# Learning reconstruction (5) – simplifying conditional expression

Time：2020-2-11

1. Decompose conditional
Scenario: you have a complex if then else statement. The independent functions are extracted from if, then and else.
Example:
if (date.before(SUMMER_START) || date.after(SUMMER_END)) {
charge = quantity * mWinterRate + mWinterServiceCharge;
} else {
charge = quantity * mSummerRate;
}
Refactoring as follows:
if (notSummer(date)) {
charge = winterCharge(quantity);
} else {
charge = summerCharge(quantity);
}
private boolean notSummer(Date date) {
return date.before(SUMMER_START) || date.after(SUMMER_END);
}
private double winterCharge(int quantity) {
return quantity * mWinterRate + mWinterServiceCharge;
}
private double summerCharge(int quantity) {
return quantity * mSummerRate;
}

2. Consolidate conditional expression
Application scenario: you have a series of conditional tests that all get the same results. Combine these tests into a conditional expression and refine the conditional expression into an independent function.
Example:
double disabilityAmount() {
if(mSeniority < 2) {
return 0;
}
if(mMonthsDisabled > 12) {
return 0;
}
if(mIsPartTime) {
return 0;
}
// compute the disability amount …
}
Refactoring as follows:
double disabilityAmount() {
if(isNotEligibleForDisability()) {
return 0;
}
// compute the disability amount …
}
private boolean isNotEligibleForDisability() {
return (mSeniority < 2) || (mMonthsDisabled > 12) || mIsPartTime;
}

3. Consolidate duplicate conditional fragments
Application scenario: there is the same piece of code on each branch of the conditional expression. Move the duplicate code out of the conditional expression.
Example:
if(isSpecialDeal()) {
total = price * 0.95; send();
} else {
total = price * 0.98; send();
}
Refactoring as follows:
if(isSpecialDeal()) {
total = price * 0.95;
} else {
total = price * 0.98;
}
send();

4. Remove control flag
Application scenario: in a series of Boolean expressions, a variable has the function of “control mark”. Replace the control tag with a break or return statement.
Example: void checksecurity (string [] people){
boolean found = false;
for(int i = 0; i < people.length; i++) {
if(!found) {
if(people[i].equals(“Don”)) {
found = true;
}
if(people[i].equals(“John”)) {
found = true;
} } } }
Refactoring to: void checksecurity (string [] people){
for(int i = 0; i < people.length; i++) {
if(people[i].equals(“Don”)) {
break;
}
if(people[i].equals(“John”)) {
break;
} } }

5. Replace nested conditional with guard clauses
Application scenario: the conditional logic in the function makes it difficult to see the normal execution path. Use guard statements to represent all special situations.
There are two forms of conditional expression: a) all branches belong to normal behavior; b) only one branch of expression is normal behavior, and the others are uncommon. If both branches behave normally, you should use conditional expressions like if… Else… If a condition is extremely rare, you should check the condition separately and return it from the function as soon as the condition is true. Such a separate check is often referred to as a “guard statement.”.
Example:
double getPayAmount() {
} else {
if(mIsSeparated) {
result = separatedAmount();
} else {
if(mIsRetired) {
result = retiredAmount();
} else {
result = normalPayAmount();
} } }
return result;
}

Refactoring as follows:
double getPayAmount() {
}
if(mIsSeparated) {
return separatedAmount();
}
if(mIsRetired) {
return retiredAmount();
}
return normalPayAmount();
}

6. Replace conditional with polymorphism
Application scenario: you have a conditional expression in your hand, which selects different behaviors according to different object types. Put each branch of this conditional expression into an override function within a subclass, and then declare the original function as an abstract function.
Example:
double getSpeed() {
switch(mType) {
case EUROPEAN: return getBaseSpeed();
case AFRICAN: return getBaseSpeed() – getLoadFactor() * mNumberOfCoconuts;
case NORWEGIAN_BLUE: return mIsNailed ? 0 : getBaseSpeed(mVoltage);
}
throw new RuntimeException(“Should be unreachable.”);
}

Refactoring as follows:
abstract class Bird {
abstract double getSpeed();
}

class European extends Bird {
double getSpeed() {
return getBaseSpeed();
} }

class African extends Bird() {
double getSpeed() {
return getBaseSpeed() – getLoadFactor() * mNumberOfCoconuts;
} }

class NorwegianBlue extends Bird {
double getSpeed() {
return mIsNailed ? 0 : getBaseSpeed(mVoltage);
} }

7. Introduce null object
Application scenario: you need to check whether an object is null again and again, and replace the null value with a null object.
Example:
if (custom == null) {
plan = BillingPlan.basic();
} else {
plan = custom.getPlan();
}
Refactoring as follows:
class Custom {
public Plan getPlan() {
return normalPlan;
}

public boolean isNull() {
return false;
}
}

class NullCustom extends Custom {
public Plan getPlan() {
return BillingPlan.basic();
}

public boolean isNull() {
return true;
}
}

8. Introduce assertion
Application scenario: a piece of code needs to make some assumptions about the state of the program. This assumption is explicitly expressed in assertions.

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