構造者模式

你是否打過如下代碼：

public class Animal{
    private String name;
 private String breed;
 private String gender;
 private int weight;

 public Animal(String name, String breed) {
 this.name = name;
 this.breed = breed;
 }


 public Animal(String name, String breed,String gender,int weight) {
 this.name = name;
 this.breed = breed;
 this.gender=gender;
 this.weight=weight;
 }

}


上述代碼，致使了在構造過程當中JavaBean可能處於不一致的狀態，也就是說實例化對象本該是一鼓作氣，但如今卻分割成了兩大步，這會致使它線程不安全，進一步引起不可預知的後果。
Effective+Java做者joshua bloch推薦利用"Builder"模式（構造者模式），其核心就是不直接生成想要的對象，而是讓客戶端利用全部必要的參數調用構造器（或者靜態工廠），
獲得一個builder對象，再調用相似setter的方法設置相關可選參數。

代碼以下：

public class Animal{
 private String name;
 private String breed;
 private String gender;
 private int weight;


 public static class Builder {
 private String name;
 private String breed;
 private String gender;
 private int weight;


 public Builder(String name, int weight){
 this.name=name;
 this.weight=weight; }

 public Builder gender(String gender){
 this.gender=gender;
 return this; }

 public Builder breed(String breed){
 this.breed=breed;
 return this; }

 public Animal build(){
 return new Animal(this); }

 }

 private Animal(Builder builder){
 this.name=builder.name;
 this.breed=builder.breed;
 this.gender=builder.gender;
 this.weight=builder.weight; }

}

　調用：
Animal animal=new Animal.Builder("Tom",10).breed("Cat").build();