When you have a method with lots of conditional logic (that is, if statements), you're asking for trouble. Conditional logic is notoriously difficult to manage, and may cause you to create an entire state machine inside a single method.
Analyzing the sample application
Here's a short example. Let's say, there is a method that calculates insurance costs based on a person's income:
package ifs;
public class IfElseDemo {
public double calculateInsurance(double income) {
if (income <= 10000) {
return income*0.365;
} else if (income <= 30000) {
return (income-10000)*0.2+35600;
} else if (income <= 60000) {
return (income-30000)*0.1+76500;
} else {
return (income-60000)*0.02+105600;
}
}
}
Let's analyze this example. Here we see the four "income bands widths", separated into four calculation strategies. In general, they conform to the following formula:
(income - adjustment) * weight + constant
Our goal is to provide separate classes to calculate each strategy, and transform the original class to make it more transparent.
Creating and running test
First of all, let's make sure that the class works. To do that, create a test class, using the JUnit4 testing framework.
Place the caret at the class name, and then press Alt+Enter (or click ). From the list of suggested intention actions, choose Create Test:
In the Create Test dialog, choose JUnit4 from the Testing library list, click Fix, if you do not have JUnit library, and then click OK:
The stub test class is created. However, you have to provide some meaningful code, using the provided quick fix to create an import statement:
import org.junit.Test;
import static org.junit.Assert.*;
public class IfElseDemoTest {
@Test
public void low() {
assertEquals(1825, insuranceFor(5000), 0.01);
}
@Test
public void medium() {
assertEquals(38600, insuranceFor(25000), 0.01);
}
@Test
public void high() {
assertEquals(78500, insuranceFor(50000), 0.01);
}
@Test
public void veryHigh() {
assertEquals(106400, insuranceFor(100_000), 0.01);
}
private double insuranceFor(double income) {
return new IfElseDemo().calculateInsurance(income);
}
}
Now let's run this test by clicking the run button in the left gutter, or by pressing Control+Shift+F10:
All 4 tests pass:
Extracting methods
Next, bring forward the original class, place the caret at the expression
The name of the superclass to be generated is InsuranceStrategy.
All the methods of the class InsuranceStrategyVeryHigh are checked – it means that they will be included in the superclass.
The method calculateInsuranceStrategyVeryHigh remains non-abstract; all the other methods are made abstract by selecting the Make Abstract checkboxes.
Agree to replace the usage of InsuranceStrategyVeryHigh class (in IfElseDemo class) with the superclass, and get the following InsuranceStrategy class:
package ifs;
public abstract class InsuranceStrategy {
public double calculateInsuranceVeryHigh(double income) {
return (income - getAdjustment()) * getWeight() + getConstant();
}
public abstract int getConstant();
public abstract double getWeight();
public abstract int getAdjustment();
}
Implementing the abstract class
Next, for this abstract class, use Implement Abstract Class intention to create implementations for all strategies:
Name the new implementation classes InsuranceStrategyLow, InsuranceStrategyMedium and InsuranceStrategyHigh.
For all new implementations provide correct return statements for the methods getAdjustment(), getWeight() and getConstant().
Thus all implementation classes should look similar to the class InsuranceStrategyVeryHigh, but with strategy-specific adjustment, weight and constant. For example:
package ifs;
public class InsuranceStrategyMedium extends InsuranceStrategy {
@Override
public int getConstant() {
return 35600;
}
@Override
public double getWeight() {
return 0.2;
}
@Override
public int getAdjustment() {
return 10000;
}
}
Note that in all newly created implementation classes the class names are grey - they are not used so far.
Next, bring forward the class IfElseDemo, and modify all the branch bodies so that they initialize the strategy field, like in the last branch:
package ifs;
class IfElseDemo {
private InsuranceStrategy strategy;
public double calculateInsurance(double income) {
if (income <= 10000) {
strategy = new InsuranceStrategyLow();
return strategy.calculateInsuranceVeryHigh(income);
} else if (income <= 30000) {
strategy = new InsuranceStrategyMedium();
return strategy.calculateInsuranceVeryHigh(income);
} else if (income <= 60000) {
strategy = new InsuranceStrategyHigh();
return strategy.calculateInsuranceVeryHigh(income);
} else {
strategy = new InsuranceStrategyVeryHigh();
return strategy.calculateInsuranceVeryHigh(income);
}
}
}
Finally, rename the method calculateInsuranceVeryHigh: bring forward the class InsuranceStrategy, place the caret at the method name and press Shift+F6. The new name should be calculate.
Happy end
And finally enjoy the code:
package ifs;
class IfElseDemo {
private InsuranceStrategy strategy;
public double calculateInsurance(double income) {
if (income <= 10000) {
strategy = new InsuranceStrategyLow();
return strategy.calculate(income);
} else if (income <= 30000) {
strategy = new InsuranceStrategyMedium();
return strategy.calculate(income);
} else if (income <= 60000) {
strategy = new InsuranceStrategyHigh();
return strategy.calculate(income);
} else {
strategy = new InsuranceStrategyVeryHigh();
return strategy.calculate(income);
}
}
}
Next, let's run the test class again. All tests should pass – we have refactored the code, but it still produces the same results.
). From the list of suggested intention actions, choose Create Test:
to create an import statement:
in the left gutter, or by pressing Control+Shift+F10:
