java instanceof运算符和类返回方法

Question

I'm writing my own TableModel implementation. As I shall need a few various implementations sharing some functionality, I decided to prepare an abstract class first. The fields of the table are represented by:

protected Object[][] lines;

Basically all elements in the same column should be of the same type, however column classes may vary among different implementations. I would like to write a common setValueAt function in the abstract class, checking whether val is of proper type or not.

@Override
public void setValueAt(Object val, int row, int col) {
    if (val instanceof this.getColumnClass(col))
        lines[col][row] = val;
}

The compiler signals error here:

Syntax error on token "instanceof", == expected

Why?

Answer 1

The right operand of instanceof must be a ReferenceType (JLS 15.20) . Use

if (this.getColumnClass(col).isInstance(val))

Answer 2

Rather than using instanceof , you might consider using a generic type in your abstract class. You could declare it with something like:

protected abstract class MyTableModel<T> implements TableModel {
    //...
    protected T[][] lines;
    //...
    @Override
    public void setValueAt(Object val, int row, int col) {
        lines[col][row] = (T) val;
    }
}

This way, you can let Java handle the type checking for the cast.

You could also just write a single generic class, if the only difference between the classes is the type of the values.