java - 我期望输出为 40，但我得到了 30

Question

class Tester {
    public static void main(String[] arg) {
        byte b=10;
        b +=(b<127)?b>-128? b+=10 :0 :5;
        System.out.println(b);
    }
}

我知道条件被评估为 true 并将控制权交给了 b+=10所以现在逻辑上 b+=b+=10; 将 b 和 10 的值相加，计算结果为 20，分配给 b。现在我无法进一步评估它。

Answer 1

JLS 15.7.1. Evaluate Left-Hand Operand First有一个类似的例子:

If the operator is a compound-assignment operator (§15.26.2), then evaluation of the left-hand operand includes both remembering the variable that the left-hand operand denotes and fetching and saving that variable's value for use in the implied binary operation.

Example 15.7.1-2. Implicit Left-Hand Operand In Operator Of Compound Assigment

In the following program, the two assignment statements both fetch and remember the value of the left-hand operand, which is 9, before the right-hand operand of the addition operator is evaluated, at which point the variable is set to 3.

class Test2 {
    public static void main(String[] args) {
        int a = 9;
        a += (a = 3);  // first example
        System.out.println(a);
        int b = 9;
        b = b + (b = 3);  // second example
        System.out.println(b);
    }
}

This program produces the output:

12
12

因此，在您的情况下，b (10) 的原始值被记住并添加到三元条件运算符的结果(即 20，因为 b+= 10 是该表达式的结果)，给你 30。