第二條 遇到多個構造器參數時要考慮用構造器
這里考慮的是參數多的情況，如果參數個數比較少，那直接采用一般的構造方法就可以了。
書中介紹了寫構造方法的時候幾種方式：
1. 重疊構造方法模式:
缺點：有許多參數的時候，客戶端代碼會很難寫，而且較難以閱讀。
2. javaBeans模式：
缺點：
在構造過程中JavaBean可能處于不一致的狀態，類本身無法判斷是否有效性。
類做成不可變的可能。
3. builder模式：
優點：
在build方法生成對象的時候，可以做檢查，判斷是否符合要求
參數靈活
缺點：
創建對象必須先創建構造器，如果對性能要求非常高的應用少用為妙
具體實現代碼：
1.重疊構造方法模式：
public class NutritionFacts {
private final int servingSize;
private final int serviings;
private final int calories;
private final int fat;
private int sodium;
private int carbohydrate;
public NutritionFacts(int servingSize, int serviings){
this(servingSize, serviings, 0);
}
public NutritionFacts(int servingSize, int serviings, int calories){
this(servingSize, serviings, calories, 0);
}
public NutritionFacts(int servingSize, int serviings, int calories, int fat){
this(servingSize, serviings, calories, fat,0);
}
public NutritionFacts(int servingSize, int serviings, int calories, int fat, int sodium){
this(servingSize, serviings, calories, fat, sodium,0);
}
public NutritionFacts(int servingSize, int serviings, int calories, int fat, int sodium, int carbohydrate){
this.servingSize = servingSize;
this.serviings = serviings;
this.calories = calories;
this.fat = fat;
this.sodium = sodium;
this.carbohydrate = carbohydrate;
}
}
2. javaBeans模式 代碼：
public class NutritionFacts {
private int servingSize;
private int serviings;
private int calories;
private int fat;
private int sodium;
private int carbohydrate;
public NutritionFacts(){}
public void setServingSize(int servingSize) {
this.servingSize = servingSize;
}
public void setServiings(int serviings) {
this.serviings = serviings;
}
public void setCalories(int calories) {
this.calories = calories;
}
public void setFat(int fat) {
this.fat = fat;
}
public void setSodium(int sodium) {
this.sodium = sodium;
}
public void setCarbohydrate(int carbohydrate) {
this.carbohydrate = carbohydrate;
}
3. builder模式
public class NutritionFacts {
private final int servingSize;
private final int serviings;
private final int calories;
private final int fat;
private final int sodium;
private final int carbohydrate;
public static class Builder {
private final int servingSize;
private final int serviings;
// 可以為空
private int calories = 0;
private int fat = 0;
private int sodium = 0;
private int carbohydrate = 0;
public Builder(int servingSize, int serviings) {
this.servingSize = servingSize;
this.serviings = serviings;
}
public Builder calories(int val){
calories = val;
return this;
}
public Builder fat(int val){
fat = val;
return this;
}
public Builder sodium(int val){
sodium = val;
return this;
}
public Builder carbohydrate(int val){
carbohydrate = val;
return this;
}
public NutritionFacts build(){
return new NutritionFacts(this);
}
}
public NutritionFacts(Builder builder) {
servingSize = builder.servingSize;
serviings = builder.serviings;
calories = builder.calories;
fat = builder.fat;
sodium = builder.sodium;
carbohydrate = builder.carbohydrate;
}
}
這個調用的時候：
NutritionFacts cocaCola = new NutritionFacts.Builder(11,22).calories(1).fat(2).calories(3).build();