How to show message which abbrevate week start letter to its full name of the day in java?

Basically i've been trying to create a program which when you enter abbreviations for days it pops up with the full name of the day, e.g M = monday, Tu = tuesday and so forth. Sounds simple enough but i've spent couple of hours constantly making changes to the code and have made a little progress being that there is no compiling error. But the current problem i have now is that the show message dialog box is not appearing after i have entered the letter so i was wondering if any of you guys could see where i've gone wrong:

Code :

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import javax.swing.JOptionPane;
public class DaysOfWeek
{
  public static void main(String[]args)
  {
	  String input1 = JOptionPane.showInputDialog("Enter the abbreviation for a day");
	  String day = (input1);
	  if (day == "M")
	  {
	  JOptionPane.showMessageDialog(null,"Monday");
      }
      else if (day == "Tu")
      {
	  JOptionPane.showMessageDialog(null,"Tuesday");
      }
  }
}

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Never compare Strings or other objects with ==, use the equals method instead.

Yes we must use equals method.
When a user inputs the abbreviation the String is constructed with the new operator and hence new String object is created. This "==" checks if both the references are pointing to the same object. For example in this case if user inputs the abbreviation as M, then a String M is created using new operator. Now when we compare --> if (day == "M"). Now this second "M" is actually another object which resides in string pool constant. As the objects are different the "if " would result in false and hence would not work. So use "equals" method