"? — Jonathan
– Jonathan, Commented Feb 11, 2015 at 8:50

Jens · Accepted Answer · 2015-02-11 08:52:04Z

Java uses CallByValue. That means that the value is transfered to the methos not the reference to the object. So a will only changed inside you function.

If you would like to change it in `main function you have to get it back via return value.

package test; import java.sql.Timestamp; import java.util.Date; public class Test { public static void main(String[] args) throws InterruptedException { Timestamp a = null; Date now = new Date(); Timestamp b = new Timestamp(now.getTime()); a = update(a, b); System.out.println(a); // returns "null", why? } /** * Sets the newer timestamp to the old if it is possible. * @param a an old timestamp * @param b a new timestamp */ private static Timestamp update(Timestamp a, Timestamp b) { if(b == null) { //nothing } else if(a == null) { a = b; } else if(a.after(b)) { a = b; } return a; } }

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