java - java中final字段初始化的问题: avoiding duplication of code using multiple constructors

Question

我有一个带有一些私有(private)最终字段的类:

public class ClassA {
  private final Object field1;
  private final Object field2;
  ...
}

该类有几个不同的构造函数，带有各种参数:

public ClassA(Object arg1, Object arg2);
public ClassA(int arg1, String arg2, boolean arg3);

这些构造函数计算要放入最终字段的值。

理想情况下，我想做这样的事情:

public ClassA(Object arg1, Object arg2) {
  ... // error check params
  Object value1 = ... // calculate value 1 based on args
  Object value2 = ... // calculate value 2 based on args

  init(value1, value2);
}

public ClassA(int arg1, String arg2, boolean arg3) {
  ... // error check params
  Object value1 = ... // calculate value 1 based on args
  Object value2 = ... // calculate value 2 based on args

  init(value1, value2);
}

private void init(Object arg1, Object arg2) {
  ... // error checks on combination of calculated arg1 and arg2 values
  ... // in reality there are more than 2 args, and this logic is fairly complex
  field1 = arg1;
  field2 = arg2;
  ... // other common initialization code, depends on field1 and field2
}

为了重用分配和公共(public)初始化代码。但是，由于这些字段是最终字段，因此只能在构造函数调用中对其进行赋值。

在这种情况下无法摆脱使用公共(public)构造函数来创建对象，因此任何类型的工厂方法都是不可能的。最好将对 arg1、arg2 值的组合错误检查保留在与将它们分配给 ClassA 字段的同一 block 中。否则，我会将 init() 分成 2 个函数，一个对最终字段进行预分配，一个对最终字段进行后分配，并使我的构造函数如下所示:

public ClassA(int arg1, String arg2, boolean arg3) {
  ... // error check params
  Object value1 = ... // calculate value 1 based on args
  Object value2 = ... // calculate value 2 based on args

  preinit(value1, value2);  // error checks combination of values
  field1 = value1;
  field2 = value2;
  postinit();  // other common initialization code
}

建议？有什么方法可以避免这种情况，或者我是否坚持拆分 init() 函数？

Answer 1

首先，我会尝试查看构建器模式，但如果您坚持将其作为构造函数并且字段应该是最终的，请使用私有(private)构造函数:

public class ClassA {
    private final Object field1;
    private final Object field2;

    public ClassA(Object arg1, Object arg2) {
        this(calc1(arg1, arg2), calc2(arg1, arg2), true);
    }

    public ClassA(int arg1, String arg2, boolean arg3) {
        this(calc1(arg1, arg2, arg3), calc2(arg1, arg2, arg3), true);
    }

    private ClassA(Object arg1, Object arg2, boolean a) {
        field1 = arg1;
        field2 = arg2;
    }

    private static Object calc1(int arg1, String arg2, boolean arg3) {
        return ... // calculate value 1 based on args
    }

    private static Object calc2(int arg1, String arg2, boolean arg3) {
        return ... // calculate value 2 based on args
    }

    private static Object calc1(Object arg1, Object arg2) {
        return ... // calculate value 1 based on args
    }

    private static Object calc2(Object arg1, Object arg2) {
        return ... // calculate value 2 based on args
    }

}