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assume i have integer variables a,b and c. 

c = a + b; or c = a - b; or c = a / b; or c = a * b;

As you can see, the computation operator needs to be passed dynamically at run time. So i have a jComboBox of operators, so a user will select either a +, -, * or / from the jcomboBox.

how do i get the jCombobox selectedItem(which will be either /, *, -, or + ) and use it in getting the value of c.

Eg. if the user selects *, then the expression should be c = a * b else if the user selected say +, then the expression should be c = a + b.

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4 Answers 4

Reset to default
4

Here is a way to 'cheat'. All the parsing is done by the ScriptEngine, we just need to assemble the parts of the expression.

ScriptEngine Calculator

import java.awt.*; import java.awt.event.*; import javax.swing.*; import javax.script.*; class ScriptEngineCalculations { public static void main(String[] args) { final ScriptEngine engine = new ScriptEngineManager(). getEngineByExtension( "js" ); String[] ops = {"+", "-", "*", "/"}; JPanel gui = new JPanel(new BorderLayout(2,2)); JPanel labels = new JPanel(new GridLayout(0,1)); gui.add(labels, BorderLayout.WEST); labels.add(new JLabel("a")); labels.add(new JLabel("operand")); labels.add(new JLabel("b")); labels.add(new JLabel("=")); JPanel controls = new JPanel(new GridLayout(0,1)); gui.add(controls, BorderLayout.CENTER); final JTextField a = new JTextField(10); controls.add(a); final JComboBox operand = new JComboBox(ops); controls.add(operand); final JTextField b = new JTextField(10); controls.add(b); final JTextField output = new JTextField(10); controls.add(output); ActionListener al = new ActionListener(){ public void actionPerformed(ActionEvent ae) { String expression = a.getText() + operand.getSelectedItem() + b.getText(); try { Object result = engine.eval(expression); if (result==null) { output.setText( "Output was 'null'" ); } else { output.setText( result.toString() ); } } catch(ScriptException se) { output.setText( se.getMessage() ); } } }; // do the calculation on event. operand.addActionListener(al); a.addActionListener(al); b.addActionListener(al); JOptionPane.showMessageDialog(null, gui); } }

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2

You will have to get the Value of the JComboBox and do a switch statement on the Char (because cant switch on a string) to see which it is, then perform the correct operation.

Edit: Cant switch on a string.... yet (Java 7)

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2 Comments

Note that you can't switch on a String yet! Java 7 finally introduces this feature.
yes, it's part of JSR 334/Project Coin.
2 
int c = compute(a,b, (String)comboBox.getSelectedItem());

...

private int compute(int a, int b, String operator) { int result = 0; if("*".equals(operator)) result = a * b; else if("/".equals(operator)) result = a / b; else if("+".equals(operator)) result = a + b; else if("-".equals(operator)) result = a - b; else throw new IllegalArgumentException("operator type: " + operator + " ,not supported"); return result;

}

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1

This similar question can help.

Alternatively, you could associate each of the operators shown in a combobox with an implementation of a custom Calculator interface that gives you the result for any two numbers. Something like:

interface Calculator { public int calculate(int a, int b); } class AddCalculator implements Calculator { public int calculate(int a, int b) {return a+b;} }

The association can be of a form of HashMap<String, Calculator>. The interface Calculator can be generic over the type you pass as parameter and return as result. That would be my approach to the problem, but I am sure there might be a simpler one.

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The similar question is probably overkill for this, but +1 for the Strategy Pattern.

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