java - 我可以通过 if else/语句声明变量吗

Question

可以通过 if/else 语句声明变量吗？

int num1;
int num2;
int num3;

if (num1 <= num2){
    num3 = (num1-num2);
}else{
    num3 = (num2-num1);
}

Answer 1

您可以在 if/else 中分配变量:

int num1 = 2;
int num2 = 3;
int num3;                // This is a declaration

if (num1 <= num2){
    num3 = (num1-num2);  // This is an assignment
}else{
    num3 = (num2-num1);  // This is an assignment
}
// We can use num3 here

您还可以在 if/else 中声明变量，但之后该变量将不可见。

int num1 = 2;
int num2 = 3;

if (num1 <= num2){
    int num3 = (num1-num2);  // This is a declaration and assignment in one.
}else{
    int num3 = (num2-num1);  // So is this
}
// We can't use num3 here.

如果您在 if/else 中分配变量，请确保保证分配该变量。

int num1 = 2;
int num2 = 3;
int num3;

if (num1 <= num2){
    num3 = (num1-num2);  
}else if (num1 == 9) {
    num3 = (num2-num1);  
}
// We can't use num3 here because the compiler can't be sure that one of the assignments happened.

使用条件运算符(也称为三元运算符)通常比在 if/else 中赋值更好

int num3 = num1 <= num2 ? num1 - num2 : num2 - num1;