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fields. Note the two text fields need to be declared as final .Iftheywerenot final ,then
they could not be accessed from the anonymous local class that inherits from the class
ActionListener .
12.5 Grid Layout
We will next build a simple calculator. Figure 12.3 shows what our calculator will look
like. We will create three panels in the window: in the north, south, and center. The panel
in the middle will use the grid layout .
Whenever components need to be placed in a grid, one can use the grid layout.
When adding the components to a grid layout, the rows are filled from left to right and
from top to bottom. In other words, the first row is filled first and so on. The call to
the constructor new GridLayout(3,5) will create a 3x5 grid (3 rows and 5 columns).
Alternatively, the call new GridLayout(2,4,3,6) will generate a grid with 2 rows, 4
columns, a 3-pixel horizontal gap, and a 6-pixel vertical gap between the components.
For the calculator application, we will create a 4x4 grid in the center. Our calculator
will have very simple behavior. When the CLEAR button is pressed, the text field in the top
will be cleared. Alternatively, when the EVALUATE button is pressed, the expression in the
text field will be evaluated and its value will be displayed in the text field. Since computing
arithmetic expressions is beyond the scope of this topic, we will use the JavaScript function
eval to evaluate an expression. Note that JavaScript is a programming language that is
different from Java. However, there is a mechanism for calling JavaScript functions from
Java. The complete code for the calculator program follows.
import java .awt . ;
import java .awt. event . ;
import javax . swing . ;
import javax . script . ;
public class Calculator {
FIGURE 12.3: Calculator program.
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