如何交换java中的两个日期

Question

This is my code. What am I missing in it?

public class Date {

    public static void main(String[] args) {
        Date dt[] = new Date[2];
          dt[0] = new Date(1,2,3);
          dt[1] = new Date(4,5,6);

        System.out.println("Before: "+dt[0]+" "+dt[1]);
        swap(dt[0],dt[1]);
        System.out.println("After: "+dt[0]+" "+dt[1]);
    }
    public void swap(Date dArr[]){
        Date temp ;
        temp = dArr[0];
        dArr[0]=dArr[1];
        dArr[1]=temp;
    }
}

input: d1 = 1/2/3 d2 = 4/5/6 output: d1 = 4/5/6 d2 = 1/2/3

Answer 1

public void swap(Date dArr[]){
    Date temp ;
    temp = dArr[0];
    dArr[0]=dArr[1];
    dArr[1]=temp;
}

Your swap method takes []Date as an argument, and swap the first and second element of the given array.

Now, you call swap method like this:

swap(dt[0],dt[1]);

You actually passed two Date object. The swap method creates an implicit []Date object as the input argument whose first element is dt[0] and second dt[1] . Note that this array of Date is not the same as your Date dt[] , they are independent of each other. You can think of Date dArr[] as containing two pointers to the Date objects in the old Date dt[] . When you call dArr[0]=dArr[1]; , you just make the pointer point to another Date object, you can't affect the elements in Date dt[] .

If you want to change the original Date dt[] , you should pass dt itself as argument:

swap(dt)

Answer 2

package le9_ps1;
//import java.util.Date;

/* **********************************************************************  *
 *                           Exercise-9                                    *
 *   Objective: Pass parameters to methods - by value and by  reference.   *
 *                                                                         *
 *   Pre-condition: Date class should be created.                          *
 *   Problem Statement-1: Use the Date class created in Lab Ex1 to Swap two*
 *      Dates(Hint:use call by value method).                              *
 *  *********************************************************************  */


public class Date {
    int d,m,y;
    public Date(int dd, int mm, int yy) {
        this.d= dd;
        this.m = mm;
        this.y = yy;
    }



    public static void main(String[] args) {
        Date dt[] = new Date[2];
                dt[0] = new Date(1,2,3);
                dt[1] = new Date(4,5,6);

                System.out.println("Before: "+dt[0].d+"/"+dt[0].m+"/"+dt[0].y+ "  "+dt[1].d+"/"+dt[1].m+"/"+dt[1].y);
                dt=swap(dt);
                System.out.println("After: "+dt[0].d+"/"+dt[0].m+"/"+dt[0].y+ "  "+dt[1].d+"/"+dt[1].m+"/"+dt[1].y);
    }
    public static Date[] swap (Date dArr[]){
        Date temp ;
        temp = dArr[0];
        dArr[0]   =  dArr[1];
        dArr[1]  =   temp;
        return dArr;
    }
}

output:

"C:\Program Files\Java\jdk-15.0.2\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.3.2\lib\idea_rt.jar=1054:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath C:\Users\Tukaram\Desktop\Exercise\ch3_le2\out\production\chapter4 le9_ps1.Date
Before: 1/2/3  4/5/6
After: 4/5/6  1/2/3

Process finished with exit code 0

Answer 3

public class Date {

    public static void main(String[] args) {
        Date dt[] = new Date[2];
          dt[0] = new Date(1,2,3);
          dt[1] = new Date(4,5,6);

        System.out.println("Before: "+dt[0]+" "+dt[1]);
        dt = swap(dt);
        System.out.println("After: "+dt[0]+" "+dt[1]);
    }
    public static Date[] swap(Date dArr[]){
        Date temp ;
        temp = dArr[0];
        dArr[0]=dArr[1];
        dArr[1]=temp;
        return dArr;
    }
}

Answer 4

Why don't you simply use Collections to reverse the values?

List<Date> dates = Arrays.asList(new Date(1, 2, 3), new Date(4, 5, 6));
Collections.reverse(dates); // swaps the dates