如果条件正常，为什么这不起作用？

Question

I created this code to check whether the text field of calculator has any content. If content is present in the text field, then it should display "." . Otherwise, it should display "0." in the text field. The problem is that the if condition always evaluates to false.

private void dotActionPerformed(ActionEvent evt){
    String dott=display.getText();
    if(dott==null)
    {
        display.setText(display.getText()+"0.");
    }
    else
    {
        display.setText(display.getText()+dot.getText());   
    }
}

Answer 1

The string will be "" , not null. Use dott.isEmpty() to see if dott has no contents. Here is another post with more details on the difference between .compareTo("") and .isEmpty().

Answer 2

我认为您可以：

if(dott.equals(""))

Answer 3

try this String method:

if(dott.isEmpty())
    //your code

or you can also use:

if(dott.compareTo("")==0)
    // your code

Answer 4

Try something like:

 String dott = ...;
 dott.isEmpty();

or

 dott.equals("");

Answer 5

String dott=display.getText();
if(dott==null) // original code; should use isEmpty()
{
    display.setText(display.getText()+"0.");
}

But you already know that display.getText() is empty, so why execute it again? Same for the "else" clause - you already know the value. So you should just write:

String dott=display.getText();
if(dott.isEmpty()) {
    display.setText("0.");
} else {
    display.setText(dott+dot.getText());   
}

Or more concise:

   String dott=display.getText();
   String text = dott.isEmpty() ? "0." : dot.getText();
   display.setText(text);

or even:

   String dott=display.getText();
   display.setText( dott.isEmpty() ? "0." : dot.getText() );

Answer 6

Replace if(dott==null) by

if(dott.equals(""))

Answer 7

You can use StringUtils of apache if you want. Link : https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/StringUtils.html

  private void dotActionPerformed(ActionEvent evt){
        String dott=display.getText();
        if( StringUtils.isEmpty(dott))
        {
            display.setText(display.getText()+"0.");
        }
        else
        {
            display.setText(display.getText()+dot.getText());   
        }
    }