java - 在匿名类中限定此名称与简单名称

Question

在下面的代码中，为什么 Java 认为简单名称 numCows 可能未初始化？为什么使用限定-this 可以防止错误？

import java.util.function.Function;

public class CowFarm {

    private final int numCows;

    public CowFarm(int numCows) {
        this.numCows = numCows;
    }

    // Fails to compile
    //
    // CowFarm.java:12: error: variable numCows might not have been initialized
    public final Function<Integer, Integer> MULTIPLY_COWS = (k -> numCows * 2);

    // Works fine
    public final Function<Integer, Integer> DIVIDE_COWS = (k -> CowFarm.this.numCows * 2);
}

Answer 1

来自Java Specification :

For every access of a... blank final field x, x must be definitely assigned before the access, or a compile-time error occurs... Such an assignment is defined to occur if and only if either the simple name of the variable (or, for a field, its simple name qualified by this) occurs on the left hand side of an assignment operator.

出现在类声明中赋值运算符 (=) 左侧的

final 字段在类构造函数中的 final 字段之前实例化。由 this 键限定的 final 字段在类构造函数之后实例化。考虑以下示例:

public class Foo{

    //Instantiated before constructor
    final Bar bar1 = new Bar();

    //Instantiated by constructor    
    final Bar bar2;

    //Instantiated after constructor
    final Bar bar3 = this.bar2;

    //Compile time error
    final Bar bar3 = bar2;

    public Foo(Bar bar) {
        this.bar2 = bar;
    }
}

就您而言，问题在于当您尝试实例化 MULTIPLY_COWS 时，numCows 未明确分配。 this 限定符确保在实例化 MULTIPLY_COWS 时明确分配 numCows。

// This works fine, as well
public final Function<Integer, Integer> DIVIDE_COWS = (k -> this.numCows * 2);