《Effective Java》笔记（一）

最新推荐文章于 2025-08-25 11:37:52 发布

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本文介绍如何通过Builder模式简化复杂对象的创建过程，提供两个示例：一个是营养成分信息类NutritionFacts的实现，另一个是User类的实现。

如果类的构造器或者静态工厂中具有多个参数，设计这种类时，应该考虑使用Builder模式。

public class NutritionFacts {
    private final int servingSize; // required
    private final int servings; // required
    private final int colories; // optional
    private final int fat; // optional
    private final int sodium; // optional
    private final int carbohydrate; // optional

    public static class Builder {
        // required parameters
        private final int servingSize;
        private final int servings;

        // optional parameters
        private int colories = 0;
        private int fat = 0;
        private int sodium = 0;
        private int carbohydrate = 0;

        public Builder(int servingSize, int servings) {
            this.servingSize = servingSize;
            this.servings = servings;
        }

        public Builder setColories(int val) {
            this.colories = val;
            return this;
        }

        public Builder setFat(int val) {
            this.fat = val;
            return this;
        }

        public Builder setSodium(int val) {
            this.sodium = val;
            return this;
        }

        public Builder setCarbohydrate(int val) {
            this.carbohydrate = val;
            return this;
        }

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

    private NutritionFacts(Builder builder) {
        this.servingSize = builder.servingSize;
        this.servings = builder.servings;
        this.colories = builder.colories;
        this.fat = builder.fat;
        this.sodium = builder.sodium;
        this.carbohydrate = builder.carbohydrate;
    }
}

新建一个NutritionFacts实例：

NutritionFacts nutritionFacts = new NutritionFacts.Builder(240, 8)
                .setCarbohydrate(10).build();

Builder模式简化get set方法，只需要定义一个静态公共的内部类即可:

public class User {
    private Integer id;
    private String name;
    private String address;

    private User() {
    }

    private User(User origin) {
        this.id = origin.id;
        this.name = origin.name;
        this.address = origin.address;
    }

    public static class Builder {
        private User target;

        public Builder() {
            this.target = new User();
        }

        public Builder id(Integer id) {
            target.id = id;
            return this;
        }

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

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

        public User build() {
            return new User(target);
        }
    }

UserExample userExample = UserExample.builder()
                .id(1)
                .name("aaa")
                .address("bbb")
                .build();