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TwoDent
Asked: 2020-12-11 16:32:07 +0800 CST 2020-12-11 16:32:07 +0800 CST

What is the difference between (String) and .toString()?

  • 772

I am using JAVA to program a replica of a program that I have in VB.NET. I was creating the "DataGrids" (using JTable's) and programming Tooltips to appear on the cells of my JTable... yep, in JAVA you have to do everything from scratch.

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Ok, my curiosity begins when writing the following lines of code an error is hung in the program:

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 DataGridMed = new JTable(){
        @Override
        public Component prepareRenderer(TableCellRenderer renderer, int row, int col){
            Component c = super.prepareRenderer(renderer, row, col);
            if(c instanceof Component){
                JComponent jc = (JComponent) c;
                jc.setToolTipText((String) getValueAt(row,col)); <--- ERROR
            }
            return c;
        }
    };

I was a little indignant, because when I do a casting like that, am I not converting that value into a String object? When I change that line to .toString(), the error disappears completely...!

jc.setToolTipText(getValueAt(row,col).toString);

I clearly know that "String" is a class, and that it does not belong to the group of primitive data types (int, boolean, float, double . . . ). That's why when comparing two Strings, the "==" operator doesn't work and you need to use ".Equals()"... But in this case I'm not distinguishing what is the difference between casting and using . toString().

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Exactly what happened here?

java

3 Answers

  1. Best Answer

    If you go to the documentation Javafor the section JTableyou can see that the method getValueAtreturns a Objeto:

    public Object getValueAt(int row, int col) {
    return rowData[row][col];
}

    therefore, when trying to cast, it is trying to generate the Stringdirectly from the object and, if it does not extend from String, it will not be possible to cast.

    However, the method toStringcalls the method toStringfound on all the data that inherits from Object, and this method does render an Object representation in the form of String.

    So with the first method you get an error if the cast can't be correct. However, with the second one you will not get an error since the data type you introduce is a Stringwhich is what the method needs setToolTipText.

    • 4

  2. First, the easy. toString()it is simply a method that returns a representation of the object as String. Since it's part of the definition of Object, all classes have that method (although the basic implementation isn't much help). That is, for any object, you can make objeto.toString()and get a String.

    (String)does a cast to String. When you have a reference to an object, when making an assignment to a subclass variable, you have to specify the cast. The class of the object doesn't change , if you try to make an casta Stringof an object that isn't a String, then at runtime it throws an exception( ClassCastException). it works like this

    String str1 = new String("Hola");
Object obj1 = str1;
String str2 = (String) obj1; // Asignas una referencia de superclase (Object) a subclase (String), cast es necesario.

Integer int1 = new Integer(1);
String str3 = (String) int1; // Error de compilación, Integer no es superclase de String

Integer int2 = new Integer(2);
Object obj2 = int2;
String str4 = (String) obj2; // No hay error de compilación porque pasa de referencia de Object a referencia de String,
                             // pero lanza ClassCastException en tiempo de ejecución porque
                             // la instancia que se asigna a str4 es un Integer y no un String.
    • 3

  3. As they already said:

    .toString() <-- is a method that returns the textual representation of an object

    String<-- is an Object (String data type)

    When using (String)objetowhat is intended is to convert said object to Stringdirectly and there may be conversion errors, when using .toString()<-- It will always convert the object to a String, although sometimes it does not return the expected value.

    • 1