构造者模式

你是否打过以下代码：

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;
    }

}


上述代码，导致了在构造过程中JavaBean可能处于不一致的状态，也就是说实例化对象本该是一气呵成，但现在却分割成了两大步，这会导致它线程不安全，进一步引发不可预知的后果。
Effective+Java作者joshua bloch推荐利用"Builder"模式（构造者模式），其核心就是不直接生成想要的对象，而是让客户端利用所有必要的参数调用构造器（或者静态工厂），
得到一个builder对象，再调用类似setter的方法设置相关可选参数。

代码如下：

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; }

}

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