Effective Java笔记：当遇到多个构造器参数时，考虑用构建器

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我们考虑一种情况，在一个类初始化时，我们要构造多个参数时，毫无疑问，我们可以这样做：

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public class BuildPattern{
	private int a;
	private int b;
	private int c;
	private int d;
	
	public BuildPattern(int a,int b,int c,int d){
		this.a = a;
		this.b = b;
		this.c = c;
		this.d = d;
	}
}

但是，如果c 和 d 是可选参数，我们并不想在构造器中将默认值传入，改怎么做呢？

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public class BuildPattern{
	private int a;
	private int b;
	private int c;
	private int d;
	
	public BuildPattern(int a,int b){
		this.a = a;
		this.b = b;
		this.c = 0;
		this.d = 0;
	}
	
	public BuildPattern(int a,int b,int c){
		this.a = a;
		this.b = b;
		this.c = c;
		this.d = 0;
	}
	
	public BuildPattern(int a,int b,int c,int d){
		this.a = a;
		this.b = b;
		this.c = c;
		this.d = d;
	}
}

三四个参数还好说，但是参数越来越多的时候，就变得很难管理了。我们也很容易想到重叠构造器模式：

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public class BuildPattern{
	private int a;
	private int b;
	private int c;
	private int d;
	
	public BuildPattern(int a,int b){
		this(a,b,0);
	}
	
	public BuildPattern(int a,int b,int c){
		this(a,b,c,0);
	}
	
	public BuildPattern(int a,int b,int c,int d){
		this.a = a;
		this.b = b;
		this.c = c;
		this.d = d;
	}
}

此时的重叠构造器模式在参数越来越多的时候也会变的难以管理，于是想到了另一种替代颁发，JavaBean模式：

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public class BuildPattern{
	private int a = -1;//必须参数，没有初始化值
	private int b = -1;
	private int c = 0;
	private int d = 0;
	
	public BuildPattern(){}
	
	public void setA(int val){a = val;}
	public void setB(int val){b = val;}
	public void setC(int val){c = val;}
	public void setD(int val){d = val;}
}

此时创建实例就容易多了，代码也容易阅读：

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BuildPattern bp = new BuildPattern();
bp.setA(3)；
bp.setB(4);
bp.setC(5);

不过，因为构造过程被分到了几个调用中，在调用过程中可能出现不一致的状态，即线程不安全。于是我们有了另一种解决以上问题的办法：我们不值接生成想要的对象，而是让客户端利用所有必要的参数调用构造器，从而得到一个builder对象，在具体实现中调用类似setter的方法，来设置每个可选的相关参数。范例如下：

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public class BuilderPattern {
	private final int a;
	private final int b;
	private final int c;
	private final int d;
	
	public static class Builder{
		//必选参数
		private final int a;
		private final int b;
		
		//可选参数
		private int c = 0;
		private int d = 0;
		
		public Builder(int tempA,int tempB){
			this.a = tempA;
			this.b = tempB;
		}
		public Builder c(int val){
			c = val;
			return this;
		}
		public Builder d(int val){
			d = val;
			return this;
		}
		
		public BuilderPattern build(){
			return new BuilderPattern(this);
		}
	}
	
	public BuilderPattern(Builder builder) {
		a = builder.a;
		b = builder.b;
		c = builder.c;
		d = builder.d;
	}
	public void show(){
		System.out.println("a="+a);
		System.out.println("b="+b);
		System.out.println("c="+c);
		System.out.println("d="+d);
	}
	public static void main(String[] args) {
		BuilderPattern bp = new BuilderPattern.Builder(1, 2).c(3).d(4).build();
		bp.show();
	}
}
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