建造者设计模式

建造者设计模式的功能：用来创建对象的

实现

public class Dept {
    private Integer deptno;
    private String dname;
    private String loc;

    //步骤5：实体类提供全参构造方法
    private Dept(Integer deptno, String dname, String loc) {
        this.deptno = deptno;
        this.dname = dname;
        this.loc = loc;
    }

    @Override
    public String toString() {  // 为了方便查看输出结果
        return "Dept{" +
                "deptno=" + deptno +
                ", dname='" + dname + '\'' +
                ", loc='" + loc + '\'' +
                '}';
    }

    public static DeptBuilder newBuilder() { //步骤 2：提供返回Builder的静态方法
        return new DeptBuilder();
    }

    public static class DeptBuilder {  //步骤 1：定义Builder类
        //步骤3：提供和实体类一样的属性
        private Integer deptno;
        private String dname;
        private String loc;

        //步骤4：提供设置属性的方法
        public DeptBuilder deptno(Integer deptno) {
            this.deptno = deptno;
            return this;
        }

        public DeptBuilder dname(String dname) {
            this.dname = dname;
            return this;
        }

        public DeptBuilder loc(String loc) {
            this.loc = loc;
            return this;
        }

        public Dept build(){ //步骤6：提供返回实体类的方法
            return new Dept(deptno,dname,loc);
        }
    }
    
}

测试代码：

public static void main(String[] args) {
    Dept dept2= Dept.builder()
            .deptno(20)
            .dname("RESEARCH")
            .loc("DALLAS")
            .build();
    System.out.println(dept2);
}

Lombok版

• 原代码：

@Builder
public class Dept {
	private Integer deptno;
	private String dname;
	private String loc;
	
}

• 编译后生成的代码：

public class Dept {
	private Integer deptno;
	private String dname;
	private String loc;

	Dept(Integer deptno, String dname, String loc) {
		this.deptno = deptno;
		this.dname = dname;
		this.loc = loc;
	}

	public static DeptBuilder builder() {
		return new DeptBuilder();
	}

	public static class DeptBuilder {
		private Integer deptno;
		private String dname;
		private String loc;

		public DeptBuilder deptno(Integer deptno) {
			this.deptno = deptno;
			return this;
		}

		public DeptBuilder dname(String dname) {
			this.dname = dname;
			return this;
		}

		public DeptBuilder loc(String loc) {
			this.loc = loc;
			return this;
		}

		public Dept build() {
			return new Dept(this.deptno, this.dname, this.loc);
		}

		public String toString() {
			return "Dept.DeptBuilder(deptno=" + this.deptno + ", dname=" + this.dname + ", loc=" + this.loc + ")";
		}

	}
}

示例：自定义Map工具类（重点）

public class MapUtil{
    public static  MapBuilder builder() {
        return new MapBuilder(new HashMap());
    }

    public static class MapBuilder  {
        private final Map map ;

        public MapBuilder(Map map) {
            this.map = map;
        }

        public MapBuilder put(K k, V v) {
            map.put(k, v);
            return this;
        }

        public Map build() {
            return map;
        }
    }

    public static void main(String[] args) {
        Map map = MapUtil.builder().put("a", "aaaa").put("b", 1111).build();
        map.forEach((k, v) -> System.out.println(k + " : " + v));
    }

}