Java:在if语句中访问字符串

Question

显示在If语句中声明的字符串时出现问题,以下是代码:

import java.util.Scanner;

// This program reads a temp and prints its current state
public class P03_01_2 {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        // get temp
        System.out.print("Enter a temperature: ");
        if (in.hasNextDouble()) {
            double temp = in.nextDouble();

            // determine temp type and determine state

            System.out.print("Enter C for Celcius or F for Fahrenheit: ");
            String type = in.nextLine();
            if (type.equals("c") || type.equals("C")) {
                if (temp <= 0) {
                    String state = "frozen";
                } else if (temp >= 100) {
                    String state = "gaseous";
                } else {
                    String state = "water";
                }
            }
            if (type.equals("f") || type.equals("F")) {
                if (temp <= 32) {
                    String state = "frozen";
                } else if (temp >= 212) {
                    String state = "gaseous";
                } else {
                    String state = "water";
                }
            } else if ((!type.equals("c") || !type.equals("C") || !type.equals("f") || !type.equals("F"))) {
                System.out.print("Not valid input.");
                System.exit(0);

            } else {
                System.out.println("Not valid input.");
                System.exit(0);
            }

            System.out.println("The state of the water is " + state);

        }
    }
}

我已经尝试在第一个if语句之前添加"String state = null之前",但是一直有错误.......谢谢.

Answer 1

你在每个块中声明变量.即每个声明都在它自己的块中并且单独作用域.

String state = null;
if (...) {
   String state = "";
   // this is a different variable to 'state' outside the block!
}

您需要在块外声明变量,然后在例如内部初始化

String state = null;
if (...) {
   state = "";
}

// state is still visible here

这是一个关于Java范围概念的简单教程.