builder模式

Posted 2021-02-09 lnas01

Builder pattern vs Constructor vs Setter

 

You could have built an object with this:

// Example 1

new FlyingMachine("Boeing 787", 2, false);

Or this:

// Example 2

FlyingMachine plane = new FlyingMachine();
plane.setName("Boeing 787");
plane.setNumEngine("Boeing 787");
plane.setHasRocketBooster(false);

 

Example one (Constructor based instantiation)

In example one, the object is created through constructor. There are multiple downsides with this

The arguments passing into the constructor are not meaningful

What the hell does 2 and false exactly mean?

In comparison the builder pattern has done a great job explaining the values .numEngine(2) and .hasRocketBooster(false)

The arguments are all passed in one line making it nearly impossible to read

 

You may arrange like this

new FlyingMachine( "Boeing 787", 2, false);

 

But I doubt it really improves readability

 

You cannot omit parameters

When one day you realize that you do not want to name that flying machine, you may choose to pass in null for name

new FlyingMachine(null, 2, false);

But using the builder pattern, you can simply remove this line: .name("Boeing 787")

 

Example two (Setter method)

In example two, the internal properties of the object are set by setters after instantiation. It essentially achieves the same effects as the two other methods, but it has a completely different semantic meanings. This method creates an empty object and fills in the details later, which bypasses the constructor completely and these properties cannot be used during initialization. In reality, you won’t build an aircraft without knowing what’s inside, right?

 

摘录自 effective java

public class DoDoContact {
    private final int    age;
    private final int    safeID;
    private final String name;
    private final String address;
 
    public static class Builder {
        private int    age     = 0;
        private int    safeID  = 0;
        private String name    = null;
        private String address = null;
        　　　
        // name必要
        public Builder(String name) {
            this.name = name;
        }
 
        public Builder age(int val) {
            age = val;
            return this;
        }
 
        public Builder safeID(int val) {
            safeID = val;
            return this;
        }
 
        public Builder address(String val) {
            address = val;
            return this;
        }
 
        public DoDoContact build() { 
            // 通过builder构建实例
            return new DoDoContact(this);
        }
    }
 
    private DoDoContact(Builder b) {
        age = b.age;
        safeID = b.safeID;
        name = b.name;
        address = b.address;
 
    }
}                

　　

DoDoContact ddc = new DoDoContact.Builder("Ace").age(10)
                .address("beijing").build();
System.out.println("name=" + ddc.getName() + "age =" + ddc.getAge()
                + "address" + ddc.getAddress());

　　

 

reference：

https://www.cnblogs.com/moonz-wu/archive/2011/01/11/1932473.html

https://hermannyung.com/2017/05/28/builder-pattern-vs-constructor-vs-setter/