int变量未初始化

Question

private int getBirthdayYear(){
        Random r1 = new Random();
        int x = r1.nextInt(1000);
        int year = 1000+ x;
        return birthdayYear;
      }
          private String getBirthday(){
        int year = getBirthdayYear();
        Random r2 = new Random();
        int y = r2.nextInt(11);
        String monthStr;
        int month = 1 + y;
          switch (month) {
            case 1:  monthStr = "January";
                     break;
            case 2:  monthStr = "February";
                     break;
            case 3:  monthStr = "March";
                     break;
            case 4:  monthStr = "April";
                     break;
            case 5:  monthStr = "May";
                     break;
            case 6:  monthStr = "June";
                     break;
            case 7:  monthStr = "July";
                     break;
            case 8:  monthStr = "August";
                     break;
            case 9:  monthStr = "September";
                     break;
            case 10: monthStr = "October";
                     break;
            case 11: monthStr = "November";
                     break;
            case 12: monthStr = "December";
                     break;
            default: monthStr = "Invalid";
                     break;
        }


        Random r3 = new Random();
        **int day;// day variable declared as int**
        if(month == 1||month == 3||month == 5||month == 7||month == 8||month == 10||month == 12){
        int z1 = r3.nextInt(30);
        day = 1 + z1;
        }
        else if(month == 2||month == 4||month == 6||month == 9||month == 11){
          int z2 = r3.nextInt(29);
        day = 1 + z2;
        }
        else if(month == 2 & year%4 ==0){
        int z3 = r3.nextInt(28);
        day = 1 + z3;
        }
        else if(month == 2 & year%4 != 0){
        int z4 = r3.nextInt(27);
        day = 1 + z4;
        }

        **birthday = monthStr + " " + day + "th " + year;// error here. day variable is not initialized
        return birthday;**
          }

Answer 1

在Java中，不能使用可能未初始化的变量。“uninitialized”表示变量可能未设置为任何值。如果编译器不能证明变量始终是初始化的，就会产生一个错误。

int a;

// a is uninitialized

if(r.nextInt(10) < 2)
   a = 5;

// a has an 80% chance of still being uninitialized

System.out.println(a); // error: a might not be initialized.  

在您的情况下：

int day;
// day is uninitialized
if(month == 1||month == 3||...){
    int z1 = r3.nextInt(30);
    day = 1 + z1;
}
else if(month == 2||month == 4||month == 6||month == 9||month == 11){
    int z2 = r3.nextInt(29);
    day = 1 + z2;
}
else if(month == 2 & year%4 ==0){
    int z3 = r3.nextInt(28);
    day = 1 + z3;
}
else if(month == 2 & year%4 != 0){
    int z4 = r3.nextInt(27);
    day = 1 + z4;
}
// if the month wasn't between 1 and 12, day is still uninitialized

编译器不够聪明，无法检测月份始终介于1和12之间。修复方法是确保day始终设置为某个值，即使month不在该范围内。

这样做的一种方法：

int day;
// day is uninitialized
if(month == 1||month == 3||...){
    int z1 = r3.nextInt(30);
    day = 1 + z1;
}
else if(month == 2||month == 4||month == 6||month == 9||month == 11){
    int z2 = r3.nextInt(29);
    day = 1 + z2;
}
else if(month == 2 & year%4 ==0){
    int z3 = r3.nextInt(28);
    day = 1 + z3;
}
else if(month == 2 & year%4 != 0){
    int z4 = r3.nextInt(27);
    day = 1 + z4;
}
// THIS PART IS NEW
else
    day = -1; // this line will never actually run

现在编译器可以知道，无论发生什么，day总是设置为在该语句结束之后的某个值。

请注意，day=-1；从未实际运行。只有这样，编译器才能证明day总是有一个值。