在Java中使用递归方法遇到麻烦

Question

The compiler says I'm not returning an integer. The getC method returns an integer. How can I fix this?

public static int calcCost(Guard[] g, int spot){
    if(spot >= 31078657) return 0;
    else {
        for(int i = 51499; i >= 0; i--){
            if(g[i].getS() == spot && g[i].getF() <= 31078657) {
                return (g[i].getC() + calcCost(g, g[i].getF() + 1));
            }
        }
    }
}

Answer 1

This problem arises when your method doesn't return a result by following all the paths. In this case, if the if inside the for loop never gets executed, you aren't returning anything.

Just add a default return statement at the bottom of your method:

public static int calcCost(Guard[] g, int spot){
    if(spot >= 31078657) return 0;
    else {
        for(int i = 51499; i >= 0; i--){
            if(g[i].getS() == spot && g[i].getF() <= 31078657) {
                return (g[i].getC() + calcCost(g, g[i].getF() + 1));
            }
        }
    }
    //here
    return 0; //or another desired default value
}

Answer 2

Simply add default value in the end

public static int calcCost(Guard[] g, int spot){
    if(spot >= 31078657) return 0;
    else {
        for(int i = 51499; i >= 0; i--){
            if(g[i].getS() == spot && g[i].getF() <= 31078657) {
                return (g[i].getC() + calcCost(g, g[i].getF() + 1));
            }
        }
    }
    return -1;// add default value here 
}

OR you can throw an Exception such as IllegalArgumentException in case of invalid value.

public static int calcCost(Guard[] g, int spot){
    if(spot >= 31078657) return 0;
    else {
        for(int i = 51499; i >= 0; i--){
            if(g[i].getS() == spot && g[i].getF() <= 31078657) {
                return (g[i].getC() + calcCost(g, g[i].getF() + 1));
            }
        }
    }
    throw new IllegalArgumentException();
} 

Answer 3

You're not returning an integer in every case . If the inner condition is never met, control will flow to the end of the method without anything being returned.

Answer 4

You need to add a 'return' for the case when your 'for' loop terminates - ie with i==0. Not knowing what this code is doing, it's entirely possible that you are certain the the inner return must occur for some value of i, but neither I nor your compiler can determine that this is the case, merely by looking at this code fragment. Modern compilers are pretty smart, but not that smart ;-)