如何正确为Resistor类搭建try-catch异常捕获块？

Hey there! Let's break down why your try-catch block isn't working and get your Resistor class tests up and running properly.

First: Fix the Logic Bug in Your Resistor Constructor #

Looking at your Resistor constructor, there's a critical logic error in the power rating check:

if (powerrating >=0){
 throw new Exception("The power rating must be greater than zero.");
}

This throws an exception when the power rating is greater than or equal to 0—the exact opposite of what you want! You should throw the exception only when powerrating <= 0, since your requirement states the power rating must be greater than zero.

Second: Your Test Code Isn't Triggering Exceptions #

The biggest issue with your test is that you're not actually creating a Resistor instance inside the try block. You just define a resistancevalue variable, but never call the Resistor constructor—so none of the exception checks in the constructor run at all. That's why your catch block never executes.

Fixed Code Examples #

Corrected Resistor Constructor #

public Resistor(double resistancevalue, double tolerancevalue, double powerrating) throws Exception {
    if (resistancevalue <= 0) {
        throw new Exception("The resistance value must be greater than zero.");
    }
    if (tolerancevalue <= 0 || tolerancevalue >= 1) {
        throw new Exception("The tolerance value must be between zero and one (not inclusive).");
    }
    // Fixed power rating check
    if (powerrating <= 0) {
        throw new Exception("The power rating must be greater than zero.");
    } else {
        this.resistancevalue = resistancevalue;
        this.tolerancevalue = tolerancevalue;
        this.powerrating = powerrating;
    }
}

// Keep your existing methods as-is
public double getResistanceValue(){
 return resistancevalue;
}
double miniResistance(){
 double Minir = resistancevalue*(1.0-tolerancevalue);
 return Minir;
}
double maxResistance(){
 double Maxr= resistancevalue*(1.0+tolerancevalue);
 return Maxr;
}

Working Test Class #

Let's write tests that cover all possible failure scenarios plus a valid case:

public class ResistorTest {
    public static void main(String[] args) {
        double validTolerance = 0.5;
        double validPower = 1.0;

        // Test 1: Invalid resistance value (<=0)
        try {
            Resistor badResistance = new Resistor(-1, validTolerance, validPower);
            System.out.println("Test 1 Failed: Exception not thrown for invalid resistance");
        } catch (Exception e) {
            System.out.println("Test 1 Passed: " + e.getMessage());
        }

        // Test 2: Invalid tolerance value (<=0)
        try {
            Resistor badToleranceLow = new Resistor(10, -0.1, validPower);
            System.out.println("Test 2 Failed: Exception not thrown for low tolerance");
        } catch (Exception e) {
            System.out.println("Test 2 Passed: " + e.getMessage());
        }

        // Test 3: Invalid tolerance value (>=1)
        try {
            Resistor badToleranceHigh = new Resistor(10, 1.0, validPower);
            System.out.println("Test 3 Failed: Exception not thrown for high tolerance");
        } catch (Exception e) {
            System.out.println("Test 3 Passed: " + e.getMessage());
        }

        // Test 4: Invalid power rating (<=0)
        try {
            Resistor badPower = new Resistor(10, validTolerance, 0);
            System.out.println("Test 4 Failed: Exception not thrown for invalid power");
        } catch (Exception e) {
            System.out.println("Test 4 Passed: " + e.getMessage());
        }

        // Test 5: Valid Resistor instance
        try {
            Resistor goodResistor = new Resistor(100, 0.05, 2.0);
            System.out.println("\nTest 5 Passed: Valid resistor created");
            System.out.println("Base Resistance: " + goodResistor.getResistanceValue());
            System.out.println("Min Resistance: " + goodResistor.miniResistance());
            System.out.println("Max Resistance: " + goodResistor.maxResistance());
        } catch (Exception e) {
            System.out.println("Test 5 Failed: " + e.getMessage());
        }
    }
}

Key Takeaways #

• Always instantiate the class you're testing inside the try block to trigger constructor-level exceptions.
• Double-check conditional logic in the constructor to ensure exceptions are thrown for the right cases (like the power rating fix we did).
• Test all possible failure scenarios plus a valid case to confirm your class behaves as expected.

内容的提问来源于stack exchange，提问作者TwilightLuigi