使用場(chǎng)景：如果一個(gè)類的構(gòu)造需要很多參數(shù)，但這些參數(shù)又不都是必須的。

Android開發(fā)中經(jīng)常使用的例子

new AlertDialog.Builder(this)
   .setTitle("Design Patterns")
   .setMessage("Builder is awesome")
   .create();

實(shí)踐

1. 假如一個(gè)User類定義如下

public class User {
   private String firstName;
   private String lastName;
   private int age;
}

2. 若想使用如下方法構(gòu)建

new User.Builder()
   .setFirstName("Leonardo")
   .setLastName("da Vinci")
   .setAge(67)
   .create();

3. 內(nèi)部構(gòu)建一個(gè)Builder類

static class Builder {
   private String firstName;
   private String lastName;
   private int age;
 
   public Builder setFirstName(final String firstName) {
      this.firstName = firstName;
      return this;
   }
 
   public Builder setLastName(final String lastName) {
      this.lastName = lastName;
      return this;
   }
 
   public Builder setAge(final int age) {
      this.age = age;
      return this;
   }
 
   public User create() {
      return new User(this);
   }
}

4. User類修改為

public class User {
   private String firstName;
   private String lastName;
   private int age;
 
   private User(final Builder builder) {
        firstName = builder.firstName;
        lastName = builder.lastName;
        age = builder.age;
   }
}

參考

• Design Patterns in Android? – ?Builder